Heating and Cooling Curves
Lab #______
Pre-Lab Discussion
In a solid the atoms have a definite position in the crystal structure. When a solid
changes to a liquid, the atoms no longer have a fixed position. In the liquid phase the
atoms or molecules, have some freedom to mover about. Energy must be added to
overcome the attractive forces that hold the molecules together in the solid. In this
experiment, we are going to observe what happens to the temperature of a liquid as it
changes to a solid, and then back to a liquid. Remember that temperature is a measure of
the average kinetic energy of the particles in a substance.
Purpose
1. To determine the melting point of p-dichlorobenzene
2. to plot a time-temperature graph for the solid-liquid and liquid solid phase change.
Equipment
Hot plate
Ring stand
watch or clock with a second hand
Thermometer
Test tube holder
400 mL Beaker
Iron ring
safety glasses
Large test tube
Materials
p-dichlorobenzene
Procedure A
1. Obtain a test tube with approximately 15g of p-dichlorobenzene.
2. There will be beakers of hot water set up throughout the lab. Place your test tube
in the hot water until the entire solid is melted.
3. Insert the thermometer into the test tube. When the temperature reaches
approximately 60oC, remove the test tube and thermometer from the hot water
bath using the test tube holder.
4. Without removing the thermometer from the test tube, take temperature reading
every 1-minute until you have a constant temperature for 5 minutes. Make a note
in the data table when the first crystals appeared.
Procedure B
1. With the thermometer embedded in the solid material, place the test tube and
thermometer back into a hot water bath.
2. Record the temperature every minute until all the material is turned into a liquid.
Make a note when the first liquid appeared in the test tube.
3. Carefully remover the thermometer from the test tube and wipe in with a paper
towel.
4. Graph the results.
Data
Time
(minutes)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Cooling
(oC)
Heating
(oC)
Conclusions and Questions
1. Referring to your graph, determine the freezing and melting points of pdichlorobenzene. How do these temperatures compare? Why?
2. Why does the temperature remain constant during the melting/freezing phase
change?