When a liquid is heated up, its molecules gain kinetic energy, leading to an increase in their average speed. As the temperature rises, the faster-moving molecules at the surface of the liquid gain enough energy to overcome the attractive forces between them and the neighboring liquid molecules. This allows them to escape from the liquid phase and enter the gas phase, resulting in the process of evaporation.
Even when the pressure is kept constant, evaporation can occur. The pressure on the liquid's surface refers to the external pressure acting on the liquid, which can include atmospheric pressure. At any given temperature, there is a distribution of molecular energies in the liquid, with some molecules having higher energies than others. Only the molecules with sufficient energy to overcome the attractive forces can escape and transition to the gas phase.
During evaporation, the high-energy molecules break away from the liquid, resulting in a decrease in the average kinetic energy and temperature of the remaining liquid. Consequently, the overall cooling effect occurs as energy is removed from the liquid. This phenomenon is known as evaporative cooling, and it explains why evaporation feels cooling on the skin.
It's important to note that the rate of evaporation is influenced by several factors, including the temperature, surface area of the liquid, humidity of the surrounding environment, and the strength of intermolecular forces within the liquid. Higher temperatures generally lead to faster evaporation rates, while factors like high humidity can slow down evaporation due to the increased number of gas molecules in the surroundings, reducing the escape of liquid molecules into the gas phase.