Water: where does it come from?
We use water for drinking, cooking, washing, farming and in factories. We get it from rivers, lakes, ponds, wells and underground water, but all of it ultimately comes from rain and snow.
About three-quarters of the Earth's surface is covered with water, yet most of it is salty sea water. Only a small fraction is fresh water we can use.
Evaporation and condensation
Evaporation is the change of liquid water into water vapour, a gas. Wet clothes dry and puddles disappear on a sunny day because of evaporation. Heat, wind and a large exposed surface make it faster.
Condensation is the change of water vapour back into liquid water when it is cooled. Drops form on the outside of a glass of ice-cold water because water vapour in the air condenses on the cold surface.
Plants also release water vapour from their leaves, a process called transpiration. A large tree can give off hundreds of litres of water in a day.
The water cycle
Water moves continuously between the oceans, the air and the land. This never-ending circulation is the water cycle.
The Sun heats water in oceans, rivers and lakes; it evaporates. Plants add vapour by transpiration.
Warm, moist air rises. Higher up it is cooler, so the vapour condenses into tiny droplets that form clouds.
Droplets join into larger drops and fall as rain, or as snow or hail when it is cold enough. This is precipitation.
Rainwater flows into rivers and back to the sea, or soaks into the ground and becomes groundwater. Snow on mountains melts in summer and feeds rivers.
The total amount of water on Earth stays almost the same; the cycle only moves it around.
Too much and too little
Floods happen when heavy rain makes rivers overflow. They damage crops, homes and roads, and drown animals.
Droughts happen when there is little or no rain for a long time. Soil dries up, crops fail and there is a shortage of food and fodder.
Conserving water
Fresh water is limited, and much of it is wasted. Ways to save it:
Turn off taps while brushing teeth; fix leaking taps and pipes.
Use a bucket instead of a hose to wash a car.
Rainwater harvesting: collect rainwater from rooftops and store it in tanks, or lead it into the ground to recharge groundwater.
Air is everywhere
We cannot see air, but we feel it as wind, see it move leaves and fill balloons, and watch bubbles rise when an "empty" bottle is pushed into water. Air occupies space.
What is air made of?
Air is a mixture of gases, along with water vapour and dust.
Component | Approximate share | Notes |
|---|---|---|
Nitrogen | about 78% | needed by plants (taken in from the soil as nitrogen compounds) |
Oxygen | about 21% | needed for breathing and burning |
Argon and other gases | just under 1% | |
Carbon dioxide | about 0.04% | used by plants to make food |
Water vapour | varies | more on humid days |
Dust and smoke | varies | more near roads and factories |
How living things use air
Animals use oxygen for respiration and give out carbon dioxide.
Green plants use carbon dioxide to make food in sunlight and give out oxygen. This keeps the balance of oxygen and carbon dioxide in the air.
Aquatic animals use oxygen dissolved in water.
Moving air turns windmills to pump water and generate electricity, and helps birds, gliders and sailing boats move.
Summary
Evaporation, condensation and precipitation drive the water cycle.
Water is limited; conserve it and harvest rainwater.
Air is a mixture, mostly nitrogen (about 78%) and oxygen (about 21%).
Plants and animals keep the balance of oxygen and carbon dioxide in the air.