organic chem/ melting pt lab
posted by Natash .
why does a soluble impurity reduce the melting point range of a pure compound ?
why is it advisable to run a mixed melting point determination on samples of two different compositions before concluding that the two compounds are identical ?
In general, a tighter packed crystal lattice of a solid will have a higher melting point. Impurities tend to disrupt the crystal lattice, so they always lower the melting point to a certain limit, which is called the eutectic point. The blend is called a eutectic mixture.
When two substances have similar or identical melting points individually, and the mixture has a substantially lower melting point, we conclude that one substance acts as an impurity of the other, hence the lower melting point. This test (melting point of the mixture) serves to confirm if two substances are identical.
An excellent PowerPoint presentation answers both of your question in a clear and concise way can be found in the following link. Microsoft Powerpoint is required to view the presentation which needs to be downloaded.
(replace each of the two * with a /)
#1. But what about reducing the melting point RANGE?
#2. But why TWO different compositions? Why won't one work just as well?