State changes in the water cycle
Water on Earth moves in a loop called the water cycle. Each part of the loop is a state change you already know.
- Evaporation: liquid water in the sea, rivers, lakes and wet soil becomes invisible water vapour (gas). The Sun gives the heat. Plants also give out water vapour.
- Condensation: as vapour rises it cools. It turns into tiny liquid drops (or ice crystals). Many drops make a cloud. Heat is given out.
- Precipitation: drops that grow heavy fall as rain. If it is very cold, they freeze (liquid to solid) and fall as snow, sleet or hail.
- Melting: snow and ice on mountains melt (solid to liquid) in warm weather and feed rivers.
- Run-off and flow: water runs over land or sinks into the ground, then reaches the sea. The cycle repeats.
The total amount of water stays the same. It only changes state and place.
Where is Earth's water? Water resources
About 97.5 per cent of Earth's water is salty sea water. Only about 2.5 per cent is fresh. Of that fresh water, about 69 per cent is frozen in glaciers and ice caps, about 30 per cent is underground (groundwater), and only about 1 per cent is in rivers, lakes and soil. So the water we can easily use is a tiny part of the whole. Numbers are rounded and may differ a little by source.
Saving water
A tap that drips once every second wastes about 4 litres a day, which is about 1,500 litres in a year. Easy ways to save water:
- Close taps tightly while brushing teeth and soaping hands.
- Fix leaks quickly.
- Use a bucket instead of a long shower, and a mug when brushing.
- Reuse water from washing vegetables to water plants.
- Collect rain from roofs in a tank (rainwater harvesting).
- Water plants in the early morning or evening so less evaporates.
- Do not pollute rivers, because polluted water is hard to clean.
Try it: leaky-tap test
Put a cup under a dripping tap (or make a tiny hole in a bottle). Count drips for one minute, and measure the water. Multiply to get the amount per day. Then write three ways your family can save water this week. In the 3D, move the sun slider and watch the speed of evaporation, then fix the tap.
Key formulas and definitions
- Evaporation: liquid → gas (heat in, from the Sun)
- Condensation: gas → liquid (heat out, in cold air) → clouds
- Precipitation: rain (liquid), snow or hail (solid)
- Melting: ice and snow → water; the water cycle never loses water
- Earth's water: about 97.5% salty, about 2.5% fresh
Worked examples
1. Name the state change when sea water turns into water vapour, and say where the heat comes from.
Evaporation (liquid to gas). The heat comes from the Sun.
2. A tap drips 4 litres a day. How many litres does it waste in 30 days?
4 × 30 = 120 litres in 30 days.
3. On a very cold mountain top, cloud drops freeze and fall. Which state changes and which kind of precipitation is this?
Condensation (vapour to drops) and then freezing (liquid to solid). The precipitation is snow (or hail).
Common mistakes
- Thinking clouds are made of gas (vapour). Clouds are made of tiny drops of liquid water or ice crystals; the vapour itself is invisible.
- Thinking only boiling makes water vapour. Evaporation happens from any water surface at any temperature.
- Thinking water is used up. In the water cycle the water is not lost; it moves and changes state.
- Saying most fresh water is in rivers. Most fresh water is frozen in glaciers and ice caps.