Which of the following does NOT describe physical properties of materials?

• A. Molecular weight
• B. Color and appearance
• C. Reactivity with acids
• D. Vapor pressure

Answer: (C)

The characteristic that does not describe a physical property of materials is reactivity with acids. Physical properties are those that can be observed or measured without changing the substance's chemical identity. Examples include molecular weight, which refers to the mass of a molecule; color and appearance, which pertain to how a substance looks; and vapor pressure, which is the pressure exerted by a vapor in equilibrium with its liquid or solid form.

Reactivity, on the other hand, relates to how a substance interacts with other chemicals, which involves a change in chemical structure and composition. Therefore, the correct answer highlights that reactivity with acids is a chemical property rather than a physical one.