How do physical and chemical properties define a substance?

Physical properties describe how a substance looks or behaves without changing its chemical makeup. Examples include color, odor, melting point, boiling point, density, and state of matter. For instance, ice melts into water - it changes state, but the chemical formula (H₂O) remains the same.

Chemical properties, on the other hand, describe how a substance reacts with other materials to form new substances. Examples include flammability, reactivity with acids, rusting, and combustion. For instance, iron reacts with oxygen and water to form rust — a chemical change.

In short, physical properties can be observed without changing what the substance is, while chemical properties can only be seen when a chemical change occurs. Together, these properties help scientists identify and classify substances. For example, knowing that gold is non-reactive and has a shiny yellow color helps distinguish it from other metals.