Latent heat refers to heat that is required to achieve what condition?

Latent heat is specifically the heat energy that is absorbed or released by a substance during a phase change, which allows the substance to transition from one state to another without a change in temperature. This means that during processes such as melting, freezing, vaporization, or condensation, energy is utilized to break or form molecular bonds rather than to raise or lower the temperature of the substance itself.

For instance, when ice is heated, it requires a certain amount of heat energy to melt into water — this energy is the latent heat of fusion. Similarly, when water is boiled, it requires latent heat to change from liquid to gas, without a rise in temperature during that phase change. This fundamental concept is crucial in understanding the behaviors of different substances during heating and cooling processes in HVAC systems and refrigeration applications.

The other choices, while related to thermal processes, do not accurately define latent heat. The increase or decrease in temperature pertains to sensible heat, and changes in pressure or volume relate more to state equations of gases rather than the specific energy transitions defined by latent heat.