Posted: 24 January 2001

Pure water is a colourless, odourless liquid that is a compound of hydrogen and oxygen (H20). Natural water in the environment is never pure, but contains a variety of dissolved substances. Sea water, for example, is a solution of sodium chloride (NaCl - common salt) and other salts; rainwater can be acidic because of the carbon dioxide (CO2) that it contains and the water in rivers may include minerals dissolved from the rocks over and through which it has flowed. Water can exist as a solid (ice), liquid (water), or gas (water vapour) and changes readily from one to the other, either releasing or taking up energy as it does so.Some 97 per cent of the world's water is in the oceans, while a further 2 per cent is in the form of ice and snow, which leaves only 1 per cent available as freshwater for plants and animals. Survival on such small amount is made possible by the natural recycling of the water in the hydrological cycle, which not only replaces the water once is has been used, but also cleans it.