The water cycle
Water is a key compound for life on Earth. All living organisms need water. Some can survive in a In a condition of inactivity, but can be activated if conditions change. state without it for long periods of time, but all organisms will quickly or eventually die without it. Water is also a A place where plants, animals and microorganisms live. for many Living entity, eg animals, plants or microorganisms. in oceans, rivers and lakes.
Water acts as the The liquid in which the solute dissolves to form a solution. for chemical reactions in the The living substance inside a cell (not including the nucleus). of all living cells. It also helps to transport dissolved substances into and out of cells. Water is also a Substances present at the start of a chemical reaction. or A substance formed in a chemical reaction. of many biochemical processes. For example:
- water is a reactant in A chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic.
- water is a product in The chemical change that takes place inside living cells, which uses glucose and oxygen to release the energy that organisms need to live. Carbon dioxide is a by-product of respiration.
- water is a reactanct/product in The breakdown of large insoluble food molecules to smaller soluble ones.
Water is found on Earth in all three states of matter:
- as a solid in glaciers and ice sheets
- as a liquid in oceans, rivers, lakes and Naturally occurring underground water stores.
- as a gas in the atmosphere
Water cycles through the environment via a number of processes (including freezing, melting, evaporating and condensing) which together make the water cycle.
The The continuous movement of water on, above and below the Earth. is easiest to understand in terms of its processes and what happens to the water in each of these. These are the key processes:
| Process | What happens to water |
| Evaporation | Water turns from a liquid to a gas when it evaporates. Energy from the Sun can evaporate water from all places on the Earth’s surface such as puddles, ponds, lakes and oceans. |
| Condensation | After evaporation water can cool and convert from gas to liquid, often forming clouds. |
| Transport | Water within clouds can be blown many miles by strong winds and so transported to other areas. |
| Precipitation | Precipitation occurs when rain, snow, hail and sleet fall from the sky. |
| Surface runoff | Much water will be absorbed into the ground after precipitation but if a large volume falls or the ground is already wet some water can run along the surface of the ground. |
| Infiltration | This occurs when water that has fallen as precipitation is absorbed into the ground. This can then be stored within underground rocks called aquifers. |
| Transpiration | Plants need to maintain a constant stream of water to their leaves for transport and support. So they allow some water to evaporate as water vapour from their leaves to mean that more is continually ‘pulled’ to their leaves from the soil. |
| Process | Evaporation |
|---|---|
| What happens to water | Water turns from a liquid to a gas when it evaporates. Energy from the Sun can evaporate water from all places on the Earth’s surface such as puddles, ponds, lakes and oceans. |
| Process | Condensation |
|---|---|
| What happens to water | After evaporation water can cool and convert from gas to liquid, often forming clouds. |
| Process | Transport |
|---|---|
| What happens to water | Water within clouds can be blown many miles by strong winds and so transported to other areas. |
| Process | Precipitation |
|---|---|
| What happens to water | Precipitation occurs when rain, snow, hail and sleet fall from the sky. |
| Process | Surface runoff |
|---|---|
| What happens to water | Much water will be absorbed into the ground after precipitation but if a large volume falls or the ground is already wet some water can run along the surface of the ground. |
| Process | Infiltration |
|---|---|
| What happens to water | This occurs when water that has fallen as precipitation is absorbed into the ground. This can then be stored within underground rocks called aquifers. |
| Process | Transpiration |
|---|---|
| What happens to water | Plants need to maintain a constant stream of water to their leaves for transport and support. So they allow some water to evaporate as water vapour from their leaves to mean that more is continually ‘pulled’ to their leaves from the soil. |
Learn more about the water cycle with Dr Alex Lathbridge.
Listen to the full series on 鶹Լ Sounds.
Dr Alex Lathbridge breaks down the key facts about the water cycle. Listen to the full series on 鶹Լ Sounds.