COOLING BEHAVIOR OF PURE SUBSTANCES AND
MIXTURES
In this experiment, you will examine the cooling behavior of a pure substance,
paradichlorobenzene, otherwise known as PDCB but best known as Moth
Nuggets. We will also examine the cooling behavior of wax, a homogenous
mixture of hydrocarbons known as Olefins. By measuring the temperature of
both as they go from LIQUID states to SOLID states, you will discover a property
of all materials as they pass through a phase change.
PROCEDURE:
1. Obtain test tubes filled with Paradichlorobenzene (PDCB) or wax. Melt both
by placing into a hot water bath. Use either a hot plate to heat the water.
3. Insert a thermometer directly into the PDCB or wax, remove the test tube
from the hot water bath and immediately clamp them onto ringstand using two
clamps. Immediately begin recording the temperature of the tube every 30
seconds until the temperature drop begins to really slow down, then you can
expand the time block between measurements to as much as 3 minutes. Be sure
to record any visual observations you may notice, such as when crystallization
first begins and ends and optical properties of the solid phase
4. When done, stick the test tubes back into very hot water to remelt the PDCB
and wax in order to remove the thermometer. Wipe the thermometer with a
paper towel to remove waste chemicals. Return the test tubes back to their
containers.
PROCESSING THE DATA:
1. Construct a graph of Temperature vs. Time for PDCB or Wax on an Excel
Spreadsheet.
2. Mark on your PDCB and Wax curves where freezing started and when it
ended.
3. Assuming that you have good data that extended from when the PDCB was
totally liquid to when it was totally solid, your graph of the PDCB should clearly
show its freezing temperature. What about the wax? It is the temperature at
which the curves plateau (levels) out. What was the freezing temperature of
the PDCB? The wax?
4. If the temperature of a material is related to the average kinetic energy of the
molecules in the material, are the materials losing kinetic energy as they change
from a liquid to solid at its freezing temperature? Explain.
5. Thus, if the PDCB and wax are constantly losing energy as they go from a
liquid to a solid what form(s) of energy can it be? How does this relate to
temperature change?
6. Think about how the results would differ if you had used TWICE the amount
of PDCB or wax. Now sketch in these results on the graph in pencil.