Metals are opaque, lustrous elements that are good conductors of heat and electricity. Most metals are malleable and ductile and are, in general, denser than the other elemental substances.
What are some applications of metals?
Metals are used in:
Transportation -- Cars, buses, trucks, trains, ships, and airplanes.
Aerospace -- Unmanned and manned rockets and the space shuttle.
Computers and other electronic devices that require conductors (TV, radio, stereo, calculators, security devices, etc.)
Communications including satellites that depend on a tough but light metal shell.
Food processing and preservation -- Microwave and conventional ovens and refrigerators and freezers.
Construction -- Nails in conventional lumber construction and structural steel in other buildings.
Biomedical applications -- As artificial replacement for joints and other prostheses.
Electrical power production and distribution -- Boilers, turbines, generators, transformers, power lines, nuclear reactors, oil wells, and pipelines.
Farming -- Tractors, combines, planters, etc.
Household conveniences -- Ovens, dish and clothes washers, vacuum cleaners, blenders, pumps, lawn mowers and trimmers, plumbing, water heaters, heating/cooling, etc.
Future Trends
In the future, we will continue to depend heavily on metals. Lightweight aluminum alloys will be utilized more in automobiles to increase fuel efficiency. New, heat resistant superalloys will be developed so that engines can operate at higher, more efficient temperatures. Similarly, ceramic coatings will be used more to protect metals from high temperatures, and to increase the lifetime of tools. New, radiation-resistant alloys will allow nuclear power plants to operate longer, and thus lower the cost of nuclear energy.
Steel will continue to be the most commonly used metal for many years to come, due to its very low cost (approximately 20 cents/pound) and the ability to customize its properties by adding different alloying elements.
Finally, as easily-mined, high grade ores are depleted, recycling will become more important. Already, half of all aluminum, copper, and steels are being recycled.
Scientific Principles
Structure of Metals:
Metals account for about two thirds of all the elements and about 24% of the mass of the planet. They are all around us in such forms as steel structures, copper wires, aluminum foil, and gold jewelry. Metals are widely used because of their properties: strength, ductility, high melting point, thermal and electrical conductivity, and toughness.
These properties also offer clues as to the structure of metals. As with all elements, metals are composed of atoms. The strength of metals suggests that these atoms are held together by strong bonds. These bonds must also allow atoms to move; otherwise how could metals be hammered into sheets or drawn into wires? A reasonable model would be one in which atoms are held together by strong, but delocalized, bonds.
Bonding
Such bonds could be formed between metal atoms that have low electronegativities and do not attract their valence electrons strongly. This would allow the outermost electrons to be shared by all the surrounding atoms, resulting in positive ions (cations) surrounded by a sea of electrons (sometimes referred to as an electron cloud).
Figure 1: Metallic Bonding.
Because these valence electrons are shared by all the atoms, they are not considered to be associated with any one atom. This is very different from ionic or covalent bonds, where electrons are held by one or two atoms. The metallic bond is therefore strong and uniform. Since electrons are attracted to many atoms, they have considerable mobility that allows for the good heat and electrical conductivity seen in metals.
Above their melting point, metals are liquids, and their atoms are randomly arranged and relatively free to move. However, when cooled below their melting point, metals rearrange to form ordered, crystalline structures.
Figure 2: Arrangement of atoms in a liquid and a solid.
Crystals
To form the strongest metallic bonds, metals are packed together as closely as possible. Several packing arrangements are possible. Instead of atoms, imagine marbles that need to be packed in a box. The marbles would be placed on the bottom of the box in neat orderly rows and then a second layer begun. The second layer of marbles cannot be placed directly on top of the other marbles and so the rows of marbles in this layer move into the spaces between marbles in the first layer. The first layer of marbles can be designated as A and the second layer as B giving the two layers a designation of AB.
Layer "A" Layer "B" AB packing
Figure 3: AB packing of spheres. Notice that layer B spheres fit in the holes in the A
layer.
Packing marbles in the third layer requires a decision. Again rows of atoms will nest in the hollows between atoms in the second layer but two possibilities exist. If the rows of marbles are packed so they are directly over the first layer (A) then the arrangement could be described as ABA. Such a packing arrangement with alternating layers would be designated as ABABAB. This ABAB arrangement is called hexagonal close packing (HCP).
If the rows of atoms are packed in this third layer so that they do not lie over atoms in either the A or B layer, then the third layer is called C. This packing sequence would be designated ABCABC, and is also known as face-centered cubic (FCC). Both arrangements give the closest possible packing of spheres leaving only about a fourth of the available space empty.
The smallest repeating array of atoms in a crystal is called a unit cell. A third common packing arrangement in metals, the body-centered cubic (BCC) unit cell has atoms at each of the eight corners of a cube plus one atom in the center of the cube. Because each of the corner atoms is the corner of another cube, the corner atoms in each unit cell will be shared among eight unit cells. The BCC unit cell consists of a net total of two atoms, the one in the center and eight eighths from the corners.
In the FCC arrangement, again there are eight atoms at corners of the unit cell and one atom centered in each of the faces. The atom in the face is shared with the adjacent cell. FCC unit cells consist of four atoms, eight eighths at the corners and six halves in the faces. Table 1 shows the stable room temperature crystal structures for several elemental metals.
Table 1: Crystal Structure for some Metals (at room temperature)
Aluminum............ FCC Nickel.................. FCC
Cadmium............. HCP Niobium............... BCC
Chromium............ BCC Platinum............... FCC
Cobalt................. HCP Silver.................. FCC
Copper................ FCC Titanium............... HCP
Gold................... FCC Vanadium............. BCC
Iron.................... BCC Zinc.................... HCP
Lead................... FCC Zirconium............. HCP
Magnesium........... HCP
Unit cell structures determine some of the properties of metals. For example, FCC structures are more likely to be ductile than BCC, (body centered cubic) or HCP (hexagonal close packed). Figure 4 shows the FCC and BCC unit cells. (See Crystal Structure Activity)
Body Centered Cubic Face Centered Cubic
Figure 4: Unit cells for BCC and FCC.
As atoms of melted metal begin to pack together to form a crystal lattice at the freezing point, groups of these atoms form tiny crystals. These tiny crystals increase in size by the progressive addition of atoms. The resulting solid is not one crystal but actually many smaller crystals, called grains. These grains grow until they impinge upon adjacent growing crystals. The interface formed between them is called a grain boundary. Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains. (See A Particle Model of Metals Activity) Figure 5 shows a typical view of a metal surface with many grains, or crystals.
Figure 5: Grains and Grain Boundaries for a Metal.
Crystal Defects:
Metallic crystals are not perfect. Sometimes there are empty spaces called vacancies, where an atom is missing. Another common defect in metals are dislocations, which are lines of defective bonding. Figure 6 shows one type of dislocation.
Figure 6: Cross Section of an Edge Dislocation, which extends into the page. Note how the plane in the center ends within the crystal.
These and other imperfections, as well as the existence of grains and grain boundaries, determine many of the mechanical properties of metals. When a stress is applied to a metal, dislocations are generated and move, allowing the metal to deform.
Mechanical Properties:
When small loads (stresses) are applied to metals they deform, and they return to their original shape when the load is released. Bending a sheet of steel is an example where the bonds are bent or stretched only a small percentage. This is called elastic deformation and involves temporary stretching or bending of bonds between atoms.
Figure 7: Elastic deformation in a bar of metal.
When higher stresses are applied, permanent (plastic) deformation occurs. For example, when a paper clip is bent a large amount and then released, it will remain partially bent. This plastic deformation involves the breaking of bonds, often by the motion of dislocations. See Figure 8. Dislocations move easily in metals, due to the delocalized bonding, but do not move easily in ceramics. This largely explains why metals are ductile, while ceramics are brittle.
Figure 8: Dislocation movement in a crystal.
If placed under too large of a stress, metals will mechanically fail, or fracture. This can also result over time from many small stresses. The most common reason (about 80%) for metal failure is fatigue. Through the application and release of small stresses (as many as millions of times) as the metal is used, small cracks in the metal are formed and grow slowly. Eventually the metal is permanently deformed or it breaks (fractures). (See Processing Metals Activity)
Processing:
In industry, molten metal is cooled to form the solid. The solid metal is then mechanically shaped to form a particular product. How these steps are carried out is very important because heat and plastic deformation can strongly affect the mechanical properties of a metal.
Grain Size Effect:
It has long been known that the properties of some metals could be changed by heat treating. Grains in metals tend to grow larger as the metal is heated. A grain can grow larger by atoms migrating from another grain that may eventually disappear. Dislocations cannot cross grain boundaries easily, so the size of grains determines how easily the dislocations can move. As expected, metals with small grains are stronger but they are less ductile. Figure 5 shows an example of the grain structure of metals.
Quenching and Hardening:
There are many ways in which metals can be heat treated. Annealing is a softening process in which metals are heated and then allowed to cool slowly. Most steels may be hardened by heating and quenching (cooling rapidly). This process was used quite early in the history of processing steel. In fact, it was believed that biological fluids made the best quenching liquids and urine was sometimes used. In some ancient civilizations, the red hot sword blades were sometimes plunged into the bodies of hapless prisoners! Today metals are quenched in water or oil. Actually, quenching in salt water solutions is faster, so the ancients were not entirely wrong.
Quenching results in a metal that is very hard but also brittle. Gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering. (See Processing Metals Activity). It results in many small Fe3C precipitates in the steel, which block dislocation motion which thereby provide the strengthening.
Cold Working:
Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is bent or shaped, dislocations are generated and move. As the number of dislocations in the crystal increases, they will get tangled or pinned and will not be able to move. This will strengthen the metal, making it harder to deform. This process is known as cold working. At higher temperatures the dislocations can rearrange, so little strengthening occurs.
You can try this with a paper clip. Unbend the paper clip and bend one of the straight sections back and forth several times. Imagine what is occurring on the atomic level. Notice that it is more difficult to bend the metal at the same place. Dislocations have formed and become tangled, increasing the strength. The paper clip will eventually break at the bend. Cold working obviously only works to a certain extent! Too much deformation results in a tangle of dislocations that are unable to move, so the metal breaks instead.
Heating removes the effects of cold-working. When cold worked metals are heated, recrystallization occurs. New grains form and grow to consume the cold worked portion. The new grains have fewer dislocations and the original properties are restored.
Alloys:
The presence of other elements in the metal can also change its properties, sometimes drastically. The arrangement and kind of bonding in metals permits the addition of other elements into the structure, forming mixtures of metals called alloys. Even if the added elements are nonmetals, alloys may still have metallic properties.
Copper alloys were produced very early in our history. Bronze, an alloy of copper and tin, was the first alloy known. It was easy to produce by simply adding tin to molten copper. Tools and weapons made of this alloy were stronger than pure copper ones. Adding zinc to copper produces another alloy, brass. Although brass is more difficult to produce than bronze, it also was known in ancient times. (See "Gold" Penny Activity) Typical composition of some alloys is given in Table 2.
Table 2: Composition of several alloys.
Alloy Composition
Brass................... Copper, Zinc
Bronze................. Copper, Zinc, Tin
Pewter................. Tin, Copper, Bismuth, Antimony
Solder.................. Lead, Tin
Alnico.................. Aluminum, Nickel, Cobalt, Iron
Cast iron............... Iron, Carbon, Manganese, Silicon
Steel.................... Iron, Carbon (plus small amounts of alloying elements)
Stainless Steel........ Iron, Chromium, Nickel
Alloys are mixtures and their percentage composition can vary. This is useful, because the properties of alloys can be manipulated by varying composition. For example, electricians need a solder with different properties than the one used by plumbers. Electrical solder hardens very quickly producing an almost immediate connection. This would not be practical for plumbers who need some time to set the joint. Electrical solder contains about 60% tin, whereas plumber's solder contains about 30%.
Pewter originally contained lead, and since pewter was used for plates and goblets, it probably was a source of lead poisoning. Pewter made today is lead-free. Increased knowledge of the properties of metals also leads to new alloys. Some brasses form shape memory alloys which can be bent and will return to their original shape when gently heated. Zinc alloys, used as a coating on steel, slow corrosion (galvanized steel). Cadmium alloys find extensive use in solar cells. The ability of cupronickel to resist the build-up of deposits makes it useful for cages in fish farming.
Iron and Steel:
Carbon steels vary in the percentage of carbon they contain. The amount of carbon affects the properties of the steel and its suitability for specific uses. Steels rarely contain more than 1% carbon. Structural steel contains about 0.1-0.2% carbon by weight; this makes it slightly more ductile and less apt to break during earthquakes. Steel used for tools is about 0.5-1 % carbon, making it harder and more wear resistant. Cast iron is between 2.5 and 4% carbon and finds use in low cost applications where its brittleness is not a problem. Surprisingly, pure iron is extremely soft and is rarely used. Increasing the amount of carbon tends to increase the hardness of the metal as shown by the following graph. In slowly cooled steels, carbon increases the amount of hard Fe3C; in quenched steels, it also increases the hardness and strength of the material.
Figure 9: Hardness of steel as a function of % carbon.
Figure 10: BCC iron showing the location of interstitial carbon atoms.
Bobby pins and paper clips are processed in much the same way but contain different amounts of carbon. Bobby pins and paper clips are formed from cold worked steel wire. The paper clip, containing little carbon, is mostly pure Fe with some Fe3C particles. The bobby pin has more carbon and thus contains a larger amount of Fe3C which makes it much harder and stronger.
The properties of steel can be tailored for special uses by the addition of other metals to the alloy. Titanium, vanadium, molybdenum and manganese are among the metals added to these specialty steels. Stainless steel contains a minimum of 12% chromium, which stops further oxidation by forming a protective oxide on the surface.
Corrosion:
Corrosion of metals can be a major problem, especially for long-term structural applications like cars, bridges, and ships. Most corrosion is electrochemical (galvanic) in nature. To have corrosion, an anode (a more easily oxidized region) and a cathode (a less easily oxidized region) must be present. These may be different types of metals or simply different regions on the same metal. Some sort of electrolyte that can allow the transport of electrons must also be present. Corrosion involves the release of electrons at the anode due to the high oxidation potential of the atoms at the anode. As the electrons are released, metal cations are formed and the metal disintegrates. Simultaneously, the cathode, which has a greater reduction potential, accepts the electrons by either forming negative ions or neutralizing positive ions.
In the case of the activity or electromotive force series, a metal such as zinc reacts with hydrogen and serves as both the anode and the cathode. (See Activity Series Activity) The equation for this reaction is:
2 Zn + 2 H+ —> 2 Zn2+ + H2
Hydrogen bubbles at the cathode while the anode is destroyed. Surface imperfections, the presence of impurities, orientation of the grains, localized stresses, and variations in the environment are some of the factors determining why a single piece of metal may serve as both electrodes. For example, the head and point of a nail have been cold worked and can serve as the anode while the body serves as the cathode. (See Corrosion of Iron Activity)
Although oxidation at the anode and reduction at the cathode are simultaneous processes, corrosion usually occurs at the anode. The cathode is almost never destroyed. In 1824, Davy developed a method of protecting the hulls of ships from corrosion by using zinc that can be periodically replaced. Zinc is more active than the steel in the hull and will serve as the anode and be corroded; it is sacrificed to protect the steel structure. The steel that would have been both the anode and cathode normally serves as the cathode. This is called cathodic protection. Pipe lines are similarly protected by the more active metal magnesium. Sometimes electric currents are maintained in short sections of pipe lines with a length of similar metal wired to serve as the sacrificial anode.
Corrosion is a major problem that must be solved in order to effectively utilize metals. Iron combines with oxygen in the air forming iron oxide (rust), eventually destroying the usefulness of the metal. (See Optional: Chemical Hand Warmer Activity) Fortunately, some metals, such as aluminum and chromium, form a protective oxide coating that prevents further oxidation (corrosion). Similarly, copper combines with sulfur and oxygen forming the familiar green patina.
Understanding the chemistry of metals leads to the development of methods to reduce and prevent corrosion. Chromium atoms are about the same size as iron atoms and can substitute for them in iron crystals. Chromium forms an oxide layer that allows stainless steel to resist corrosion. Metals can be painted or they can be coated with other metals; galvanized (zinc coated) steel is an example. When these two metals are used together, the more active zinc corrodes, sacrificing itself to save the steel.
Metal Ores:
Gold, silver, and copper were the first metals used because they are found in the free or elemental state. Most metals found in nature are combined with other elements such as oxygen and sulfur. Energy is needed to extract metals from these compounds or ores. Historically, the ease with which a given metal could be extracted from its ore, along with availability, determined when it came into use, hence the early use of copper, tin, and iron. The formulas for some ores are given below:
Hematite Fe2O3 Rutile TiO2
Magnetite Fe3O4 Zircon ZrSiO4
Pyrite FeS2 Cassiterite SnO2
Chalcocite Cu2S Bauxite Al2O3
Cinnabar HgS Galena PbS
These ores are ionic compounds in which the metals exist as positive ions. For example the oxidation state of iron in hematite is +3; the oxidation state of copper in chalcocite is +1. Extracting metals from their ores is an oxidation-reduction (Redox) reaction. In the elemental state, metals consist of atoms not ions. Since atoms have no overall charge the metal ions gain electrons in the reaction; they are reduced.
The overall reaction for the reduction of copper from chalcocite is:
Cu2S + O2 + Energy —> 2 Cu + SO2
This is the overall reaction only. The complete process is not this simple. The reduction of metals from their ores typically requires a series of chemical and mechanical processes. These are usually energetically expensive, consuming large amounts of heat and/or electrical energy. For example, about five percent of the electricity consumed in the United States is used to produce aluminum. It costs about one hundred times as much to make an aluminum pop can, starting with the ore, as it does to melt and form recycled aluminum. Extracting metals from ores may also produce pollutants such as the sulfur dioxide above. Whenever possible, recycling and reprocessing metals makes sense.
The relative difficulty of extracting metals from their ores indicates that this is their preferred state. Once removed from their ores, and in the elemental state, most metals display considerable tendency to react with oxygen and sulfur and return to their natural state; they corrode! In corrosion, the metal is oxidized. It loses electrons, becoming a positive ion. (See Corrosion of Metals Activity)
Metals Summary
Metals have useful properties including strength, ductility, high melting points, thermal and electrical conductivity, and toughness. They are widely used for structural and electrical applications. Understanding the structure of metals can help us understand their properties.
Metal atoms are attached to each other by strong, delocalized bonds. These bonds are formed by a cloud of valence electrons that are shared between positive metal ions (cations) in a crystal lattice. In this arrangement, the valence electrons have considerable mobility and are able to conduct heat and electricity easily. In the crystal lattice, metal atoms are packed closely together to maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals called grains that touch at grain boundaries.
Due to the delocalized nature of the bonds, metal atoms are able to slide past each other when the metal is deformed instead of fracturing like a brittle material. This movement of atoms is accomplished through the generation and movement of dislocations in the lattice. Processing techniques that change the bonding between atoms or affect the number or mobility of dislocations can have a large effect on the mechanical properties of a metal.
Elastic deformation of a metal is a small change in shape at low stress which is recoverable after the stress is removed. This type of deformation involves stretching of the metal bonds, but the atoms do not slide past each other. Plastic deformation occurs when the stress is sufficient to permanently deform the metal. This type of deformation involves the breaking of bonds, usually by the movement of dislocations.
Plastic deformation results in the formation of more dislocations in the metal lattice. This can result in a decrease in the mobility of these dislocations due to their tendency to become tangled or pinned. Plastic deformation at temperatures low enough that atoms cannot rearrange (cold-working), can strengthen a metal as a result of this effect. One side effect is that the metal becomes more brittle. As a metal is used, cracks tend to form and grow, eventually causing it to break or fracture.
Dislocations cannot easily cross grain boundaries. If a metal is heated, the grains can grow larger and the material becomes softer. Heating a metal and cooling it quickly (quenching), followed by gentle heating (tempering), results in a harder material due to the formation of many small Fe3C precipitates which block dislocations.
Mixing of metals with other metals or nonmetals can result in alloys that have desirable properties. Steel formed from iron and carbon can vary substantially in hardness depending on the amount of carbon added and the way in which it was processed. Some alloys have a higher resistance to corrosion.
Corrosion is a major problem with most metals. It is an oxidation-reduction reaction in which metal atoms form ions causing the metal to weaken. One technique that has been developed to combat corrosion in structural applications includes the attachment of a sacrificial anode made of a metal with a higher oxidation potential. In this arrangement, the anode corrodes, leaving the cathode, the structural part, undamaged. The formation of a protective coating on the outside of a metal can also resist corrosion. Steels that contain chromium metal form a protective coating of chromium oxide. Aluminum is also corrosion resistant due to the formation of a strong oxide coating. Copper forms the familiar green patina by reacting with sulfur and oxygen in the air.
Only a few pure metals can be found in nature. Most metals exist as ores, compounds of the metal with oxygen or sulfur. Separating the pure metal from the ore often involves large amounts of energy as heat and/or electricity. Due to this large expenditure of energy, it makes sense to recycle metals when possible.
Discussion Questions
1. How are ores extracted from the earth?
2. Name 4 alloys and the metals from which they are made.
3. What impact does "cold working" have on metals?
4. What process makes metals hard, but brittle?
5. What process makes metals softer and easier to work?
6. Give three methods used to reduce corrosion.
7. Give 2 valuable impacts of recycling.
Problem
Assume the radius of one iron atom is 1.24 angstroms (1 angstrom = 1 x 10-8 cm). What would be the density of body centered cubic (BCC) iron in grams/cubic centimeter? Hint: Find the mass and volume of one unit cell. Remember to count only the fraction of each atom in the cell.
Extension:
The maximum solubility of carbon in BCC iron is one atom for every 5000 atoms of iron. What would be the density of steel with the maximum amount of carbon dissolved?
Solution
r = m/V = # atoms x (mass/atom) / cell volume
In BCC iron, there are two iron atoms per unit cell. (8 x 1/8 + 1)
One iron atom has a mass of 55.85 amu or 9.27 x 10-23 grams.
The total mass of one unit cell is 1.85 x 10-22 grams.
Let (r) be the radius of an iron atom. The atoms at the corners contact the atom in the middle, making the diagonal of the box equal to (4r).
If we call one side of the box (L), a diagonal of the cube face would be equal to (square root of 2) times (L).
One side, the diagonal of the cube face, and the diagonal of the box make a right triangle. Using the Pythagorean theorem, (L)2 + (square root 2 x (L))2 = (4r)2.
Solving for L and plugging in for (r), we find that L = 2.86 angstroms or 2.86 x 10-8 cm.
The volume of the cube (unit cell) is (L)3 = 2.34 x 10-23 cm3. Dividing the mass by the volume we get :
Density = 7.91 grams/cm3.
References
Bogner, Donna, Starting at Ground Zero, vol. 4, Genie Publications, Hutchinson, KS, (1989), pp. 41-42.
Borgford, Christie L., and Summerlin, Lee R., Chemical Activities: Teacher Edition, American Chemical Society, Washington, DC, (1988), pp. 77-78.
Carter, Giles F., Paul, Donald E., Materials Science and Engineering, ASM International, United States, (1991).
Ciardullo, C. V., C. M., Micro Action Chemistry, Princeton, NJ, (1990), pp. 63-64.
"Corrosion and Corrosion Prevention", a lesson from Metallurgy for the Non-Metallurgist, Course 3, Lesson, Test 12, ASM International, (1987).
Cortez, James A., Powell, Dick, and Mellon, Ed, "Test Tube Geology: A Slowly Developing Redox System for Class Study", J. Chem. Ed., 65-4, April (1988), pp. 350-351.
Future Metal Strategy, The Metals Society, London, (1980).
LaQue, F. L. (supervisor), Corrosion in Action, The International Nickel Company, Inc., New York, (1955).
Louthan, McIntyre R. Jr., Metals: A History, ASM International, (1987).
Loyd, Lowell, Plastic Deformation and Annealing of Metals, ASM International, (1987).
McCabe and Bauer, Metals, Atoms, and Alloys, National Science Teachers Association, Inc., (1964).
Mendenhall, J. Howard, Understanding Copper Alloys, Olin Corporation, (1977).
Neely, John E., Practical Metallurgy and Materials of Industry, 2nd Ed., John Wiley and Sons, New York, (1984).
Shackelford, James R., Introduction to Materials Science for Engineers, 3rd Ed., Macmillan, New York, (1992).
Schrager, Arthur M., Elementary Metallurgy and Metallography, 3rd Ed., (1969).
Smith, J. Denny, Heat Treatment of Steel , ASM International, (1977).
Summerlin, Lee R., Borgford, Christie L., Ealy, Julie B., Chemical Demonstrations: A Sourcebook for Teachers, Volume 2, 2nd Ed., American Chemical Society, Washington, DC, (1988), pp. 101-102.
Tylecote, R. F., A History of Metallurgy, 2nd Ed., The Institute of Materials, Brookfield, VT, (1992).
Weaver, Elbert C. (editor), Scientific Experiments in Chemistry, Holt, Rinehart, and Winston, Inc., New York, (1966), pp. 96-98.
Resources
American Society for Metals (ASM International)
Materials Park, OH 44073
(216) 338-5151
The Metallurgical Society of AIME (TMS-AIME)
420 Commonwealth Drive
Warrendale, PA 15086
(412) 776-9050
American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME)
345 East 47th Street
New York, NY 10017
American Iron and Steel Institute
1101 17th Street Northwest
Suite 1300
Washington, DC 20036-4700
(202) 452-7100
Master Materials and Equipment Grid
Material |
Lab 1 |
Lab 2 |
Lab 3 |
Demo1 |
Lab 4 |
Demo |
Lab 5 |
Lab 6 |
Styrofoam (balls 26/lab) |
S/DS |
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Toothpicks (round) |
G |
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Plastic petri dish |
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LE |
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BB's |
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H/DS |
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"Bobby" pins (hair pins) |
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DS |
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Bunsen burner or hot plate |
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LE |
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LE |
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Pair of 3" C-clamps |
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H/LE |
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Hammer |
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H |
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Beaker, 400 ml, 100 ml |
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LE |
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LE |
LE |
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Tongs/ forceps |
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LE |
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LE |
LE |
LE |
Wire gauze |
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LE |
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LE |
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16-18 gauge wire: aluminum, steel, copper, nickel, etc. |
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LE |
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LE |
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0.016" diameter piano wire, 10 feet |
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H/S |
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2 Ring stands |
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LE |
LE |
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2 Burette clamps, single |
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LE |
LE |
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Meter stick |
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LE |
LE |
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3 Alligator clip lead wires |
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LE |
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VARIAC |
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LE |
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40 - 60 gram weight |
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LE |
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AC ammeter |
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LE |
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Extension cord |
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H/DS |
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2 - #3 or #4, 1 hole stoppers |
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LE |
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2 - 6" strips, 14 gauge copper wire |
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H |
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small paper cup (1 oz.) |
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G/DS |
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variable masses (lead shot or small |
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LE/H |
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6 and 3 molar HCl |
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LE |
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LE |
pre- and post-1983 pennies |
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O |
O |
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triangular file |
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LE/H |
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centigram balance |
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LE |
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acetone |
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LE |
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powdered tin |
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LE |
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steel wool |
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LE |
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metal samples: Zn, Cu, Mg, Pb, Fe |
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LE |
0.1 M AgNO3, CuSO4, MgSO4, ZnSO4, and Pb(NO3)2 |
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LE |
24 hole cell well plate and beral pipettes |
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LE |
KEY FOR TABLE: E = ELECTRONIC STORE
H = HARDWARE, G = GROCERY,
DS = DISCOUNT STORE,
LE = LAB EQUIPMENT / SCIENTIFIC CATALOG,
S = SPECIALTY SHOP, O = OTHER
Materials |
Demo 3 |
Lab 7 |
Lab 8 |
2.5 - 5.0 grams CuSO4.5H2O |
LE |
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NaCl |
LE/G |
LE/G |
LE/G |
Filter paper |
LE |
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Small nails |
H |
H |
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Test tubes |
LE |
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Test tube rack |
LE |
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Stirring rod |
LE |
LE |
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Agar |
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LE |
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ring stand |
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LE |
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Plastic petri dish |
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LE |
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Beaker, 400 ml |
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LE |
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Bunsen burner or hot plate |
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LE |
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Wire gauze |
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LE |
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3 molar HCl |
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Foil: zinc, copper, aluminum, tin |
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LE |
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Magnesium ribbon |
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LE |
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Phenolphthalein |
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LE |
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.1 molar K3Fe(CN)6 |
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LE |
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Battery (1.5 or 9 volt) |
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H/DS |
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"Clip lead" wires |
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LE |
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Iron powder, 25 g |
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LE |
Small vermiculite, 1 tbs. |
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S |
plastic baggy |
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G |
KEY FOR TABLE: E = ELECTRONIC STORE,
H = HARDWARE, G = GROCERY,
DS = DISCOUNT STORE,
LE = LAB EQUIPMENT / SCIENTIFIC CATALOG,
S = SPECIALTY SHOP, O = OTHER
Experiment 1
Crystal Packin' Mama
Crystal Structure/Packing Exercise
Objective:
The objective of this lab is to learn more about the basic crystal structures that metal atoms form.
Review of Scientific Principles:
To maximize the bonding, atoms in metals pack together as closely as possible. Several packing arrangements exist such as face centered cubic (FCC) and hexagonal closest packing (HCP).
Applications:
The properties of metals are very dependent on their crystal structure. The metal structure can be altered by processing treatments to make them more useful in various applications.
Time: 50 minutes
Materials and Supplies:
26 Styrofoam balls, about 1.5" diameter
16 toothpicks(round)
Procedure:
1. Each of the Styrofoam balls will represent an atom and the toothpick will
represent bonds. Attach 10 of the balls together with toothpicks to form a
triangle with four balls at the base. This will form the first layer of the packing
model. Draw a diagram of the arrangement of the atoms in the space below.
2. Attach 6 of the balls together with toothpicks to form a triangle with 3 balls at
the base. This will form the second layer of the packing model. Draw a
diagram of the arrangement of the atoms in the second layer in the space below.
3. Form another triangle of Styrofoam balls like the one in procedure 1 with the
remaining 10 balls.
4. Place the second layer on top of the first one with "atoms" of the second layer
nesting in the hollows between the "atoms" of the first layer. This creates the
closest possible packing of atoms.
5. The third layer can be placed on top of the second layer in one of two positions.
It can be placed so that its "atoms" are directly over those in layer one. This
gives the ABABAB arrangement which corresponds to hexagonal closest
packing (HCP). The third layer can also be placed on top of the second layer so
that its "atoms" are not directly over those in the first layer. This gives the
ABCABC arrangement which corresponds to face centered cubic (FCC). Try
both arrangements with your layers.
Questions:
1. Which packing arrangement, FCC or HCP, is more dense?
2. What is the difference in FCC and HCP arrangements?
3. About how small would an atom have to be to fit in an interstitial hole in an FCC
or HCP crystal structure?
Teacher Notes:
• It would be beneficial for the teacher to have a completed model constructed with the layers painted different colors to help the students visualize the two types of packing arrangements.
• To paint the Styrofoam balls use water based paint diluted slightly and add a small amount of detergent.
• Below are the arrangements for the triangles the students are to construct.
Answers to Questions:
1. Actually, FCC and HCP packing arrangements have the same atomic density.
They each have approximately 26% empty space.
2. FCC has an ABC arrangement while HCP is ABA.
3. Depending on the type of hole, an interstitial atom should be approximately one
third the size of the atom which makes up the crystal structure in order to "fit"
well.
Experiment 2
Atomic Bb's
A Particle Model of Metals
Objective:
The objective of this lab is to learn more about the basic particle model for metals.
Review of Scientific Principles:
Metallic crystals are not perfect. Sometimes there are empty spaces, vacancies, where an atom should be. There are also small mismatches, dislocations, in the rows of atoms, and these are found in all metals.
Applications:
Defects in the crystal structure of metals control many of their properties including hardness and ductility.
Time: 40 minutes
Materials and Supplies:
Plastic Petri Dish
Bb's
Procedure:
Your team will be given a covered Petri dish containing Bb's. Keep the dish flat on the table so no Bb's spill out! Answer the Questions which correspond to a given procedure. Try to write complete idea answers; that increases understanding.
1. The Bb's represent the atoms in a metal.
2. If you move the Petri dish back and forth the atoms move. This simulates the
movement of atoms in a metal when it is heated.
3. Move the dish back and forth and try to get the atoms arranged as neatly as
possible. Slowing the motion of the dish and gradually stopping it simulates the
formation of the crystal.
4. Make a sketch below showing how the Bb's are arranged. You don't have to
draw all the Bb's!
5. Make a sketch below showing the arrangement of Bb's around an empty space.
When this happens in the metal it is called a vacancy. In a real crystal when
atoms are out of line it is called a dislocation.
6. Move the dish back and forth somewhat rapidly. (Don't spill the Bb's). This
simulates heating the metal. (Heating the metal gives the atoms more kinetic
energy.)
Questions:
1. Describe the bonding between the atoms in a metal.
2. What type of energy do "moving" atoms possess?
3. Do the atoms in a crystal move?
4. Describe the arrangement of the Bb's. Are there any empty spaces, i.e., places
where a Bb is missing?
5. Are the Bb's arranged perfectly? Would you expect atoms to be perfectly
arranged?
6. Do more or less defects exist in the metal when it is heated?
Answers to Questions
1. The valence electrons of a metal atom are loosely held and considered to be
shared by all the atoms in the crystal. This is called the electron sea model.
2. Kinetic energy.
3. Yes, they vibrate about an equilibrium position.
4. Student answer.
5. It is unlikely that the atoms are perfectly arranged. Some disorder is expected.
6. More defects exist at higher temperatures.
Experiment 3
Making Metals Strong
Processing Metals
Objective:
The objective of this lab is to demonstrate the effect of cold-working (strain-hardening) and annealing on the ability of wires of the same metal to support a load.
Review of Scientific Principles:
Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is deformed, new dislocations are produced. As dislocations are generated and move, the metal can be bent or shaped without cracking. As the number of dislocations in the crystal increases, they will get tangled or pinned and will not be able to move. This will strengthen the metal, making it harder to deform. When this is done at or near room temperature, the process is known as cold-working. When cold-worked metals are annealed (heated gently), new grains form from the cold worked structure and grow until they replace it with new, soft crystals. Steels (alloys of iron with up to 1% carbon) can also be hardened by heating and quenching. At high temperatures (red hot), iron has an FCC structure which can dissolve carbon. At low temperature, the iron changes to BCC which cannot dissolve carbon, so it precipitates as an iron-carbon compound. If quenched, this compound does not have time to form, the carbon is trapped and distorts the BCC crystal structure to create a new, hard and brittle structure called Martensite. If Martensite is gently heated, the carbon can precipitate giving a strong, tough structure.
Applications:
The properties of metals can be altered by processing. Since the properties of a material are dependent upon its structure on the atomic level, altering its structure should alter its properties. Common treatments include cold-working and heat treating.
Time: 50 minutes, part I; 50 minutes, part II; 30 minutes, part III
Materials and Supplies:
Hammer
Bunsen burner and tongs
16 or 18 gauge solid wire of copper (or aluminum)
16 or 18 gauge solid wire of other metals
high carbon steel wire or bobby pins
Pair of 3" C-clamps (or other size if 3" not available)
Wire gauze
General Safety Guidelines:
• Take precautions to avoid burns when using the Bunsen burner to heat the metal.
• Make sure all fingers are out of the way when hammering the wires.
Procedure (Part I):
1. Hammer one of the pieces of copper wire until it is about half its original
thickness.
2. Bend it and the other piece of wire back and forth several times. Observe.
3. Heat the flattened (work hardened) piece of copper in the burner flame until red
hot.
4. Let it cool slowly on the wire gauze.
5. Label and save for experiment 4.
6. Repeat procedures 1 - 5 for the other wires.
7. Label and save the wires for later.
Procedure (Part II):
1. Obtain 7 samples each of high carbon steel wire (bobby-pins) and other metals.
2. Bend one of the wires until it breaks. Count and record the number of bends
needed to break the wire.
3. Heat the second and third steel wires in the middle until they are red hot. Let
them cool slowly in air.
4. When the wires are cool, bend one of them back and forth as before. Count and
record the number of bends needed to break this heat treated wire. Label and
save the other wire for later.
5. Fill the beaker with cold water.
6. Heat the fourth and fifth wires in the flame until they are red hot and
immediately plunge it into the water in the beaker.
7. When the wires are cool, bend one of them as before and record the number of
bends needed to break it. Label and save the other.
8. Heat and quench the last two wires as in procedure 6. Heat them again but cool
them slowly in air. This process is called tempering. As before, note the
properties of the tempered wire. Label and save one.
9. Repeat steps 1 - 8 for the other metal wires.
10. Save the extra wires for Experiment 4.
Procedure (Part III):
1. Using a pair of 3" C-clamps attached to the ends of the wire stretch a section of
annealed copper wire by 5% and another by 10%.
2. Repeat for the other metal wires.
3. Save these wires also for Experiment 4.
Questions:
1. What is the hammering in Part I, procedure 1 called?
2. In Part I, procedure 2, what did you observe about the ease of bending for each
wire? Why were they different?
3. In Part II, procedure 2, how many bends were required to break the wire? Did
it break easily? Briefly describe the mechanical properties for this sample.
4. What term describes the heat treating method used in Part II, procedure 3
(heating, slow cooling)?
5. In Part II, procedure 4, how many bends were required to break the wire? Did
it break easily? Briefly describe the mechanical properties for this sample.
6. What is cooling the hot metal rapidly as in Part II, procedure 6 called?
7. In Part II, procedure 7, how many bends were required to break the wire? Did
it break easily? Briefly describe the mechanical properties for this sample.
8. In Part II, procedure 8, what were the properties of the tempered wire?
Teacher Notes:
• You might have good success in stretching the wire if you wrap the ends of the wire around the treads of the C-clamp and then tighten the clamp on the ends of the wire.
• A convenient way to stretch the right amount is to mark a 10 cm section of wire with a dark colored marker. Then you can easily measure the amount of stretch between the marks (1 cm increase = 10%). Make sure the students understand that this is another way to cold-work metals.
• Processing metals with heat followed by quenching and cold-working should harden them. However, strong heating and quenching will only affect steel. Some aluminum is precipitate hardened with small amounts of copper. Heating these alloys strongly will soften them by causing the copper to form large precipitate particles which have little hardening effect. Most aluminum wire, however, is soft.
Answers to Questions:
1. Cold-working.
2. The hammered wire was harder to bend, but broke more easily. The hammering
produced many dislocations which became tangled, inhibiting the sliding of
planes of atoms.
3. Answers will vary. The unworked wires should be easier to bend and bend
more times before breaking.
4. Annealing.
5. Annealing the wires should soften the metal allowing it to bend more easily and
more times before breaking.
6. Quenching.
7. The quenched wires should be harder and bend fewer times before breaking.
8. The tempered wire should bend more times than the quenched wire did before
breaking.
Demonstration 1 (Caution; use only with extreme care)
Stand Back!
Phase Transition of High Carbon Steel
Time: 15 minutes
Materials and Supplies:
Piano wire 10" length, 0.016" diameter, of high-carbon steel, obtained from an instrument store (or hardware store)
VARIAC- source of variable AC voltage, 0-120 V, 0-5 amps
3 alligator clip lead wires
2 ring stands
2 adjustable single burette clamps
1 meter stick
metal weight - of about 40-60 grams
AC ammeter - 0-5 amps
Extension cord - UL rated at over 5 amps (heater cord)
2- #3 or #4 1 hole rubber stoppers
2- 6" strips of copper wire, #12 or #14 gauge (household wire)
Diagram of Set-up:
Procedure:
Set up the equipment as shown in Figure A. Attach one rubber stopper to each clamp. Thread one end of the piano wire up through the hole in the rubber stopper, and bend the ends over, or otherwise secure the wire to the stopper. The rubber stopper acts as an insulator between the electric circuit and the metal ring stand to prevent electric shock. Slide the metal weight over the other end of the wire, and attach this end of the wire to the rubber stopper. The metal weight should slide to the middle of the wire, causing the wire to hang down. Tape a meter stick to the table so you can read the height of the weight from the floor.
Make sure the VARIAC is unplugged, off, and the dial set at zero. The VARIAC will have one wire with a standard 120 V male plug on the end. This wire will be plugged into the wall receptacle. (LATER) The VARIAC will also have a standard 120 V female receptacle or a second wire. If the VARIAC has a standard female receptacle, you can plug the extension cord into it. Force the two pieces of copper wire into the female end of the extension cord. (Strip off the insulation.) Alligator clip lead wires will attach these wires to the rest of the circuit. Attach one lead wire to each of the copper wires. Attach the other end of one lead wire to the piano wire, about 2" below the rubber stopper. Attach the other end of the second lead wire to one terminal on the ammeter. Then attach the third lead wire to the other terminal on the ammeter and to the other end of the piano wire (about 2" below the rubber stopper). The purpose of the ammeter is to allow you to adjust the amount of current flowing through the circuit, so neither the VARIAC nor the circuit breaker is overloaded.
Now you have a complete circuit from the VARIAC to the piano wire. Check your circuit with Figure A. READ THE FOLLOWING EXPLANATION OF THE PROCEDURE COMPLETELY BEFORE YOU ATTEMPT TO PERFORM THIS DEMONSTRATION.
Plug the extension cord into the VARIAC. Make sure the dial on the VARIAC is set to zero and the switch is in the OFF position. Plug the VARIAC into the wall receptacle. Turn the switch on the VARIAC on. Now you are ready to heat the piano wire by making an electric current flow through it. As the voltage is increased, the current increases (I = V/R), and the wire gets hot (Power = I2R). However, in the beginning, the cold wire has a low resistance, so only a small voltage is needed to produce the 5 amp current needed to heat the wire red hot. This is why you SLOWLY turn the dial on the VARIAC, carefully watching the ammeter so as not to exceed 5 amps. As the wire heats, you will have to increase the voltage (turn the dial clockwise) to approximately 40 - 60 volts to maintain a current of 4.6 to 4.8 amps. The wire should be red hot, and will have expanded. An observer can watch the wire expand, by watching the metal weight lower along the meter stick. At this point, the weight will be at its lowest point. If you turn off the VARIAC (FLIP THE SWITCH) and turn the dial to zero, the wire will cool and contract, moving the weight up the meter stick. When the temperature of the wire cools to 720˚C, the weight will hesitate, then continue rising as the wire continues to cool and contract. The hesitation should be very noticeable; this is when the metal undergoes the transformation from FCC to BCC crystalline structure. Heating the wire takes less than 2 minutes, and cooling the wire takes less than 30 seconds.
NOW YOU ARE READY TO PERFORM THIS DEMONSTRATION.
1. Check your connections.
2. Plug in the VARIAC; MAKE SURE THE SWITCH IS OFF.
3. Choose one person to observe the weight and read the meter stick.
4. Warn all the observers not to touch any part of this apparatus due to the
possibility of electric shock or burns.
5. Turn on the VARIAC.
6. Slowly turn the dial of the VARIAC clockwise, checking the ammeter, so as not
to exceed 5 amps.
7. Continue to increase the voltage, until the wire is red hot, and approximately 4.6
to 4.8 amps of current is flowing through the wire.
8. Have the meter stick reader record the lowest position of the weight.
9. Tell the observer to watch the weight. Tell the meter stick reader to record the
height of the weight at the time that the weight stops moving.
10. Turn off the switch, and turn the dial to zero.
11. Ask the observer to describe the motion of the weight.
12. You can repeat this demonstration by repeating steps 3 through 11.
Note: If 5 amps are insufficient to heat the wire above 750˚C, use a finer gauge wire.
Note: As the wire is cycled through the experiment several times, the carbon content will be reduced as the carbon burns off at high temperatures. This decreases the sharpness of the phase change (which occurs over a wide range of temperatures rather than a single temperature, and is obscured by the thermal expansion). Thus, it will be necessary to replace the wire occasionally.
Optional Apparatus:
You may be able to use a toaster or other electric heating appliance (with a heater coil) wired in series with the piano wire. The darkness control on the toaster can be used as a potentiometer to control the voltage drop across the piano wire. This way you can use a single pole light switch (120 V), and eliminate the need for a VARIAC. The success of this method depends on reducing the voltage drop across the toaster enough, so the voltage drop across the piano wire will be large enough to heat the wire above the transformation temperature. This sounds challenging however.
Discussion
The crystalline structure of iron is different at different temperatures, and high-carbon steel alloys undergo a transformation from BCC to FCC in a very small temperature range. An interesting application of this theory is in determining the maximum temperature to which a piece of steel has been heated before it changes phases. By studying the crystal structure of a piece of metal, you can determine what transformations have occurred. Since these phase transformations occur over specific temperatures ranges, you can determine the heat history of the metal. This information can be useful in determining how close the material was to failure. One result of the Three-Mile Island incident was that some of the nuclear fuel was so hot, that it melted. Small pieces of molten nuclear fuel dropped to the floor of the containment vessel. As the fuel cooled, the steel floor became very hot. By examining the crystal structure of samples of the floor, metallurgists could determine the maximum temperature the metal reached. This information gave the design engineers an idea of the maximum amount of heat and stress the steel containment vessel will experience, if part of the cooling system of the reactor fails.
Experiment 4
Stretching Wires
Tensile Strength Test for Various Metals
Objective:
The objective of this experiment is to demonstrate the elastic and plastic properties of metals.
Time: 50 minutes
Review of Scientific Principles:
Wires of the same gauge, but made of different metals typically support different loads (masses) before going through the point at which they change from being elastic to being plastic. Elastic deformation is recoverable after the load is removed. Plastic deformation is not recoverable.
Applications:
In order to use metals in particular applications, it is sometimes necessary to know their tensile strength. To avoid failure, the right metal must be used.
Materials and Supplies:
15 to 20 cm long pieces of 16 or 18 gauge wire of copper, aluminum, steel, etc.
wires saved from experiment 3
ring stand
adjustable single-burette clamp
meter stick
small paper cup (a 1-oz) paper cup works well)
variable masses (lead shot or small ring-washers)
Procedure:
1. Clamp the solid copper wire in the clamp and attach the small cup to the end of
the metal wire. See diagram of setup.
2. Adjust the wire so it extends horizontally about 8 to 10 cm beyond the edge of
the clamp.
3. Measure the height of the end of the wire above the surface of the work area.
This height, ho, will be your reference height.
4. Carefully place 3 small washers (approximately 3 g) into the cup and again
measure the height of the end of the metal wire above the surface of the work
area. Record this mass and the new height.
5. Using your hand, gently support the cup and show the students how to check to
see that the wire returns to approximately its original height, ho.
6. Continue increasing the mass in the cup by 3 washers at a time, recording both
the mass in the cup and the height of the end of the wire above the surface of the
work area, until the wire no longer returns to approximately ho. Record this
value of mass. We shall refer to this maximum number of washers (mass) that
the wire can elasticity support as its critical number, Wc.
7. Now take 3 or 4 more sets of data on this wire after straightening it.
8. Replace the original copper wire with annealed copper wire of the same gauge.
9. Replace the annealed copper wire with the piece of copper wire that has been
annealed and then stretched (cold-worked) by 10%.
10. Repeat procedures 1-8 above for the other wires, time permitting. Be sure the
distance from the clamp to the point where the weights are attached is the same
for the annealed wire as it was for the original wire.
Diagram of Set-up:
Data:
Sample table
mass |
ht. |
disp. |
|
|
|
|
|
|
|
|
|
Sample Graph:
Analysis:
1. From your data collected in Part III, generate a data table for each wire;
including the values for mass, height, and displacement (height - ho).
2. Plot a graph of mass (on vertical axis) versus displacement for each of the
different types of wires. Note that the slope of the curve represents the relative
stiffness of the wire represented by the curve. See sample graph.
3. For each of the types of metal wires, find the maximum mass for which the
curve remained basically a straight line (the slope was constant). This defines
the yield strengths of the metal.
Questions:
1. What is happening to the bonded metal atoms during elastic deformation?
2. What is happening to the bonded metal atoms during plastic deformation?
3. Give the maximum mass placed on each wire before permanent deformation
occurred.
4. Why would an engineer be interested in the yield strength of a metal for a
particular application?
Teacher Notes:
• It is advisable to test this experiment beforehand to be sure the particular gauge wire that you have chosen undergoes sufficient deformation with the masses you are making available to the students.
• One way to save time on this lab is to have different groups do different metals.
Answers to Questions:
1. The bonds between the atoms are stretching.
2. Metal atoms are sliding past each other.
3. Student answers.
4. In most applications, it is not desirable to exceed the yield strength of the
product. If the yield strength is exceeded the object will be permanently
deformed and likely will no longer be useful.
Demonstration 2
Floating Pennies
Removal of Zinc from Pennies
Objective: The different reactivities of copper and zinc to hydrochloric acid will be used to separate the two metals in a post-1983 penny. The percent of zinc and of copper can be calculated as well as the economic value of each.
Time: This activity must be performed over a period of two days. Only a few minutes of student time are required each day.
Materials and Supplies:
one penny (newer than 1983)
20 ml 6 M HCl
100 ml beaker
triangular file
centigram balance
forceps or tongs
acetone (optional)
General Safety Guidelines:
• Goggles are necessary for this experiment.
• The beaker containing the 6 M HCl should be set in a fume hood overnight.
Procedure:
1. Use a triangular file to make four 1 mm deep scores in the penny's edge at 90˚
apart.
2. Determine the mass of the penny on a balance and give this value to the
students.
3. Place the penny in a 100ml beaker and add 20ml of 6M HCl.
4. Place this beaker in the fume hood or other safe area overnight.
5. The penny should be floating the next day. Decant the HCl off into the sink,
flushing with plenty of water, add about 50ml of tap water to the beaker
containing the penny to rinse it off, pour the water off, and tip the penny out
onto a paper towel. Leave the penny on the towel to dry a few minutes.
6. When the penny is dry, determine the mass again. Report this value to the
students.
7. Determine the mass of a pre-1982 penny and report this value to the students
also.
Questions and Calculations:
1. From the masses of the penny before and after the reaction, calculate the mass of
zinc in the penny.
2. Calculate the percentage of zinc and the percentage of copper in the penny.
3. From a newspaper look up the prices of copper and zinc at the present time.
Note the units. If the units are dollars per ounce, convert the value to dollars
per gram. (1 pound = 454 grams)
4. From the mass of zinc and the current market value of zinc, calculate the value
of zinc in the penny. Do the same for the copper in the penny.
5. Determine the value of the copper in the pre-1982 penny.
Why do you think our government switched to a copper-clad zinc penny?
Experiment 5
"Gold" Penny Lab
Forming Brass from Zinc and Copper
Objective: The objective of this lab is to use a post-1983 penny to produce a thin layer brass alloy and a pre-1983 penny to make a bronze alloy.
Scientific Principles:
In order to save expensive copper, penny coins, starting in 1983, were made of zinc with a thin layer of copper plated on the surface. If these coins are heated, the zinc will diffuse into the copper layer, producing a surface alloy of zinc and copper. These alloys are brasses. Not only does the zinc change the properties of copper, but also the color of the brasses changes with zinc content - reaching a golden yellow color at around 20% zinc and golden at 35-40% zinc. Copper also oxidizes when heated in air, producing a black layer of copper oxide (CuO). Thus when heated, there is a competition between the rate of oxidation (making the surface black) and the rate of diffusion (making the surface a golden-yellow color). Bronzes are alloys containing tin and copper.
Applications:
Brasses are used in many industries because they are fairly corrosion resistant but harder and cheaper than pure copper. Bronzes are sometimes used for the same purposes and are also used to make bearings. Bronzes are generally harder and more corrosion resistant than brasses.
Time: 40 minutes
Materials and Supplies:
3, pre-1982 penny
5, post-1983 pennies
powdered tin (Sn)
steel wool
hot plate or Bunsen burner
wire gauze
forceps or tongs
General Safety Guidelines:
• Hot metals can cause severe burns.
Procedure I:
1. Obtain five post-1983 pennies. Thoroughly clean them using the steel wool.
2. Pre-heat the hot plate using the setting which is 75% of the maximum value.
For Bunsen and wire gauze, place the Bunsen to produce maximum heating and
pre-heat.
3. Start timing and place four of the post-1983 pennies on the hot surface in a ring
around the center.
4. FOR HOT PLATE: Using forceps, remove one of the pennies at each of the
following time intervals:
1 minute
5 minutes
10 minutes
20 minutes
FOR BUNSEN AND WIRE GAUZE: Use the following time intervals:
15 seconds
25 seconds
35 seconds
45 seconds
5. While these pennies cool, place the pre-1982 penny on the hot plate for 10
minutes or wire gauze for 40 seconds.
Observations:
Record the color and anything else you observe about the pennies.
Post-1983, no heat
Post-1983, 1 min/15 sec
Post-1983, 5 min/25 sec
Post-1983, 10 min/35 sec
Post-1983, 20 min/45 sec
Pre-1982, 10 min/40 sec
What causes the color variations among the coins?
Procedure II:
1. Place 3 or 4 small grains of tin on the two remaining pre-1983 pennies. Place
these pennies on the pre-heated surface.
2. When one of the pennies develops a silver color in the area of the grains remove
it from the surface. Remove the second penny in a time period equal to twice
that of the first penny.
Observations:
1. Describe how the zinc alloy (brass) differs from the tin alloy (bronze).
Questions:
1. What is an alloy? What distinguishes an alloy from a compound?
2. Why did the color of the post-1983 penny change as you heated it longer?
3. What would happen if you heated other coins?
Teacher Notes:
• Brass can also be made by heating a mixture of copper and zinc pellets if good contact exists between the pellets.
• CAUTION! Zinc powder is very flammable, only use pellets of zinc.
Answers to Questions:
1. An alloy is a mixture (solution) of different types of atoms, generally metals.
An alloy can have a variable composition, but a compound has a specific
composition.
2. The color continued to change as more zinc diffused into the copper. Also, the
longer it was heated, the more oxidation occurred.
3. If the coins were made of pure metal, oxide might form on the surface. If the
metals were a mixture (dime or quarter), an alloy might form.
Experiment 6: (Optional)
Which one Reacts?
Activity Series
Objective: An activity series of six substances can be developed from performing and observing a series of single replacement reactions.
Review of Scientific Principles:
When two different metals are in contact with each other or simply wetted with a solution containing sufficient ions to carry an appreciable electric current, an electrochemical cell is formed. The more active of the two metals will be consumed by the reaction.
Applications:
Understanding the chemistry of metals leads to the development of methods to reduce and prevent corrosion. Two metals may be used together; the more active one corrodes, sacrificing itself to save the other.
Time: 30 minutes
Materials and Supplies:
6 pieces of zinc, 1 cm by 1/2 cm
6 pieces of copper, 1 cm by 1/2 cm
6 pieces of magnesium, 1 cm by 1/2 cm
6 pieces of lead, 1 cm by 1/2 cm
optional - 6 pieces of iron
0.1 M AgNO3 about 1 ml - 1.7g/100 ml
0.1 M CuSO4 about 1 ml - 2.5 g/100 ml
0.1 M MgSO4 about 1 ml - 1.2 g/100 ml
0.1 M ZnSO4 about 1 ml - 2.9 g/100 ml
0.1 M Pb(NO3)2 about 1 ml - 3.3 g/100 ml
3 M HCl
24 hole cell well plate or 13 x 100 mm test tubes, rack, and test tube brush
beral pipettes filled with the six solutions above
forceps
General Safety Guidelines:
• Performing this experiment as a micro chemical lab requires less chemical and is
generally safer.
• Proper eye protection should be worn.
Procedure:
1. Obtain a cell well plate and six pieces of each of the following metals: lead,
copper, magnesium, and zinc.
2. Obtain six Beral pipettes each filled with a different solution. The needed
solutions are 0.1 M silver nitrate, 0.1 M zinc sulfate, 0.1 M magnesium sulfate,
0.1 M lead (II) nitrate, 0.1 M copper (II) sulfate, and 3 M hydrochloric or
sulfuric acid.
3. Note that the cell well plate has a number and letter grid. Place 20-25 drops of
silver nitrate solution in cell well A1. Repeat this for B1, C1, and D1. It may
be necessary to refill the pipette. Place 20-25 drops of zinc sulfate solution in
each hole A2, B2, C2, and D2. Repeat this process remaining solutions until all
of the cells are filled appropriately.
4. Using forceps, drop a sample of lead into cell well A1. Add another sample of
lead to A2. Repeat this process until each of the cells in the A row contains a
sample of lead. Record your observations.
5. Add a piece of copper to each of the cells in the B row, zinc to the C row cells,
and magnesium to the D row. Record your observation after adding the metal
samples to each row.
6. Using forceps remove and rinse each remaining piece of metal. Follow your
teacher's directions as to disposal. Do not put the metal pieces down the drain.
Add water to the plate and dump the contents in the receptacle provided.
Questions:
1. Draw a chart similar to the cell well plate and record your observations in that
chart.
2. Write single replacement equations for those reactions that occurred.
3. Draw and fill in a chart showing which substance in each of the above reactions
is more active and which is less active. The six substances for comparison are
magnesium, silver, hydrogen, copper, lead, and zinc.
4. Which of the six substances always ended up less active? Which always ended
up more active?
5. Use the chart you created in question 3 to rank the six substances from most
active to least active.
Teacher Notes:
• If equipment is not available for micro-chemical analysis, test tubes can be substituted for the cell well. As mentioned previously, micro-chemical analysis is safer and results in less chemical to dispose of after the experiment.
Questions:
1. Student chart. The students should note reactions between any more active
metal with a less active cation.
2. A sample reaction would be one between magnesium and copper (II) sulfate.
Mg (s) + CuSO4 (aq) --> MgSO4 (aq) + Cu (s)
3-5. The order of activity, from most to least active, is: Mg Zn Fe Pb H Cu
Ag
Demonstration 3
Test Tube Geology
Corrosion of Iron
Objective: The objective of this experiment is to observe over a period of several days the corrosion of iron nails in a test tube.
Time Required: The initial set up will require fifteen to twenty minutes which includes time to mass the copper(II) sulfate. Five to ten minutes each day can then be spent recording observations until no more changes are observed.
Materials and Supplies:
2.5 to 5.0 g CuSO4. 5H2O
NaCl (twice as much volume as that of copper (II) sulfate)
two circles of filter paper or paper toweling
2 to 3 small nails
test tube
test tube rack
long stirring rod
General Safety Guidelines:
• Wear goggles when using copper (II) sulfate.
• Wash hands well.
• Discard test tube contents in trash or other receptacle when finished.
Procedure:
1. Use the test tube as a template and cut out two small filter paper disks.
2. Mass 2.5 to 5.0 g of copper (II) sulfate pentahydrate and place into a standard
size test tube. See diagram of setup.
3. Place one of the paper disks in the test tube and use the stirring rod to push it flat
against the copper(II) sulfate.
4. Add Sodium chloride to twice the depth of that of the copper (II) sulfate.
5. Place the second paper disk on top of the sodium chloride layer.
6. Place two or three nails on top of the paper disk and slowly add enough water to
cover the entire contents.
7. Sketch and label the contents of the test tube each day. Record any
observations.
Diagram of Set-up:
Questions:
1. Which is the more active metal, sodium or iron?
2. Which is the more active metal, copper or iron?
3. Write the reaction which allows the iron to corrode.
Answers to Questions:
1. Sodium is the more active metal. The iron does not react with the sodium.
2. Iron is more active than copper. The iron is oxidized and the copper is reduced.
3. 2Fe + 3Cu++ --> 2Fe+++ + 3Cu
Experiment 7
Rust!
Corrosion of Iron
Objective:
The objective of this lab is to observe the electrochemical nature of the changes in an iron nail when it corrodes and to investigate methods to protect it.
Review of Scientific Principles:
An understanding of the activity series investigated in experiment 6 suggests that one way of preventing the corrosion of iron is to protect it with a more active metal. Another way to prevent the corrosion of iron is to exclude oxygen and moisture from its surface with a protective coating.
Applications:
When iron is exposed to the weather, it tends to corrode (rust). Understanding of how this occurs leads to ways of preventing the corrosion.
Time: If the solutions are already prepared, each day will require 15 to 20 minutes.
Materials and Supplies:
Agar 2 to 5 grams/250 ml H2O, enough for 6 to 8 petri dishes
7.5 g NaCl
Iron nails
Copper foil, 2 inch by 1/8 inch, or copper wire
Zinc foil, 2 inch by 1/8 inch
Aluminum foil, 2 inch by 1/8 inch
Tin foil, 2 inch by 1/8 inch
Magnesium ribbon, 2 inch
Magnesium ribbon, 1 inch piece
0.1% phenolphthalein (0.1 g to 50-50 water-alcohol mixture)
0.1 M potassium ferricyanide, K3Fe(CN)6 (0.33 g/100 ml)
3 M HCl (if galvanized nails are used)
1 400 ml beaker (for heating agar solution)
6 petri dishes, either glass or plastic
1 stirring rod
Bunsen burner or hot plate
ring stand, ring clamp, wire gauze if Bunsen burner is used
beaker tongs (optional)
clip lead wires
1.5 V or 9 V battery
General Safety Guidelines:
• The potassium ferricyanide and phenolphthalein should be made by the teacher.
• Potassium ferricyanide is not dangerous unless heated to very high temperatures which is not done in this experiment.
Procedure:
1. Add 7.5 g of sodium chloride to 250 ml of distilled water in a 400 ml beaker.
Heat this to boiling. Turn off the flame if using a Bunsen burner or turn down
the heat if using a hot plate. Add slowly, with constant stirring, 5.0 g of agar.
2. After the agar has been dissolved, add 5 to 10 drops of the 0.1 M potassium
ferricyanide solution and 5 drops of 0.1% phenolphthalein solution.
3. While the agar mixture is cooling in the beaker, prepare the nails for the petri
dishes. Obtain twelve nails that have been soaked in 3 M HCl to remove any
zinc coating and six petri dishes. Each dish will have two nails. The following
are suggestions for each dish. Your teacher may wish to change the contents of
each dish. Get the nails ready but do not place them in the dishes yet.
• For dish 1: one straight nail and one bent nail
• For dish 2: one straight nail with copper foil wrapped with two turns
one straight nail with zinc foil wrapped with two turns
• For dish 3: straight nail with aluminum foil wrapped with two turns
straight nail with tin foil wrapped with two turns
• For dish 4: straight nail with magnesium ribbon wrapped with two turns
straight nail with one in. of Mg metal set nearby but not touching nail
• For dish 5: straight nail hammered in middle
straight nail heated red hot in the middle and allowed to cool slowly
• For dish 6: two straight nails with tips bent upward, battery attached by wires
4. Pour the agar mixture into the petri dishes to a depth of a little less than a
centimeter. Allow it to cool until it just begins to set. Place each nail in the
agar.
5. Use alligator clip and any other available method to attach lead wires to the nails
in the dish.
6. Attach the other ends to a battery. Note which nail is attached to the positive
end and which is attached to the negative end of the battery. Observe the
reaction and sketch and explain what is happening. After the reaction has been
noted, detach the wires.
7. Allow the dishes to react overnight. Observe, sketch, and explain the changes
observed.
Questions:
1. Compare the colors observed on the straight nail, the bent nail, the hammered
nail, and the heated nail.
2. Explain the differences observed between the copper wrapped nail and the zinc
wrapped nail.
3. Was there any difference observed between the aluminum wrapped nail and the
tin wrapped nail? If so, what was it?
4. Did the magnesium that was near, but not touching, the nail show any protective
tendencies?
5. Which end of the battery, the positive or negative, was connected to the nail that
turned pink? Was this nail the cathode or the anode?
Teacher Notes:
• This may be performed as an overhead demonstration using a water mixture containing a few drops of phenolphthalein and potassium ferricyanide solutions. Any 1.5 V battery should work but a 9 V battery will give almost immediate results.
• Attach two dissimilar pieces of metal to a volt meter using either the agar or water solutions.
• For an extension, ask students to design and perform other possible
combinations.
• In preparing agar, it is important to get the mixture hot enough to allow the agar to
dissolve, but if it gets too hot the agar will burn. If you have not made agar before, you might want to start out with a small batch for practice.
• It is very convenient to use a microwave to heat the agar mixture, if one is available.
• During the section using the batteries, you might ask the students to switch the electrodes from the battery and notice the fading of the original colors and the creation of the other colors.
• Blue coloration is evidence of the reduction of the ferricyanide ion and pink is the oxidation of the iron nail.
Answers to Questions:
1. Where the nail was stressed, the agar will turn pink and rust will form on the
nail. Other locations around the nail will be blue. Near the tip, end, bent
portion, hammered portion and heated portion the agar should be pink.
2. When the nail is removed, less rust should be seen where it was in contact with
the zinc. The copper is less active than iron and should not show protective
tendencies.
3. The aluminum should show better protective tendencies than the tin.
4. Yes. Since the magnesium is much more active than iron and there is an
electrical connection (ion laden agar), the magnesium can show protective
tendencies.
5. The agar near the nail connected to the positive end of the battery should turn
pink due to the oxidation of the nail. This nail should also show more rust.
This nail is the anode. The agar near the nail connected to the negative side of
the battery should turn blue, showing the reduction of the ferricyanide. This
nail is the cathode.
Experiment 8: (Optional)
Chemical Hand Warmer
Oxidation of a Metal
Objective:
The objective of this lab is to observe the heat energy that is given off during the oxidation of iron.
Review of Scientific Principles:
When a reaction is endothermic (absorbs energy - heat) in one direction, such as the reduction of iron oxide to elemental iron, it will be exothermic (give off energy) in the reverse direction, the oxidation of elemental iron back to iron oxide.
Applications:
The converting of iron ore (iron oxide) in the blast furnace requires tremendous amounts of heat energy. When iron spontaneously oxidizes back to iron oxide (rust) in the air the heat released is not noticeable. When this reaction is sped up, the amount of heat is noticeable and usable in the form of a hand-warmer.
Time: 20 minutes
Materials and Supplies:
25 g Iron powder
1 g sodium chloride
1 Tbs. small vermiculite (might try sand)
plastic baggy
General Safety Guidelines:
• Avoid burns from the warm chemicals.
• Use normal precautions for a laboratory experiment.
Procedure:
1. Mass 25 g of iron powder or very fine iron filings and 1 g of sodium
chloride. Place these in a small plastic bag. Shake the bag to mix.
2. Add about a tablespoon of vermiculite to the bag and shake well.
3. Add 5 ml of water and seal the bag. Shake it. The reaction will start after about
a minute.
Questions:
1. The individual commercial sporting goods store hand warmers are purchased in
a plastic bag. Inside the plastic bag is a cloth bag. The directions state that the
plastic bags should not be removed until you are ready to activate the hand
warmer. Why should the plastic bag not be removed?
2. In this reaction, iron metal was oxidized to form iron (III) oxide and heat was
released. Mills that reduce ores to metals take a large amount of energy to form
metals. Explain why the second process requires so much energy.
Teacher Notes:
• You might want to bring in a commercial hand warmer to show the students.
Answers to Questions:
1. The plastic bag keeps humidity (moisture) away from the chemicals until they
are ready to be activated.
2. The reaction to form elemental iron from iron (III) oxide is very endothermic.
Review Questions
1. Why did it take society so long to develop metals?
2. Define an alloy.
3. Why do metals break even though they are not stressed beyond their elastic limit? What conditions cause this type of failure?
4. What would be the advantage of alloys that would withstand higher temperatures?
5. Why does recycling save so much energy?
6. Removing elemental metal from its ore is called .
7. What impact does quenching have on ferrous metals?
8. What effect on tensile strength does stretching copper have?
9. Does rusting of steel occur at the anode of cathode?
10. What material is used as a "sacrificial" anode of steel?
11. What metal is alloyed with iron to make stainless steel?
12. What mechanical process is accomplished by stretching copper?
13. What happens to dislocations when a wire is bent?
14. Give the words for the following acronyms: FCC, BCC, HCP.
15. How does the metal composition differ in a paper clip and a bobby pin?
16. Compare the grain differences in normal steel and quenched steel.
Answers to Review Questions
1. Why did it take society so long to develop metals. It is very difficult to form elemental metals from their ores. It often requires very high temperatures. The technology for this process took many years to develop.
2. Define an alloy. A substance that has metallic properties and is made up of two or more chemical elements, of which at least one is a metal. The two types of alloys are in
3. Why do metals break even though they are not stressed beyond their elastic limit? What conditions cause this type of failure? Under repetitive stresses, cracks in a metal can develop and grow.
4. What would be the advantage of alloys that would withstand higher temperatures? They could be used for many applications, such as higher temperature gasoline engines, nuclear reactor containment vessels, etc.
5. Why does recycling save so much energy? Because of the large amount of energy required to form elemental metals from their ores.
6. Removing elemental metal from its ore is called Extracting or Reduction.
7. What impact does quenching have on ferrous metals? If the metal contains carbon, the carbon will not be able to separate during the FCC to BCC transition and will be trapped, resulting in a distorted BCC structure. This hard, brittle form of steel is called Martensite.
8. What effect on tensile strength does stretching copper have? Stretching copper increases its tensile strength due to the formation of dislocations which become pinned.
9. Does rusting of steel occur at the anode of cathode? Rusting is an oxidation process and occurs at the anode.
10. What material is used as a "sacrificial" anode of steel? Zinc is used in galvanized steel.
11. What metal is alloyed with iron to make stainless steel? Chromium.
12. What mechanical process is accomplished by stretching copper? Cold-working.
13. What happens to dislocations when a wire is bent? More dislocations form and they get tangled or pinned.
14. Give the words for the following acronyms: FCC, BCC, HCP. FCC - Face centered cubic, BCC - Body centered cubic, HCP - hexagonal closest packed
15. How does the metal composition differ in a paper clip and a bobby pin? The bobby pin contains more carbon and is harder and stronger.
16. Compare the grain differences in normal steel and quenched steel. Normal steel contains separate grains of BCC arranged Fe and Fe3C. In quenched steel, the carbon remains in the BCC iron crystals distorting its structure.
Glossary
activity series: also known as the Electromotive Force Series. This is a listing of the
elements according to their potential differences and ability to place other elemental
ions in solution.
alloy: a substance that has metallic properties and is made up of two or more chemical
elements, of which at least one is a metal.
annealing: a heat treatment of a metal designed to produce a soft, ductile condition.
Typically the metal is heated and allowed to cool slowly.
anode: electrode at which electrons are released during corrosion. The half reaction at the anode is called oxidation and the metal is said to be oxidized. The anode is the
electrode that disintegrates during corrosion.
bronze: An alloy composed of tin and copper.
cathode: electrode which accepts electrons during corrosion. The half reaction at the
cathode is called reduction and the metal is said to be reduced. The cathode is not destroyed during corrosion.
cathodic protection: a more active metal is placed next to a less active metal. The more
active metal will serve as an anode and will be corroded instead of the less active metal. The anode is then called a sacrificial anode.
cold-working: a permanent deformation of a metal below its crystallization temperature.
Deforming the metal creates more dislocations which entangle, pinning them and thereby strengthening the metal.
corrosion: oxidation-reduction reaction where electrons are released at the anode and
taken up at the cathode.
dislocations: linear defects in a crystal.
ductile: can be drawn or stretched into wire and other shapes.
elastic deformation: materials return to their original shape after a small load or stress
is applied.
face-centered cubic: crystal arrangement of close-packed layers of particles where three
layers of particles alternate positions. This layering is known as ABCABC.
failure: ultimate separation of metal parts due to applied loads. i.e. it breaks.
fatigue: the application and release of stresses as metal is used which cause small cracks
to grow, during many cycles of application, until they fracture.
grain: a crystal (ordered arrangement of atoms).
grain boundary: the interface between the grains or crystals.
Hall Process: an electrolytic technique to refine aluminum from its ore.
hardening: heating and rapidly cooling steel.
heat treating: modification of properties and structure of alloys by specific heating and
cooling cycles.
hexagon closest packing (HCP): crystal arrangement of layers of particles where
two layers alternate positions. The layering is known as ABAB.
malleable: can be hammered into a sheet.
martensite: a super-saturated solid solution of carbon in ferrite. The carbon atoms
distort the BCC ferrite into a BC-tetragonal structure.
metallic bonding: bond formed by positive ions surrounded by a sea of valence electrons.
ore: a natural mineral deposit that contains enough valuable minerals to make it profitable
to mine at the current technology.
oxide: a compound of oxygen with some other chemical element.
oxidation: the half of an electrochemical reaction where electrons are released. Oxidation
occurs at the electrode called the anode.
pinned: the dislocations in a crystal get tangled or attached to atoms of an alloying agent.
plastic deformation: materials remain deformed after a load is added and then removed.
quenched: cooled rapidly.
reduction: the half of an electrochemical reaction where electrons are taken up. Reduction occurs at the cathode.
reduction of metals: changing a metal ion to a neutral atom by the addition of electrons.
steel: an iron-carbon alloy, malleable in some temperature range as initially cast. Steel
usually contains some other alloying elements such as silicon, manganese, etc. as
well as impurities such as sulfur and phosphorus.
strength: a measure of the ability of a material to support a load.
stress: the internal forces produced by application of an external load, tending to displace
component parts of the stressed material. It is defined as the force (load) divided by
the area on which it acts.
toughness: the ability to aborb energy of deformation without breaking. High toughness requires both high strength and high ductility.
unit cell: The smallest repeating array of atoms in a crystal.
Source: http://matse1.matse.illinois.edu/metals/metals.doc
Web site to visit: http://matse1.matse.illinois.edu/
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