Specifity of the melting points of pure substances?

Question

Specifity of the melting points of pure substances?

Answer 1

The melting point of a pure substance is defined as the temperature at which the solid and liquid phases are at equilibrium, at a given pressure (usually one atmosphere). The melting point is an important physical property of a pure substance which is relatively easy to measure. The value of the melting point is enhanced by the fact that even small amounts of impurities seriously depress (and broaden) the melting point, affording a melting range. In this experiment we will look at the melting points of two pure solids and the impact of a mixture on those values.

Both urea and cinnamic acid have melting points of about 133 degrees Celsius, hence a single melting point determination of an unknown would not distinguish between the two. If you had an unknown and pure samples of each, mixed melting points could be used to determine what the unknown was. For example, you mix your unknown with pure urea, and determine the melting point to be 133 (sharp). You then mix your unknown with cinnamic acid and obtain a melting range of 115-128. These results indicate that your unknown must have been urea.