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Water is a chemical substance that is essential to all known forms of life. It appears colourless to the naked eye in small quantities, though it is actually slightly blue in colour. It feels wet to the touch. It covers 71% of Earth's surface. more...
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The United Nations Environment Programme estimates there are 1.4 billion cubic kilometres (330 million mi3) of it available on Earth, and it exists in many forms. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, and sea ice. Water in these bodies perpetually moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human life. In many parts of the world, it is in short supply. Significant quantities are believed to exist on the moons Europa and Enceladus.
Water as a chemical
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Water is the chemical substance with the chemical formula H2O; meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. Water is a colorless, tasteless, and odorless liquid at ambient temperature and pressure. It is a very important solvent, dissolving many other chemical substances, such as salts, sugars, acids, alkalis, some gases and many organic molecules, to name a few.
Water is unusual in that it is a liquid under normal conditions, when relationships between other analogous hydrides of oxygen's column in the periodic table suggest it should be a gas, as is hydrogen sulphide. If the periodic table is examined, it will be noted that the elements surrounding oxygen are nitrogen, fluorine, phosphorus, sulphur and chlorine. All of these elements combine with hydrogen to produce gases at normal temperature and pressure. The reason that oxygen forms a liquid is that it is more electronegative; oxygen pulls on electrons much more strongly than hydrogen, leaving a net positive charge on the hydrogen side of the molecule, and a net negative charge on the oxygen side. The electrical attraction 'pulls' separate molecules closer together and raises the boiling point. This attraction is known as hydrogen bonding.
Water has been referred to as the universal solvent, and the only real pure substance found naturally in all three states of matter. It is in dynamic equilibrium between the liquid and vapor states at standard temperature and pressure. Ionically, water can be described as as a hydrogen ion (H+) that is bonded to a hydroxide ion (OH-).
Solvation
Water is a very good solvent, dissolving many types of substances. The substances that will mix well and dissolve in water (e.g. salts) are known as "hydrophilic" (water-loving) substances, and those that do not mix well with water (e.g. fats and oils), are known as "hydrophobic" (water-fearing) substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between themselves. If the ability of a substance to dissolve in water cannot, the molecules are "pushed out" from amongst the water and do not dissolve.
Read more at Wikipedia.org
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