Question

After recrystallizing an impure sample of benzoic acid, a student scratched it off the filter paper and took a melting point. Most of the compound melted between 121 and 122 degrees Celsius. A small amount remained un-melted even at temperatures above 200 degrees celsius. Explain this behavior.

Answer 1

The reason for this depression/broadening is that contaminants disrupt the consistency and organization of the crystal lattice at the molecular level. Contaminants don’t “fit” correctly into what would be the normal pure lattice.

1. The disruption weakens the lattice, so that the lattice can be broken down more easily; the weakened structure melts more easily at reduced temperature (depression).
2. Disruption of the lattice makes it non-uniform. At the molecular level, the molecules closest to the impurities melt fastest. Further away from the impurities, the crystal lattice is relatively undisturbed and therefore melts at or nearer the normal temperature.

When two chemicals are mixed, the resulting melting point is not the average of the two mp’s. It is always depressed from the melting point of the major component in the mixture. This is true even if the impurity is higher melting (when pure) than the major component. For example, if a chemical that normally melts at 130º is contaminated by a small amount of material that when pure melts at 200º, the resulting mixture will not melt between 130º and 200º. Rather, the melting point of the major component will be depressed, and the observed melting range will begin lower than 130º.