Evaporation is the change of state from a liquid to its vapour at any temperature. Although both boiling and evaporation involve the change of state from a liquid to a vapour, there are important differences.
Evaporation of liquid occurs due to molecules, at the surface, with greater energy than the average kinetic energy escaping from the rest of the liquid. At any temperature , the molecules of a liquid are in a continuous random motion with different speeds. The liquid absorbs heat from the surroundings, and an increase in air over the substance increases the rate of heat absorption. At the surface of the liquid the more energetic molecules escape into the atmosphere, leaving behind the less energetic molecules and less heat energy. A liquid with slow moving molecules is a cooler liquid because the temperature is directly proportional to the average kinetic energy of the molecules. Therefore the temperature of the liquid falls during evaporation.
Evaporation causes a cooling effect to the object in contact due to absorption of heat by the liquid from the objects surface, also by the energetic molecules escaping along with heat energy.
Factors effecting the rate of evaporation
Raising the temperature of a liquid will increase the rate of evaporation. A warmer liquid means that there are more energetic molecules to escape the surface.
The rate of evaporation decreases with increased humidity. This is because as the air already holds a high percentage of water vapour there is little room for excess vapour.
Movement of air above the surface (or draught) increases the rate of evaporation, because the moving air removes the molecules of the liquid as soon as it has evaporated.
Decreasing atm pressure increases the rate of evaporation. If air pressure is high on the surface of the water then the water will not evaporate easily, as the pressure pushing down on the water makes it difficult for the molecules to escape.
The nature of the liquid is also important, as substances with lower boiling points evaporate easier than those with higher.