What can be categorized as a chemical property of a substance?

The ability to rust is indeed categorized as a chemical property because it describes how a substance reacts with oxygen in the presence of moisture to form rust, which is a different chemical substance (iron oxide). This property is inherently tied to the chemical identity of the material and involves a transformation that changes the substance at a molecular level.

In contrast, melting point, color change during freezing, and density under pressure are physical properties. These properties can be observed or measured without changing the chemical structure of the substance. For example, melting point indicates the temperature at which a solid becomes a liquid, but it does not involve any change in the chemical composition of the substance itself. Similarly, a color change during freezing may indicate a physical change (like crystallization) rather than a chemical transformation. Density is a physical characteristic that can vary with changes in pressure, but again, it does not involve any alteration in the chemical composition of the material.