Evaporation, transpiration and condensation
When water evaporates, the particles at the top need energy to escape as vapour. They take that energy as heat from the water and the air around, so evaporation takes in heat and the surroundings cool. This is why wet skin feels cool in the breeze. When water vapour condenses, the particles slow down and join as liquid, and they give out heat to the air around. Plants also add vapour to the air. Water leaves their leaves as vapour: that is transpiration, which is evaporation from leaves, and it makes up about a tenth of the water vapour in the air. Clouds are not vapour and not steam; a cloud is made of billions of tiny liquid droplets that float in the air.
| Change | Heat | Example |
|---|---|---|
| evaporation (liquid to vapour) | takes in heat | wet cloth drying |
| condensation (vapour to liquid) | gives out heat | drops on a cold glass |
| transpiration | takes in heat | water leaving leaves |
Mist and drops. With a grown-up, breathe on a cold mirror or cold glass and watch. Then put a cold drink can on a plate in a warm room and look after ten minutes. Draw both. Where did the drops come from? Tell a family member. Do not use hot water.