The water cycle

The water on Earth is used again and again. The Sun heats water so that it evaporates; the vapour rises, cools and condenses into clouds; water falls as rain or snow; and it collects in rivers, lakes and seas, ready to evaporate again. This is the water cycle.

Key facts

  • Evaporation: the Sun heats water in seas, lakes and rivers, and it turns into water vapour (a gas).
  • Condensation: water vapour rises, cools and turns into tiny droplets that make clouds.
  • Precipitation: the droplets join, get heavy and fall as rain, snow, sleet or hail.
  • Collection: water runs off the land into rivers, lakes and seas, or soaks into the ground (infiltration).
  • Plants also give off water vapour from their leaves (transpiration).
  • The Sun provides the energy. When salty sea water evaporates, the salt is left behind, so rain is fresh water.
  • Evaporation is faster when it is warm, when it is windy and when the surface area is large.

Worked examples

Example 1

Name the parts of the water cycle at A, B, C and D.

  1. A: the Sun heats the sea and water vapour rises: evaporation.
  2. B: the vapour cools high up and condenses into a cloud: condensation.
  3. C: water falls from the cloud as rain: precipitation.
  4. D: the water flows back to the sea in rivers: collection. Then the cycle starts again.

Answer: A evaporation, B condensation, C precipitation, D collection

Example 2

Drops of water form on the outside of a cold can of drink. Where does the water come from?

  1. The air always contains some invisible water vapour.
  2. When the vapour touches the cold can, it cools down.
  3. Cooled water vapour condenses into liquid droplets: the same thing that happens when clouds form.

Answer: water vapour in the air touches the cold can, cools and condenses into liquid droplets

Example 3

A dish holds 300 ml of water. It loses 25 ml a day by evaporation. How much is left after 4 days?

  1. Work out how much evaporates in 4 days. 4 × 25 = 100 ml
  2. Take it from the starting volume. 300 − 100 = 200 ml

Answer: 200 ml

Example 4

Much of the rain that falls evaporated from the salty sea. Why is rain not salty?

  1. When sea water is heated by the Sun, water particles escape into the air as water vapour.
  2. Salt does not evaporate, so it stays behind in the sea.
  3. So the water vapour, the clouds and the rain are all fresh water.

Answer: only the water evaporates; the salt is left behind in the sea