A liquid left to stand in a saucer eventually evaporates because, at any time, a proportion of its molecules will be fast enough (have enough kinetic energy) to escape from the attractive intermolecular forces at the liquid surface into the atmosphere. The temperature of the liquid tends to fall because the evaporating molecules remove energy from the liquid. The rate of evaporation rises with increased temperature because as the mean kinetic energy of the liquid's molecules rises, so will the number possessing enough energy to escape.

A fall in the temperature of the liquid, known as the cooling effect, accompanies evaporation because as the faster molecules escape from the surface, the mean energy of the remaining molecules falls. The effect may be noticed when sweat evaporates from the skin. It plays a part in temperature control of the human body.

The evaporation of liquid water to form water vapour is responsible for the movement of water from the Earth's surface to the atmosphere.

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Evaporation and Boiling