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What is water?

The scientific name for water is H2O. This means that each water molecule or particle has two hydrogen atoms joined to an atom of oxygen.

One litre of pure water (which weighs 1kg) contains about 33,000,000,000,000,000, 000,000,000 (or 34 x 1024) water molecules. A tiny drop of water will contain about 1,650,000,000,000,000,000,000 (or 1.65 x 1021) water molecules.

Water has many unique properties which are very important for the existence of life as we know it on the earth. Water exists in three states on the earth.

Water is found as a:

  • solid (ice, hail, snow or frost)
  • liquid (in lakes, oceans, rain, dew, fog or mist)
  • gas (steam or water vapour).

Its low freezing point (0°C) and high boiling point (100°C) means that most of the water found on the earth's surface is in liquid form. In the liquid state, water has no definite shape. Instead it takes the shape of any container it is poured into with the upper surface of the water always horizontal.

Liquid water expands rather than contracts when it freezes. Water, like other materials will contract as it is cooled. At 4°C however, water starts to expand as it begins to solidify. As a result, ice is less dense than liquid water. This means that ice floats on top of liquid water and bodies of water freeze from the top down. Lakes and streams usually do not freeze solid in cold climates as the layer of ice on the top acts as an insulator. Without this feature, there would be significantly less aquatic plants and animals.

Because water expands on freezing, it can crack pipes, engine blocks, roads and rocks.

Water is the only common, safe substance which can exist in all three states at the same time naturally on the surface of the earth. Most chemical reactions need water.

Liquid water dissolves a wide range of substances. This means that water can carry dissolved materials around living things, flush out wastes, act as a cleanser and dilute many wastes. As a consequence, pure water is rarely found in nature.

Liquid water changes temperature very slowly. This is because water can store a large amount of heat without a large change in temperature. This means that large bodies of water, like lakes, do not change temperature quickly and living things are protected from rapid and large temperature changes.

It also takes large amounts of heat to evaporate water. Evaporation occurs when a water particle gains enough heat to 'escape' from the other water molecules in liquid water.

The hottest water molecules move fast enough to break free from the other water molecules. Evaporation has a cooling effect as the water molecules left behind are the slower moving, cooler ones. It also means that water helps even out the earth's climate a little and makes water an effective coolant for many purposes, for example in a car's radiator.

It takes almost nine times as much heat energy to increase the temperature of water 1°C than it does to increase the temperature of the same weight of sand by 1°C. No wonder the water feels so much cooler!