MHS Chemistry
Melting/Freezing Point of an Unknown Solid
Purpose
To examine the melting/freezing behavior of an unknown substance.
To determine the melting/freezing temperature of the unknown substance.
To identify the unknown solid based on its melting/freezing point.
Procedure
Assemble the lab apparatus as demonstrated by your teacher. Read these
directions completely before continuing.
Select a test tube with unknown material from the rack on the lab cart.
Re-read these directions again!
Immerse your test tube in the water bath and heat the water until the unknown
melts.
Raise the test tube from the bath and lower the thermometer into it.
Record the temperature of the unknown every 30 seconds until 3 minutes after
it is completely solidified. Wiggle the thermometer gently once in a
while to keep the solid and liquid mixed. Remember to read AND record
the temperature correctly.
Return the test tube with the thermometer to the hot water bath. Lower
the second thermometer into the water bath. Record both temperatures
every 30 seconds until 3 minutes after the unknown is entirely melted.
Do not let the temperature of the hot water get more than 10 degrees above
the “level” temperature from the cooling.
Remove the thermometer and rinse it in hot water. Return it to the
rack, and leave the unknown at your bench to cool.
Analysis
Make a graph with time across the x-axis and temperature up the
y-axis. Use the whole page, and make sure the scales stretch
only to include your numbers. Remember to use graph paper if you make your
graph by hand.
Plot all the temperature data on the same graph. Include a key to
show which line goes with which thermometer.
Based on your graphs, what was the melting/freezing point of your material?
Explain how you chose this number.
Based on your melting/freezing point, which of the materials in the table
below was your unknown? Explain your reasoning.
If you used ice instead of your unknown in this experiment, how would your
graph and results differ? Be specific.
If you used twice as much unknown in this experiment, how would your graph
and results differ?
Explain why there is a level spot on your graph(s). Describe what
happened in terms of kinetic and potential energy, and phase changes.
Refer to your notes, a text book, or any other printed source.
Name
MP
(C)
ethanethiol
35.1
lauric acid
44.0
urethane
50.0
p-dichlorobenzene
53.0
diphenylamine
53.0
heptacosane
59.5
procaine
61.0
oxythane
69.5
1,10-decanediol
74.0
naphthalene
80.5
Look up your unknown in the Merck Index (there are two copies in the chemistry
lab and one in the library). Draw the structure shown for your chosen compound.
If you do not get it from the Merck index, include your source.