What defines the heat of vaporization?

The heat of vaporization is specifically defined as the amount of energy required to convert a unit mass of a liquid into vapor without a change in temperature, typically measured at the substance's boiling point. This process involves overcoming the intermolecular forces that hold the liquid molecules together, allowing them to escape into the gaseous state.

In this case, the correct answer highlights the concept that this energy requirement is specific to the phase change from liquid to gas, emphasizing that it occurs at the liquid's boiling point. The other options reflect different phase changes or thermal processes, but they do not pertain to the transition from liquid to gas, which is the essence of boiling and vaporization.