Which of the following best defines a physical property?

A physical property is defined as a characteristic of a substance that can be observed or measured without altering the substance's chemical identity. This means that when you measure a physical property, you can describe aspects like color, boiling point, melting point, density, mass, and volume without transforming the substance into something else. For instance, boiling water will produce steam, but the water remains H2O and hasn’t changed into a different chemical entity.

The other choices don't accurately capture the definition of physical properties. Observable characteristics that change identity involve chemical properties, where a substance undergoes a process that changes its chemical composition. For example, rust formation alters the identity of iron. Reactions that result in new substances pertain to chemical properties as well, emphasizing transformations at a molecular level that are not seen in mere observations or measurements. Lastly, intrinsic qualities of a compound only suggest a limited view, as physical properties can encompass both intrinsic and extrinsic characteristics, including those dependent on the amount of substance present. Therefore, the focus on qualities that can be observed and measured without changing identity distinctly defines physical properties.