What is defined as the gas state of a substance that is usually a solid or liquid at room temperature?

The correct answer is vapor, which refers specifically to the gaseous state of a substance that is predominantly found as a solid or liquid at room temperature. For instance, water is a liquid at room temperature, but when it is heated, it can turn into vapor. This transition occurs when the molecules gain enough energy to break free from the liquid state and enter the gaseous state without any chemical change to the substance itself.

In contrast, aerosol refers to a suspension of tiny particles or droplets in a gas, which involves solid or liquid particles dispersed in air or another gas but doesn’t describe the pure gaseous state of a substance. Gas is a broader term that includes all substances in the gaseous state, whether they are usually solid, liquid, or gas at room temperature. Steam specifically refers to water vapor but usually in contexts involving boiling water, making it a more specific situation compared to vapor. Thus, vapor is the term that captures the essence of a substance transitioning from solid or liquid to an observable gas phase.