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EFFECT OF COMPOSITION ON THE MELTING POINT OF AN ALLOY

* PURPOSE

In this experiment students will explore the effect of changing the proportions of the metallic components on the melting point of an alloy.

* DESCRIPTION

This experiment is appropriate for a first-year college-prep or advanced course. Students prepare an alloy sample and then test the melting points of that and other samples of different percentage composition. While the actual measurements may be relatively crude, it will be evident that varying the composition of an alloy has an effect on the melting point.

* TIME REQUIRED

Two lab Periods.

* MATERIALS

Chemicals:
solder--50% Pb/50% Sn, nonacid core*
bismuth lead*
mineral oil*
Equipment:
ring stand
clay triangle
Bunsen burner
crucible tongs or pot holders
400-mL beaker
thermometer, to 200°C*
hot plate with magnetic stirrer and magnetic stir bar*
wire
aluminum weighing pan*
tin snips
pliers
* See Modifications / Substitutions

* HAZARDS

The crucibles and molten metals will be extremely hot; crucible tongs must be used. The mineral oil will be very hot. Allow it to cool to room temperature before moving it and before removing the alloy samples. Mineral oil is combustible; heat only on a hot plate. Do not substitute powdered or granular lead since lead dust or powder that can be inhaled is hazardous; lead is poisonous. Goggles must be worn throughout the experiment.

* MODIFICATIONS/SUBSTITUTIONS

  1. Solder can be purchased at a hardware store. Lead in the form of lead sinkers can be obtained from bait and tackle shops. Mineral oil is available from drugstores.
  2. Since the relative rate of the melting points compared to the alloy composition are what is of interest in this experiment, it is not necessary to use a thermometer. The melting points of the alloys are sharp and can easily be observed.
  3. A stirring rod can be used instead of a magnetic stirring bar although it is not as convenient.
  4. If aluminum weighing pans are not available, metal aspirin tins or fuse tins are a suitable substitute.

* PROCEDURE

  1. Support a crucible on a pipestem triangle on an iron ring attached to a ring stand.
  2. Mass one of the following sets of metal combinations assigned by the teacher. Each group should mass a different set.
    1. 12.8 g solder + 6.4 g bismuth
    2. 9.0 g solder + 4.6 g lead + 13.5 g bismuth
    3. 9.2 g solder + 4.6 g lead + 4.6 g bismuth
    4. 6.1 g solder + 12.1 g bismuth
  3. Place the metals in the crucible. Heat the crucible to melt the metals together. Use the crucible tongs to gently swirl the contents to evenly mix the metals as they melt.
  4. Use the crucible tongs to carefully pour the molten metal into an aluminum weighing pan. The alloy layer should be fairly thin.
  5. Allow the metal to cool and harden for later use.
  6. Collect one gram samples of each of the four alloy sets by using the tin snips to cut small pieces. Identify each of the samples.
  7. Pliers should be used to bend each piece of alloy so that it can be suspended from a 6-cm piece of wire. Hook all four wires containing the samples on a stirring rod. Be sure that the alloy samples are the same distance from the rod. Place this assembly into a 400-mL beaker containing 200 mL of mineral oil.
  8. Put the beaker on a hot plate and begin heating the oil. Watch the thermometer carefully after the temperature reaches 70°C and record the temperature at which each of the metal-alloy samples melts.

* DISPOSAL

The alloy samples can be removed from the cooled mineral oil and saved for reuse. The cooled mineral oil can be stored for reuse as a high melting point medium.

* DISCUSSION

Alloys are solid solutions made by melting two or more metals together. If the melt is cooled rapidly, a homogeneous noncrystalline metal results. The physical properties of alloys are often quite different than the properties of the metals making up the alloy. The alloying of metals is important commercially because it is one of the primary ways of altering the properties of the pure metallic elements.

Wood's metal is a low melting alloy made of tin, lead, bismth and cadmium. It is commonly used in sprinkler systems for fire protection in buildings. The alloy combinations in this experiment do not use cadmium because of the toxicity of cadmium. The alloys formed here from bismuth, lead, and tin have melting temperatures considerably below those of the pure metals: bismuth, 271.3°C; tin 231.8°C; and lead 327.5°C.

* TIP

This activity could be carried out as a demonstration. The melted samples can be recovered from the mineral oil and reused each year. To save time, make one alloy sample, pour it into the mold and then use the previously prepared samples to demonstrate how composition affects the melting point.

* REFERENCES

Brown, T.L. and LeMay, H.E., Jr., Chemistry: The Central Science, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981, p. 700. This work describes the chemistry of alloys.

McDuffie, T.E. and Anderson, J., Chemical Experiments From Daily Life, J. Weston Walch Publisher, Portland, ME, 1980, p. 4. A similar experiment is describes that uses powdered metals; powdered metals pose a safety hazard.

Weast, R.C., editor, CRC Handbook of Chemistry and Physics, CRC Press, Inc., Boca Raton, FL. This reference can be used to obtain melting points of alloys and pure metals.

Submitted by Judith Bazler, John Ihde, Carolyn Lucas,Harry Palin and Joe Trebella


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