Final answer:
A characteristic observed or measured during a reaction that changes a substance's composition is a chemical property, while a physical property can be observed without such a change.
Step-by-step explanation:
A property or characteristic of a substance that is observed or measured during a reaction in which the chemical composition or identity of the substance is changed is called a chemical property. A chemical change occurs when one substance is transformed into another, often producing matter with a different identity and different properties than the original matter. By contrast, a physical property is a characteristic that can be observed or measured without causing a change in the substance's chemical composition, like melting and boiling points, which may be observed during a physical change.