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This illustration of a geyser shows water in all three states: solid (ice on a distant glacier), liquid (the lake, and in the...Read More >>This illustration of a geyser shows water in all three states: solid (ice on a distant glacier), liquid (the lake, and in the spray from the geyser) and gas (the vapour in the air as the tiny water droplets in the steam evaporate). The circular insets show the molecules made up of oxygen (blue) and hydrogen (green) atoms. In solid ice the molecules are tightly packed; as a liquid the joins are looser; as a gas they move far apart. Water is a molecule with the chemical formula H2O. It contains two chemical elements—hydrogen (H) and oxygen (O)—with each atom of oxygen bonded to two hydrogen atoms. Water occurs in nature in all three states of matter. As a solid (ice), it makes up, for example, hail, icebergs and glaciers. As liquid water it occurs as raindrops, sea water and fresh water. As a gas, it is water vapour, an invisible component of the air. All known forms of life depend on water. All living things are made up mostly of water: our own bodies are 70% water. Water is fundamental to the way photosynthesis works in plants and plays a vital role in digestion and breathing in animals. It also acts as a solvent for essential nutrients.

Properties of water

Pure water has neither taste nor smell. It is transparent and colourless. Water can dissolve a large variety of chemical substances. The boiling point of water, as with all liquids, depends on the surrounding pressure. On the summit of Mt Everest, for example, where air pressure is low, water boils at 68°C (154°F), compared to 100°C (212°F) at sea level. Water deep in the oceans, where pressure is extremely high, can reach temperatures of hundreds of degrees near hydrothermal vents and yet still remain liquid.
An iceberg off the coast of GreenlandAs it freezes, water expands (about 9%), whereas most other substances shrink. So water occupies a greater volume as both a solid and a gas (about 1700 times) than in its liquid state. Because water expands as it freezes, it becomes less dense, not more. This explains why ice cubes—and massive icebergs—float in liquid water
Pond skater. Its feet make tiny depressions in the "skin" on the water's surface: the result of surface tension.

Hydrogen bonds

Water is one of the commonest compounds on Earth, and the only one found naturally in solid, liquid and gaseous forms.

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