Which of the following is NOT a physical property?

Flammability is indeed not a physical property; it is a chemical property. Physical properties are characteristics of a substance that can be observed or measured without changing the substance's chemical identity. For instance, boiling point indicates the temperature at which a substance transitions from liquid to gas, solubility describes how well a substance can dissolve in a solvent, and color refers to the visual characteristic of a substance.

In contrast, flammability refers to the ability of a substance to catch fire and burn, which involves a chemical change. This means that while observing flammability, the original substance undergoes a transformation into new substances, thus categorizing it as a chemical property rather than a physical one. Recognizing the difference between physical and chemical properties is fundamental in understanding how substances interact and behave under different conditions in chemistry.