How does the presence of an impurity in a substance typically affect its melting point?

The presence of an impurity in a substance generally lowers the melting point due to a phenomenon known as freezing point depression. When a pure substance melts, its constituent particles are tightly packed and have a specific arrangement. Introducing impurities disrupts this orderly structure, which can weaken the intermolecular forces holding the particles together.

As a result, less energy is required to overcome these disrupted forces, leading to a decrease in the temperature at which the substance transitions from solid to liquid. This effect can vary depending on the nature and amount of the impurity present, as some impurities can significantly affect the melting behavior more than others.

Additionally, in practice, this means that mixtures or solutions often have a melting point range rather than a sharp melting point, leading to variability in melting behavior. This contrasts with a pure substance that has a definite and consistent melting point. Thus, the typical consequence of adding an impurity to a solid is a reduction in its melting point.