Water (H2O) is a colorless (appearing blue outside because of skylight reflection), odorless, tasteless (contained minerals give it taste) liquid that at low elevations freezes at 0°C and boils at 100°C, by definition of the Centigrade temperature scale. It covers 3/4 of the earth's surface, although it is unevenly distributed on land; deserts have little and anarctic regions have much. The clouds in the atmosphere contain droplets of water that eventually precipitate as rain. The air around us contains water called humidity, which is measured by a hygrometer. No one knows how water came to be on earth, but its constant cycling between the earth and the atmosphere (the "water cycle") is essential to life.
Like air, water is a part of animal respiration and plant photosynthesis and therefore it is essential to life. Because people depend on water, they cannot live where water is unavailable. Therefore, cities depend on large quantities of potable water for their inhabitants and many of them were located at rivers, lakes, and oceans where water provided convenient transportation before the inventions of steam and internal combustion engines that provided more convenient land transportation. Fishes, whales, sea lions, and other seaborne animals can live only in water. Many plant species depend on a water habitants for survival.
Many important elements and compounds, such as salt, come from water. Acquifers are stores of underground water that may be pumped to provide water for people, animals, and crops, the last using irrigation ditches and canals for distribution. However, excessive pumping of water from acquifers causes land subsidence, which can harm overlying structures and farms. Animal and chemical wastes are often dumped into moving water to prevent them from polluting the land, but this creates water pollution, the source of some lethal diseases. Water pollution also deteriorates the water for animal and plant habitation. When wastes in water are excessive, people, animals, plants and crops are deprived of useful water. Therefore, waste and polluted waters must be treated, contained, and disposed of properly. Water also is invaluable for transportation and communications in oceans, lakes, rivers, and canals. Frozen water, ice, was an important refrigerant used for homes and transportation to preserve perishable foods in the days before the invention of electrical and chemical refrigeration. Because it is portable in small quantities, we still use it to temporarily preserve food at picnics and parties.
Water is essential in the chemical, food, and clothing industries because it is a part of many chemical reactions and because it is the "universal solvent" (dissolves many solutes). Some of these aqueous solutions, called electrolytes, are useful conductors of electricity. (Pure water is a non-conductor.) Falling water, usually provided by river dams, has energy that can be used to drive water turbines to produce electrical energy. Dams are controversial because they can hinder fish migrations. On the other hand, their contained lakes provide boating, fishing, and picnic recreation.
Water dissociates into hydronium and hydroxide ions, which form highly important compounds called acids and bases:
Water is an integral part of the crystal structure of many
compounds where it is called water of hydration. An example is borax
(sodium tetraborate decahydrate), the chemical formula for which is
Interestingly, the molecules of compounds in most rocks contain minute amounts of water. Water is just about everywhere!