water

Chemical compound of hydrogen and oxygen elements – H₂O. It can exist as a solid (ice), liquid (water), or gas (water vapour). Water is the most common compound on Earth and vital to all living organisms. It covers 70% of the Earth's surface, and provides a habitat for large numbers of aquatic organisms. It is the largest constituent of all living organisms – the human body consists of about 65% water. It is found in all cells and many chemicals involved in processes such as respiration and photosynthesis need to be in solution in water in order to react. Pure water is a colourless, odourless, tasteless liquid which freezes at 0°C/32°F, and boils at 100°C/212°F. Natural water in the environment is never pure and always contains a variety of dissolved substances. Some 97% of the Earth's water is in the oceans; a further 2% is in the form of snow or ice, leaving only 1% available as freshwater for plants and animals. The recycling and circulation of water through the biosphere is termed the water cycle, or ‘hydrological cycle’; regulation of the water balance in organisms is termed osmoregulation. Water becomes more dense when it cools but reaches maximum density at 4°C/39°F. When cooled below this temperature the cooler water floats on the surface, as does ice formed from it. Animals and plants can survive under the ice.

The recycling and circulation of water on Earth is called the water cycle. Water occurs on the Earth's surface as standing water in oceans and lakes, as running water in rivers and streams, as rain, and as water vapour in the atmosphere. Together these sources comprise the hydrosphere which is in a constant state of flux – water vapour condenses as it cools to form clouds, droplets of water in the clouds merge to form raindrops that fall to earth (precipitation), and after flowing through rivers and streams into lakes and oceans water is returned to the atmosphere by evaporation; and so the cycle continues. Since the hydrological cycle is a closed system, the amount of water in the Earth's hydrosphere is constant. The cycle is powered by solar radiation which provides the energy to maintain the flow through the processes of evaporation, transpiration, precipitation, and run-off.