Water (H2O)
Pure water is an
odourless and tasteless liquid. It has a bluish tint, which may be detected only
in layers of considerable depth. Under standard atmospheric pressure (760 mm of
mercury, or 760 torr); the freezing point of water is 0° C (32° F) and its
boiling point is 100° C (212° F).
Water attains its maximum density at a temperature of 4° C (39° F) and expands upon freezing. Like most other liquids, water can exist in a supercooled state; that is, it may remain a liquid although its temperature is below its freezing point. Water can easily be cooled to about -25° C (-13° F) without freezing, either under laboratory conditions or in the atmosphere itself. Supercooled water will freeze if it is disturbed, if the temperature is lowered further, or if an ice crystal or other particle is added to it. Its physical properties are used as standards to define temperature scales and in the metric system for the original definition of the unit of mass, the gram.
Water is one of the best-known ionizing agents. Because most substances are somewhat soluble in water, it is frequently called the universal solvent. Water combines with certain salts to form hydrates. It reacts with metal oxides to form acids. It acts as a catalyst in many important chemical reactions.