The Water Cycle (Edexcel GCSE Combined Science: Biology): Revision Note

The Water Cycle

• Water molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes

• This is possible because water changes state at a relatively low temperature

The water cycle

• Water enters the atmosphere as water vapour in one of two processes

  • Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and lakes

  • Transpiration from plants releases water vapour into the air

• The warmer air of the lower atmosphere rises, taking the water vapour with it

  • The moist air cools down as it rises

  • Water vapour condenses back into liquid water, forming clouds

• Water returns to earth in the form of precipitation

  • As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet

  • This is called precipitation

The importance of the water cycle

• All life on earth depends upon water for a variety of reasons, this includes photosynthesis

• The water cycle, therefore, is a fundamental process for all living things as it distributes fresh water globally providing us with clean water for drinking

Dealing with drought

• In some areas, drought may become an issue as it means that populations living in those areas would not have access to the potable water they require

• One way to manage this is through desalination of salty water such as sea water

  • Desalination means removing the excess mineral ions (salts) from the water to make it drinkable

  • There are several ways of removing these salts

    • Distillation - saline water is boiled, the water vapour is funnelled through a tube before it is condensed and the pure water is collected

    • Reverse osmosis - saline water is forced at high pressure through a partially permeable membrane which filters out all the mineral ions leaving pure water