How do impurities affect the melting point of a substance?

Impurities generally lower the melting point and broaden the melting point range of a substance.

In a pure substance, all the particles are identical and hence, they all require the same amount of energy to change from the solid state to the liquid state. This means that a pure substance has a sharp melting point, i.e., it melts at a specific temperature. However, when an impurity is present in the substance, it disrupts the regular arrangement of particles in the solid. This disruption means that less energy is required to break the intermolecular forces holding the particles together, resulting in a lower melting point.

Moreover, the presence of impurities also broadens the melting point range of a substance. In a pure substance, all the particles start to melt at the same temperature and finish melting at the same temperature, giving a sharp melting point. But in an impure substance, different particles may require different amounts of energy to melt due to the presence of impurities. This means that melting starts at a lower temperature and finishes at a higher temperature, giving a broader melting point range.

In addition, the effect of impurities on the melting point of a substance is a key principle used in the process of purification known as recrystallisation. In this process, an impure solid is dissolved in a hot solvent and then allowed to cool and recrystallise. As the solution cools, the pure substance crystallises out first, because it has a higher melting point than the impure substance. The impurities remain dissolved in the solvent, resulting in a purer substance.

IGCSE Chemistry Tutor Summary: Impurities in a substance lower its melting point and expand the range at which it melts. Pure substances melt at a specific temperature, but impurities disrupt this, causing some parts to melt at lower energies. This results in the substance starting to melt at a lower temperature and finishing at a higher one, thus broadening the melting range.