Wet clothes dry in the sun, even though the water is far below its boiling point. A drop of perfume on the skin feels cold. In this course we study evaporation, what makes it faster, and why it cools the surroundings.
3 lessons, about 1 h 20 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabEvaporation is the change of a liquid to a gas at the surface, at any temperature below the boiling point, when particles with enough energy escape. It happens only at the surface, is slow, and causes cooling, while boiling happens in the whole liquid at one temperature with bubbles. Evaporation is faster when the surface area is larger, the temperature is higher, the humidity is lower and the wind is stronger. It cools because the escaping particles take the most energetic particles away.
When a liquid evaporates, the particles with the most energy leave, so the rest of the liquid cools. Evaporation takes the latent heat of vaporisation from the surroundings, and this cools them.
Humidity is the amount of water vapour in the air, and relative humidity compares it with the most that the air can hold at that temperature. When the humidity is high, evaporation is slow, so sweat and wet clothes dry slowly, and we feel hotter because the body cannot lose heat. Warm air can hold more vapour than cool air. An evaporative cooler works well in dry weather and poorly in humid places. Evaporation and condensation drive the water cycle: water evaporates, the vapour condenses into clouds, and the drops fall as rain.
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