MHS Chemistry
Melting/Freezing Point of an Unknown Solid


  1. To examine the melting/freezing behavior of an unknown substance.
  2. To determine the melting/freezing temperature of the unknown substance.
  3. To identify the unknown solid based on its melting/freezing point.


  1. Assemble the lab apparatus as demonstrated by your teacher. Read these directions completely before continuing.
  2. Select a test tube with unknown material from the rack on the lab cart.  Re-read these directions again!
  3. Immerse your test tube in the water bath and heat the water until the unknown melts.
  4. 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.
  5. 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.
  6. Remove the thermometer and rinse it in hot water.  Return it to the rack, and leave the unknown at your bench to cool.
  1. 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.
  2. Plot all the temperature data on the same graph.  Include a key to show which line goes with which thermometer.
  3. Based on your graphs, what was the melting/freezing point of your material? Explain how you chose this number.
  4. Based on your melting/freezing point, which of the materials in the table below was your unknown?  Explain your reasoning.
  5. If you used ice instead of your unknown in this experiment, how would your graph and results differ? Be specific.
  6. If you used twice as much unknown in this experiment, how would your graph and results differ?
  7. 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

  8. 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.

    [Melting Point score sheet][MHS Chem page]