Impurities typically lower the melting point of a substance and expand its melting point range.

In a pure substance, all particles are identical, which allows them to interact uniformly. Consequently, they begin to melt at the same temperature, resulting in a sharp and well-defined melting point. However, the presence of impurities alters this scenario. The introduction of impure particles disrupts the ordered arrangement of the pure substance’s particles, diminishing their ability to interact effectively. This disruption lowers the energy required for the substance to start melting, leading to a decrease in the melting point.

Furthermore, not all impurities affect the structure of the substance in the same manner or to the same degree. Some impurities may cause significant disruption, while others might have a more minor effect. As a result, different sections of the substance may melt at varying temperatures, which contributes to the broadening of the melting point range.

Interestingly, the influence of impurities on the melting point is not always detrimental. In specific cases, impurities can actually raise the melting point of a substance. This phenomenon occurs when the impurities form a solution with the substance that possesses a higher melting point than that of the pure substance. However, such instances are relatively rare and typically arise only with specific combinations of substances and impurities.

In summary, impurities can significantly impact the melting point of a substance. They generally lower the melting point and broaden the melting point range, although there are exceptions where they may increase the melting point. Understanding these effects is crucial in various fields of chemistry, including the purification of substances and the identification of unknown materials.