The melting temperature of an organic solid is a classic physical property which is reported in tables. Usually tables list this as a melting "point" though often the temperature at which the melting occurs will be small range of 1-2° C. Since impurities both depress the melting temperature and broaden the melting range, melting point determination is often used as a qualitative determination of the purity of a solid.

Note that there are many reasons why a melting point determination of a material might yield results which differ from those listed in a table. For example:

Sample may be impure (causes values to be lowered)
Sample heated too quickly (causes values to be artificially high)
Too much sample (causes values to be artificially broad)
Table values sometimes differ
Material may be pure, but not what we think it is!

In addition to judging the quality of a solid sample, melting point determination may also be used to identify a material if one already has a good idea of the material's identity. For example, suppose a chemist was performing an experiment using six different chemicals during the previous week. Suppose further that this chemist found a small sample of a familiar looking chemical in an unlabelled vial on the bench (something that never should have happened!). The chemist might identify the unknown as follows. First determine its melting point to see if it comes close to any of the materials it is suspected to be. This allows one to hypothesize the identity of the material. Next mix the unknown with a known sample and see if the melting point is depressed. If it was not, the hypothesis is correct otherwise the identity is still unknown.

1. Determination of the melting points of several known samples
   
   In this experiment we will observe the effect known as melting point depression. The presence of impurities in a solid sample causes the melting point to be reduced and the range of melting temperatures to be much broader than normal. We will observe this effect using two materials with very similar melting points.
   
   
   1. Obtain the melting point of urea
   2. Obtain the melting point of cinnamic acid
   3. Obtain the melting point of 1:1 urea/cinnamic acid
   4. Find the melting point of urea and of cinnamic acid from a literature source
   5. Present the results of this experiment in a table and write a short conclusion
   
   
   
2. Determination of the identity of a pure unknown sample
   
   Melting point determination is not enough in itself to identify an unknown sample since many materials have similar melting points. If one believes that they know the identity of a compound, however, the melting point can be a useful tool. In this experiment you will determine the melting point of an unknown material from a small list of possibilities. After you make this measurement you will propose the identity of the unknown and then prove your hypothesis by performing a mixed melting point between the unknown and a known sample of the same material.
   
   
   1. Obtain an unknown compound from your instructor
   2. Obtain an accurate melting point for your sample
   3. Your unknown consists of either:
      
      
      • Acetanilide
      • Benzoic Acid
      • Cinnamic Acid
      • Fluorene
      • Salicylic Acid
      • Urea
      • Vanillin
      
      
   4. Find the structure and melting point for each of these molecules from a literature source and present the data in tabular form
   5. After forming a hypothesis as to the composition of your unknown, perform a mixed melting point between your unknown and a genuine sample of the hypothesized material.
   6. Write a brief conclusion which states your "Unknown Number" and the hypothesized identity of the unknown