The water cycle and potable waterSpec 9.14
All organisms need water to survive, and plants need it for photosynthesis. The water cycle is important because it keeps supplying fresh water to the land for plants, animals and humans. The main processes are:
- Evaporation: water from seas, lakes and soil changes to water vapour, using heat from the Sun.
- Transpiration: water vapour is lost from the leaves of plants.
- Condensation: water vapour cools and turns into liquid water droplets, forming clouds.
- Precipitation: water falls as rain, snow or hail, and then runs over the land or soaks into the soil and flows into rivers, lakes and the sea.
Potable water and desalination
Potable water is water that is safe to drink. In areas of drought there may not be enough fresh water. Sea water contains too much dissolved salt to drink, but desalination removes the salt to produce potable water.
- Distillation: sea water is evaporated, leaving the salt behind, and the water vapour is condensed back to pure water.
- Reverse osmosis: sea water is forced through a partially permeable membrane that lets water through but not salt.
Desalination needs large amounts of energy, so it is expensive.