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A simple chemical compound that is liquid at room temperature and pressure. Has the chemical formula H2O. Found almost everywhere on Earth. Predominantly in seas and oceans. Vital to most living organisms. The physical properties of water are used to define the Celsius scale for measuring temperature.

The solid state of water is known as ice; the gaseous state is known as steam.

Bodies of water, generic: sea, lake, ocean, river, pond, etc.

The dipolar nature of water

An important feature of water is its polar nature. This is caused when some areas of molecules attract electrons more than others - they have a higher electronegativity. In covalently bonded water, this causes a negative dipole on the oxygen which causes it to be attracted to positive dipoles on other molecules. Water also has hydrogen bonding, where the lone pair of electrons on the oxygen causes it to be even further negative increasing the attraction between molecules.

This strong attraction results in properties such as a very high boiling point, as a lot of heat is neccesary to break the bonds between molecules, and also a large specific heat capacity.

Water as a solvent

Another property emerging from strong intermolecular attraction is water being a good solvent. When soluble compounds enter water they are surrounded by water molecules (the small size of a water molecule allows typically many water molecules to surround one solute molecule). The negative dipoles on water are attracted to the positive dipoles of the solute, and likewise for the positive dipoles of water. These pull apart the compounds until each ion of the original solute molecule is surrounded by respectively (depending on the charge of the ion) cations and anions . The ions are then free to move about on their own. If the solute molecules can form hydrogen bonds with water, dissolving occurs even faster.

The property of water as a solvent is vital in biology, as many biochemical reactions can only take place in solution (eg. reactions in cytoplasm and blood).

Surface tension

The strong hydrogen bonds give water high surface tension and cohesiveness. This is evident when small quantities of water are put on to a non-soluble surface and the water stays together as drops. This feature is important when water is carried through xylem up stems in plants; The strong intermolecular attractions hold the water column together during transpiration whereas in other liquids with lower surface tension this would not be possible.

see also precipitation