Lab: Energy and Changes of State
Introduction
Temperature is a measure of the average kinetic energy of the molecules in a system. This
means that the molecules of a solid must have the same kinetic energy as the molecules of a
liquid if both are at the same temperature. But if that’s so, why aren’t their states the same?
Why aren’t they either both solids or both liquids. The answer has to do with the potential
energy of the molecules and with the forces pulling those molecules toward each other.
You can investigate the roles of kinetic and potential energy during a change of state by making
a cooling curve, in which the substance is heated until it melts, then is allowed to cool below its
freezing point. In this experiment, you will make two curves at once. You will begin by melting
a pure solid in a test tube. This will be done by heating the tube in a beaker of water on a
hotplate. The substance, phenyl salicylate (known as salol) will melt by the time the water has
started to visibly steam. Then you will remove the beaker and tube from the hotplate and follow
the cooling of both the salol in the tube and of the water in the bath.
Materials
250 mL beaker for water bath
hot plate
5-6 g phenyl salicylate in 16x150 mm test
tube
2 thermometers
hot hands or test tube clamps
Procedure
1. Prepare a hot water bath: place 150 mL tap water in the beaker and set on the hot plate. Put
the test tube containing the salol in the water bath and turn on the hot plate to mid-range.
Insert a thermometer in both the test tube and the water bath.
2. As soon as the salol is nearly melted, transfer the water bath containing the test tube to your
bench top (it will finish melting on its own).
3. When the temperature drops below 50°C, begin recording both temperatures once every
minute.
Salol has a tendency to exhibit a behavior known as super-cooling, in which its temperature
will fall several degrees below the normal freezing point, then quickly rise back to the
freezing point and level off there. This results in a V-shaped dip in the normal cooling curve.
To help prevent super-cooling, gently use the digital thermometer to stir the contents of the
tube. When the thermometer no longer moves easily, stop stirring to avoid breaking the test
tube.
While one partner is attending to the salol tube, the other should gently stir the water so it
cools at a steady rate.
4. Stop recording when the temperature of the salol and the water bath are the same.
5. At the end of the run, turn off the digital thermometer and return the test tube of salol with
the thermometer to the teacher. Wash your hands before leaving the lab.
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Names:
Date:
Lab: Energy and Changes of State
Prelab Assignment.
1. Consider two solids. One has a melting point of 40°C, while the other melts at 60°C. What does this
tell you about the relative strengths of the intermolecular forces present in each one? Explain.
2. Is freezing of a substance an endothermic process or an exothermic one? Explain.
3. Describe the changes in intermolecular forces of a substance when it
changes from liquid to solid?
4. The diagram to the right is a heating curve for water. Which segment
corresponds to (a) melting ice and (b) heating water vapor?
Temperature, °C
F
0
E
D
100
B
C
A
Heating
Data Table
Time, min
Salol Temp,
°C
Water Temp,
°C
Time, min
0
16
1
17
2
18
3
19
4
20
5
21
6
22
7
23
8
24
9
25
10
26
11
27
12
28
13
29
14
30
15
31
Salol Temp,
°C
Water Temp,
°C
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Analysis and Conclusions
1. Graph your data; label both curves. On the salol curve, indicate the phases present in each
portion of the curve. Attach your lab to this report.
2. Describe the shapes of the two curves on your graph.
3. As the salol changed from liquid to solid, its temperature was constant.
a. At what temperature did this occur? The temperature at which you observed the plateau
is the freezing point of salol, as determined by your experiment.
b. What can you say about the kinetic energy of the salol molecules along the temperature
plateau? Explain.
4. During the time that the temperature of the solid remained constant, the temperature of the
water in the beaker continued to fall. Was this a change in kinetic energy or potential energy
for the water? Explain.
5. During the same time period, the salol temperature was constant, however it must have been
losing energy also.
a. Why?
b. If its temperature is constant, what type of energy must it have been losing? Explain.
6. While a liquid is boiling its temperature remains constant even though you continue to apply
heat. What happens to all that energy you are adding as you heat the liquid?
7. Explain which would give you a more severe burn: steam or (liquid) water at 100°C. Why?
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