Brendan Kelly 21-2-12
Mrs. Sutherland Lab 18

Heating and Cooling Curves

Purpose Study the effects of heating and cooling a pure substance through a change of phase. Construct heating and cooling curves of a pure substance using experimental data. Determine the freezing and melting point temperatures of the pure substances.

Procedure

PART A COOLING CURVE 1. Fill a 250-mL beaker three-quarters full of cold tap water. 2. Obtain a corked test tube, containing a sample of the substance (lauric acid) to be studied. Clamp the test tube to a ring stand as shown in Figure 18-1. 3. Remove the cork. Heat the sample CAREFULLY by moving the burner gently back and forth as shown in Figure 18-1. CAUTION: Before heating, check to see that the mouth of the test tube is pointing away from yourself and others. 4. As soon as the sample begins to melt, remove the heat and place a thermometer in the sample. Using the thermometer to stir the sample, resume heating gently until the sample in completely melted. DO NOT OVERHEAT. The final temperature of the sample should be about 60°C. If it is higher, wait until the temperature drops to this temperature before proceeding with step 5. 5. When the temperature of the sample is 60°C, set the time at 0 minutes. Immediately immerse the test tube into the cold water bath, making sure the entire sample, but not the entire test tube, is below the surface of the water. 6. The recording partner will call out the time every ½ minute. The second partner will use the thermometer to stir the sample constantly as long as some liquid remains. At each half minute interval, this partner will record the temperature. 7. Continue this procedure until the temperature of the sample reaches 25°C. Remove the test tube from the water and stand it in the test tube rack.

PART B HEATING CURVE 8. Using a setup like that shown in Figure 18-3 (but without the test tube in the beaker), heat the water in the beaker to a temperature of about 70°C. Remove the heat. 9. While the water was being heated, the sample in the test tube was cooling to approximately room temperature. Set the time at 0 minutes when you record the exact temperature of the sample and immerse it below the water level in the hot water bath. Read and record the temperature of the sample every ½ minutes as in Part A. 10. As soon as the thermometer if free to move, it should be used to stir the solid-liquid mixture. Continue stirring and recording the temperature at half-minute intervals until the temperature of the sample reaches 50°C/ 11. Remove the thermometer and clean it. Remove the test tube from the water bath and allow it to cool. Replace the cork in the test tube and return the sample to your teacher.

Questions 1. Referring to your graph,