Evaporation is the escape of the most energetic particles from the surface

1. The particles in a liquid do not all move at the same speed, and at any moment there is a spread of kinetic energies across them.
2. A particle can only leave the liquid from the surface, because a particle deeper down is surrounded on all sides and has nowhere to go.
3. To escape, a surface particle must be travelling upwards and fast enough to break away from the attractive forces of the particles it is leaving behind.
4. Only the most energetic particles manage it, so evaporation is always the fastest particles leaving and the slower ones staying.
5. Because it needs no bubbles and no fixed temperature, evaporation goes on quietly at any temperature, which is why washing on a line dries on a cold day.
6. The escaped particles join the air above the liquid as a gas, and the liquid left behind gradually loses mass.

Boiling and evaporation are not the same process

1. Boiling happens at one fixed temperature, the boiling point, while evaporation happens at any temperature below it.
2. Boiling happens throughout the liquid, with bubbles forming at the bottom and sides, while evaporation happens only at the surface.
3. Boiling is fast and obvious, whereas evaporation is slow and produces nothing to see.
4. Boiling needs a continuous supply of energy from a heater, while evaporation draws its energy from the liquid and its surroundings, so it needs no heater at all.
5. The temperature of a boiling liquid stays constant, while an evaporating liquid cools down unless something keeps warming it.

Three things change how quickly a liquid evaporates

1. Raising the temperature speeds evaporation up.
   1. A warmer liquid has a higher average kinetic energy, so a larger fraction of its surface particles have enough energy to break away.
2. Increasing the surface area speeds evaporation up.
   1. A wider surface puts more particles at the surface at once, so more of them are in a position to escape, which is why a shirt spread out on a rack dries faster than the same shirt bundled up.
3. Moving air across the surface, from a breeze or a fan, speeds evaporation up.
   1. Escaped particles hover just above the liquid, and some of them fall back in; a draught carries them away so fewer return and the net loss is larger.
4. All three factors work through the same two questions: how many particles have enough energy to leave, and how many of those that leave are stopped from coming back.