Question

What 2 things happen to the melting point of a solid when an impurity is present?

Answer 1

An impurity causes the melting point to deviate from the standard. Actually, it makes the magnitude of the melting point lower. For example, the melting point of benzoic acid is 122.4 degree Celcius. When we did this in the laboratory, we got 120 degree Celcius because of the impurity present which is sodium sulfate.

An impurity also causes the melting point to become a melting range. The melting point is not anymore sharp, but a range of values. The more impurities present, the greater the range of values. For example, when we purified crude acetanilide via recrystallization, we obtained a melting range of 105-116 degree Celcius. By the way, the standard melting point of acetanilide is 113-115 degree Celcius.

So basically, those are the two effects that impurities have on melting point iof a pre compound. I learned that it is because the impurities "defect" the crystal lattice of the compound. A pure compound is a structure of high molecular symmetry. An impurity is rather assymetrical, or is a poorly organized structure. If the impurites are present with the pure compound, the melting point will deviate because melting point is also dependent on the high symmetry and organization of molecular structures, just as the vapor pressure. This is the principle involved in Sublimation and Melting point determination