Why do pure substances have fixed melting and boiling points?

Pure substances have fixed melting and boiling points because they are made up of identical particles—atoms or molecules of a single kind. This uniformity means all particles require the same amount of energy to break their intermolecular bonds when changing from one state to another.

For example, when pure water is heated, every water molecule (H₂O) behaves the same way. As the temperature reaches 100°C at normal pressure, the energy supplied is just enough to overcome the hydrogen bonds holding the molecules together, and boiling begins. The temperature remains constant during the entire phase change because all the added energy goes into converting liquid to vapor, not raising the temperature further.

In contrast, mixtures contain different particles with varying bond strengths. As a result, different parts of the mixture melt or boil at different temperatures, producing a range instead of a fixed point.

Thus, a fixed melting and boiling point is a hallmark of purity. Scientists often measure these points to determine whether a substance is pure or impure—any deviation from the expected value suggests the presence of other substances or impurities.