Melting Point Determination
An organic compound's melting point is one of several physical properties by which it is identified. A physical property is a property that is intrinsic to a compound when it is pure. Since melting points are relatively easy and inexpensive to determine, they are handy identification tools to the organic chemist. If you want to use the melting point to identify a solid compound which you have isolated in the lab, you will need to compare its melting point with that of the true compound. Melting points are listed in various sources of scientific data, as referenced on the Chemical Information page on this orgchem site.
If you look up the melting point of a compound in more than one source, you may find that the values reported differ slightly. Keep in mind that the melting point ranges listed in the company catalogues (Acros, Baker, Sigma-Aldrich, etc.) are the melting points of the compounds as they are sold. If the compounds are sold slightly impure, the melting point range will reflect this fact. The melting point listed in the CRC, Merck Index, or on the MSDS is the melting point of the pure compound. While theoretically these should be constant from source to source, in reality the reported melting points sometimes vary. Therefore, always reference the source of the physical data which you write in your lab report.
Descriptions and Use of Melting Point Apparati
Three types of melting point apparati are available in the Organic Chemistry teaching labs, the Fisher-Johns, the Mel-Temp, and the DigiMelt. Explanations (including pictures) of how to take a melting point on each type of apparatus are linked on this website.
Melting Point Movie
CU Organic Chemistry has produced a movie about recrystallization which contains a section on taking melting points. You can see a melting point determination if you advance to 10:45 into the movie.
Melting Point Technique Quiz
See how well you understand melting point measurement by taking the online Melting Point Technique Quiz!