Question

When water vapor condenses to a liquid, must heat be added or removed to maintain a

constant temperature?

Answer 1

Let us take the case of liquid water. If we want to convert liquid water to water vapour, then we need to provide heat to the system. As system is absorbing heat from the surrounding, it is an endothermic process.

The reverse process, that is, converting water vapour to liquid water (condensation) will be exothermic. The water vapour has to lose heat to the surrounding in order to become liquid water. As this is an exothermic process (the system is losing heat), in order to maintain constant temperature, we need to supply heat to the system.