What is necessary for observing a chemical property of a substance?

Observing a chemical property of a substance requires a change in the substance's composition, which typically occurs when it reacts with another substance. When a chemical reaction takes place, the original substances are transformed into new substances with different chemical properties. For example, rusting iron is a chemical property that can only be observed through its reaction with oxygen and moisture, resulting in the formation of iron oxide.

In contrast, the other options involve physical changes or conditions that do not necessarily lead to observing a chemical property. Heating a substance can cause physical changes without altering its chemical identity. Mixing with air might lead to a reaction, but it does not guarantee one will occur and could also simply involve physical mixing. Removing heat might change a substance's state but won’t change its chemical properties. Therefore, the necessity of a reaction with another substance is key to observing a chemical property.