EFFECT OF COMPOSITION ON THE MELTING POINT OF AN ALLOY
In this experiment students will explore the effect of changing the proportions of the metallic components on the melting point of an alloy.
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
Two lab Periods.
* See Modifications / Substitutions
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
- solder--50% Pb/50%
Sn, nonacid core*
- bismuth lead*
- mineral oil*
- ring stand
- clay triangle
- Bunsen burner
- crucible tongs or pot holders
- thermometer, to 200°C*
- hot plate with magnetic stirrer
and magnetic stir bar*
- aluminum weighing pan*
- tin snips
- 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.
- 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.
- A stirring rod can be used instead of a magnetic stirring
bar although it is not as convenient.
- If aluminum weighing pans are not available, metal aspirin
tins or fuse tins are a suitable substitute.
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.
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.
- Support a crucible on a pipestem triangle on an iron ring
attached to a ring stand.
- Mass one of the following sets of metal combinations assigned
by the teacher. Each group should mass a different set.
- 12.8 g solder + 6.4 g bismuth
- 9.0 g solder + 4.6 g lead + 13.5 g bismuth
- 9.2 g solder + 4.6 g lead + 4.6 g bismuth
- 6.1 g solder + 12.1 g bismuth
- 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.
- Use the crucible tongs to carefully pour the molten metal
into an aluminum weighing pan. The alloy layer should be fairly
- Allow the metal to cool and harden for later use.
- Collect one gram samples of each of the four alloy sets by
using the tin snips to cut small pieces. Identify each of the
- 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.
- 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.
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.
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.
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.
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
Woodrow Wilson Leadership Program in Chemistry
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281