The Water Cycle (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

The Water Cycle

• Water is found in three states across the Earth:

  • Solid: in glaciers and ice sheets

  • Liquid: in oceans, rivers, lakes and underground aquifers

  • Gas: as water vapour in the atmosphere

• Water continuously cycles between these states through several processes

Evaporation

• Energy from the Sun causes liquid water to evaporate from oceans, rivers and lakes, turning into water vapour in the atmosphere

• As water vapour rises, the air cools

Condensation

• Cooling water vapour condenses back into tiny liquid droplets, forming clouds

Precipitation

• As water droplets in clouds get bigger and heavier, they fall back to Earth as precipitation

• Precipitation can take the form of rain, sleet, snow or hail

Melting and freezing

• Solid water in glaciers and ice sheets can melt, flowing as liquid into rivers and oceans

• Under cold conditions, liquid water freezes back into a solid

Why is water important?

• Life on Earth depends on water, both on land and in the seas

Water as a solvent

• Water acts as the solvent for chemical reactions in cells

• Many substances dissolve in water, allowing biochemical reactions to take place

Water as a transport medium

• Water transports dissolved compounds into and out of cells

• It carries nutrients, waste products and other substances around organisms

Water as a reactant and product

• Water is either a reactant or a product in many biochemical reactions, including:

• Photosynthesis:

carbon dioxide + water → glucose + oxygen

• Respiration:

glucose + oxygen → carbon dioxide + water

• Digestion: large food molecules are broken down with water

Water as a habitat

Rivers, lakes and seas provide habitats for a huge variety of living organisms