Question

A certain substance, melting at 188-189°C, is suspected to be 4-aminobenzoic acid. How could you be reasonably certain that the substance is actually 4-aminobenzoicacid and not another compound with the same melting point?

Answer 1

As we know impurities can lower melting point of a compound.This is called coligative property. In short, it means that pure compound melt over a narrowrange of 0.5 to 1.5 degrees C, whereas impure substances often melt over a much larger range. Also the presence of even small amount of impurities usuallydepresses melting point a few degrees. This behavior can be helpful not only in evaluating compound's purity but also in identification of thecompound.

Assume that two compounds have virtually identical melting ranges. Are the compounds identical? Possibly, but not necessarily, because some compounds havethe same melting point even though they are different compounds. So what you would do to figure out whether they are the same compound is to use themelting point of a mixture.

First you would take a sample of known 4-aminobenzoic acid and measure its melting point or melting range. Then you would finally ground the compound youare looking to identify and thoroughly mix it with a sample of known 4-aminobenzoic acid (roughly equal amounts). Then measure the mixture's melting range.If there is a melting point depression or if the melting range is extended by a number of degrees, it is reasonably safe to conclude that two compounds arenot identical. In other words the substance is not 4-aminobenzoic acid. Why? Because one compound has acted as an impurity toward the other by lowering themelting range. If there is no lowering of the mixture's melting range relative to that for each separate compound, the two are probably the samecompound.