What indicates a physical property that can be observed without changing the substance?

A physical property is one that can be observed or measured without altering the chemical composition of a substance. Color is a clear example of a physical property because it can be seen and described without changing the substance itself. For instance, when you note that a liquid is blue, you are observing its color and not causing any transformation in its chemical structure.

In contrast, flammability, reactivity, and acidity are all properties that involve chemical changes. Flammability refers to a substance's ability to burn, which would change the substance into different products. Similarly, reactivity describes how a substance engages in chemical reactions, leading to the formation of new substances, and acidity pertains to a substance’s ability to donate protons in a chemical reaction, which also results in a change. Thus, observing color does not carry any risk of changing the substance, making it the correct identification of a physical property.