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On Earth, water can exist as a liquid,
solid (ice) or gas (water vapour). Pure water has no-colour,
taste or smell. It turns to solid at 0°C and turns to vapour
at 100°C. The most important quality of water is that many
chemicals will dissolve in it. For example, plants need water
to take in dissolved minerals in their roots, and animals
rely on water in their lungs to absorb oxygen from the atmosphere.
Humans dissolve ingredients in water to make food, drinks,
medicine, etc.
Most of the Earth's water supply is
found in the oceans. In fact, 97 percent of all the water
in the world is found in the oceans. This is salt water so
it cannot be consumed by humans or animals, or used for growing
crops. Three percent of water on Earth is fresh water. However,
only 0.003 percent of this amount can be used by people or
animals. The rest of the water is either polluted, located
too far underground to be removed easily, or contained in
glaciers, polar ice caps, the atmosphere or soil. If the world's
water supply were 100 litres of water, the amount that could
be used would only equal one half a table-spoon.
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