water, Inorganic compound composed of hydrogen and oxygen (H2O), existing in liquid, gas (steam, water vapour), and solid (ice) states. At room temperature, water is a colourless, odourless, tasteless liquid. One of the most abundant compounds, water covers about 75% of Earth’s surface. Life depends on water for virtually every process, its ability to dissolve many other substances being perhaps its most essential quality. Life is believed to have originated in water (the world’s oceans or smaller bodies), and living organisms use aqueous solutions (including blood and digestive juices) as mediums for carrying out biological processes. Because water molecules are asymmetric and therefore electric dipoles, hydrogen bonding between molecules in liquid water and in ice is important in holding them together. Many of water’s complex and anomalous physical and chemical properties (high melting and boiling points, viscosity, surface tension, greater density in liquid than in solid form) arise from this extensive hydrogen bonding. Water undergoes dissociation to the ions H+ (or H3O+) and OH−, particularly in the presence of salts and other solutes; it may act as an acid or as a base. Water occurs bound (as water of hydration) in many salts and minerals. It has myriad industrial uses, including as a suspending agent (papermaking, coal slurrying), solvent, diluting agent, coolant, and source of hydrogen; it is used in filtration, washing, steam generation, hydration of lime and cement, textile processing, sulfur mining, hydrolysis, and hydraulics, as well as in beverages and foods. See also hard water; heavy water.
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