Micro – Melting Point
Matter is made up of many tiny particles called molecules. These molecule arrange themselves in the order which is lowest in energy at the temperature of the immediate environment. The physical form which occurs at this temperature is referred to as the “phase” of the matter. Phases of matter are characteristic of that matter at a particular temperature. If the temperature of the surroundings changes, then the phase of the matter may also change. The change of matter from the solid to the liquid phase, for example is referred to as MELTING.
In this experiment the melting point of several pure substances will be determined. The results of your experimentation can be verified by consulting the accepted or theoretical values in a chemistry text or Handbook of Chemistry and Physics.
Since the lab procedure requires the use of micro-sized quantities of samples, you must make careful observations.
Sample of one of the following: Hotplate (if available) or
p-dichlorobenzene bunsen burner, ring stand,
(moth balls) iron ring, wire gauze
naphthalene 600 mL beaker
closed end capillary tubes
1. If commercial grade samples of naphthalene and p-dichlorobenzene are used, the large lumps or crystals must first be powdered in a mortar and pestle.
2. Place a small sample of the desired solid on a piece of paper. Tap the open end of the capillary tube into the sample of the powdered crystals.
3. When a small amount of powder has been transferred to the open end of the tube, invert the tube and carefully tap the closed end on the lab table. If the solid does not drop to the bottom of the tube, it may be necessary to push the sample of the bottom with a piece of fine wire.
4. Seal the other end of the capillary tube in an open flame (a match or butane lighter will work)
5. Strap the capillary tube containing the sample solid to the thermometer with a rubber band.
6. Place the whole assembly into a water bath as shown in Diagram #1.
7. Heat the water bath with constant stirring. Observe the crystals of the solid sample.
8. When the crystals first show signs of clearing or melting, note the temperature.
9. Add some cool tap water to the water bath to decrease the bath temperature by approximately 10°C.
10. Begin heating the bath SLOWLY. Note the exact temperature at which the sample begins to liquefy. Take this temperature to be the melting point.
11. Record the melting point temperature of the solid.
12. Obtain the theoretical melting point temperatures of the solid and calculate the percent error using the following equation:
% error = observed temperature – theoretical temperature
1. How would a contaminant change the melting point?
2. What is the purpose of heating the solid, cooling the solid, and reheating the solid to find the actual melting point?
3. What are some sources of error?