What is a physical property of a substance?

A physical property of a substance is defined as a characteristic that can be observed or measured without changing the substance's chemical identity. This means that when you examine a physical property, such as color, melting point, boiling point, density, or solubility, the substance itself remains unchanged at the molecular level.

The chosen answer emphasizes that a physical property does not involve any alteration or transformation of the substance's chemical structure. For instance, when ice melts into water, its physical state changes from solid to liquid, but it is still H2O, and no chemical bonds are broken or formed in the process.

In contrast, other options describe properties or changes associated with chemical reactions or transformations. Observing changes that require a chemical reaction, such as combustion, or noting features that only appear during such changes, do not define physical properties. Thus, the focus on characteristics that can be observed without altering the substance itself makes the selected answer correct.