Which of the following best describes vapor in terms of phase change?

Vapor is best described as a gaseous state of a substance that is typically liquid at room temperature. This definition highlights the phase change from liquid to gas that occurs when a substance is heated. For example, when water is heated, it transitions into water vapor, which is its gaseous form. This phenomenon is crucial in understanding various processes, such as evaporation and boiling, where substances gain enough energy to overcome intermolecular forces and enter the vapor phase.

In contrast, the other options do not accurately define vapor. A solid substance that has melted pertains to the transition from solid to liquid, which is not relevant to the concept of vapor. The description of a state of matter at high density does not apply to vapor, as gases, including vapors, are generally less dense than their liquid counterparts. Lastly, a liquid that has cooled does not represent vapor either; instead, this would refer to a liquid that may be close to freezing or condensing, further emphasizing that vapor specifically refers to the gaseous state of a typically liquid substance.