CHEMISTRY
WARMING AND COOLING BEHAVIOR OF A PURE SUBSTANCE
INTRODUCTION
In this experiment, you will study the warming and cooling behavior of
paradichlorobenzene, which is a pure substance. You will begin with a solid sample.
You will melt it and then record temperatures as you watch it refreeze. After it is frozen,
you will heat it again; this time recording temperatures until it is all melted. The heating
and cooling temperatures will be graphed in one plot so that you will be able to predict
the freezing and melting points (both should be the same temperature). Periodically in
this lab, the temperatures may level off for a reading or two. The freeze/melt
temperatures are identified when the temperatures level off for several minutes.
Paradichlorobenzene is poisonous and is highly combustible. It has a very strong odor,
so no contact lenses may be worn. Since you will be heating it, shoes must cover your
feet.
PROCEDURE 1
Safety goggles must be worn from now till the end of the lab.
Obtain a 600 mL beaker, a test tube containing paradichlorobenzene, two
thermometers, and a hot plate. Turn on your hot plate. Fill the beaker with about 3/4 full
of water and place the beaker on the hot plate. Using a thermometer clamp, suspend a
thermometer in the water and heat until the water temperature is about 65 º C. Using a
utility clamp, place the test-tube containing the chemical into the water and allow the
chemical to melt completely. When the entire chemical is melted, turn off the hot plate.
Leave the warm water on the hot plate. Be sure the test tube is secured in the clamp.
Using a second thermometer, observe and record the temperature of the chemical. Every
30 seconds record the temperature in the data table. Continue recording temperatures,
every 30 seconds, until you reach a temperature in the 40's. Remove the test tube from
the beaker of water.
PROCEDURE 2
Turn on the hot plate again. Monitor the thermometer suspended in the water and heat
until the water temperature is about 70 º C. Turn the hot plate off. Place the test tube
with the solid chemical from Procedure 1 into the warm water using the utility clamp.
Again, with the second thermometer, read and record the temperature of the chemical.
Record temperatures every 30 seconds in the data table. Continue recording temperatures
until you reach a temperature in the 60's.
PROCESSING THE DATA
Construct a graph of temperature and time for both sets of data. Both sets of data will
be on the same graph paper, but use two different colors. Temperature will go on the
vertical axis. Plot your procedure 1 data first and draw a “connect the dot line” in black
ink. Now, plot your procedure 2 data, draw a “connect the dot line,” but this time in blue
ink. Your graph must have a title as well as the x and y-axis labeled properly.
Name:__________________________________________Clean-Up Initials__________
Heating Lab Data Table
Cooling Data
Procedure 1
Time
Temperature
Heating Data
Procedure 2
Time
Temperature
QUESTIONS (Answer on lined paper.)
1. Observe the two lines. How are they similar? How are they different?
2. If your test tube was filled with more chemical, would that have any effect on the
shape of the lines in the graph?
3. Based on your graph, what is the freezing/melting point of this chemical?
4. What safety measures did you follow in this lab?