How much heat energy is needed to change 1 lb of liquid into 1 lb of vapor at the same temperature?

The correct choice is the latent heat of vaporization, which refers to the amount of energy required to convert a unit mass of a substance from a liquid phase to a vapor phase at a constant temperature and pressure. This process occurs without a change in temperature, thus the term "latent" indicates that the heat energy is absorbed or released during the phase change rather than raising the temperature of the substance.

For water, for example, the latent heat of vaporization is quite significant, indicating that a considerable amount of energy is necessary to transform liquid water into steam. This energy is needed to overcome the intermolecular forces holding the water molecules together in the liquid state.

In contrast, the latent heat of fusion pertains to the energy required to change a substance from solid to liquid, and the latent heat of condensation is the energy release that occurs when a vapor condenses into a liquid. The heat of compression relates to the process of compressing a gas and does not specifically describe the phase change from liquid to vapor. Understanding these concepts is crucial for working with refrigeration and air conditioning systems, where phase changes are integral to their functioning.