Why does a pure substance maintain the same melting and boiling points?

A pure substance maintains the same melting and boiling points because it consists of only one type of particle. This homogeneity allows the substance to have a uniform molecular structure, which results in consistent physical properties. When a pure substance is heated or cooled, it will transition at specific temperatures, known as its melting point and boiling point.

For instance, the melting point is the temperature at which a solid becomes a liquid, and the boiling point is when a liquid becomes a gas. These transitions occur at distinct temperatures for a pure substance due to the uniform interactions between its identical particles, whether they be atoms, molecules, or ions. The lack of different types of particles means there are no variations in the forces acting between them, resulting in predictable and consistent phase change points.

In contrast, other options suggest factors that do not apply to the characteristics of pure substances. For example, molecular weight can affect physical properties, but it does not determine the constancy of melting and boiling points. Additionally, temperature fluctuations are irrelevant to the definition of a pure substance's phase change characteristics, and variable composition applies to mixtures rather than pure substances, leading to a lack of consistent melting and boiling points.