In what condition are liquid and vapor said to be in equilibrium?

The condition where liquid and vapor are said to be in equilibrium occurs when they maintain the same temperature. This state reflects that the rate of evaporation of the liquid equals the rate of condensation of the vapor. In this scenario, the two phases coexist stably, and although individual molecules are transitioning between the liquid and vapor states, there is no net change in the amount of liquid or vapor present in the system.

This equilibrium condition is essential in understanding phase transitions and the behavior of substances in various engineering applications, as it indicates a balance between the thermal energies of both states. The maintenance of the same temperature signifies that the system is in a stable state, avoiding either an increase in vaporization from heat addition or a reduction through cooling or condensation that would disrupt this balance.