Final answer:
A chemical property cannot be observed without changing a substance's composition, while physical properties can be observed without any change in identity.
Step-by-step explanation:
The type of property that cannot be observed without changing the composition of a substance is a chemical property. Physical properties, such as color, density, hardness, melting and boiling points, can be determined without altering the substance's identity. In contrast, chemical properties describe a substance's capability to undergo a chemical change, which does involve altering the substance's composition. Examples of chemical properties include flammability, reactivity with acids, and the ability to oxidize. Observing a chemical property requires a substance to transform into a different substance, thereby experiencing a chemical change.