Water

Background

Water is a chemical compound needed by most plants and animals on Earth in order to sustain life. Pure water is a tasteless, odorless, transparent liquid. In small amounts it is colorless, but it takes on a bluish tint in larger amounts. Water is an excellent solvent and as a result it usually contains a wide variety of dissolved minerals and other chemicals. It can also carry and support bacteria. Most of the water distributed through municipal water systems is treated to remove harmful substances. Some bottled waters undergo even further treatment to remove almost all impurities. The English word water is derived from the German word wasser, which in turn is derived from an ancient Indo-European word meaning to wet or wash.

Water is the most abundant chemical in the body, making up roughly 60 per cent of body weight. It is an essential nutrient although it provides no energy. It has excellent solvent properties enabling it to act as a transport medium for many chemicals. It is involved in many chemical reactions including digestion of food.

Early water treatment was surprisingly advanced, although rarely practiced. Even with water treatment, water contamination remained a serious concern as an increasing amount of industrial wastes poured into rivers and lakes.

Pure water is an almost non-existent entity. Most water contains varying amounts of dissolved minerals and salts, plus an abundance of suspended particles such as silt and microscopic organic material. Different types of water are classified by the presence or absence of these impurities.

Evaporation of water as sweat is essential for cooling the body. However, failure to replace water losses results in dehydration. This can adversely affect physical performance even if relatively slight.

Hardness of water varies with geographical location. There is statistical evidence that heart disease is more common in areas with soft drinking water than in those with hard water. However, the link between type of water and heart disease is not proven.

Chemical and Physical Properties

Chemically, water is a compound of hydrogen and oxygen, having the formula H₂O. It is chemically active, reacting with certain metals and metal oxides to form bases, and with certain oxides of nonmetals to form acids. It reacts with certain organic compounds to form a variety of products, e.g., alcohols from alkenes. Because water is a polar compound, it is a good solvent. Although completely pure water is a poor conductor of electricity, it is a much better conductor than most other pure liquids because of its self-ionization, i.e., the ability of two water molecules to react to form a hydroxide ion, OH^-, and a hydronium ion, H₃O⁺. Its polarity and ionization are both due to the high dielectric constant of water.

Water Cycle

The water cycle (known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the hydrosphere, between the atmosphere, soil water, surface water, groundwater, and plants.

Water moves perpetually through each of these regions in the water cycle consisting of following transfer processes:

• evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into air.
• precipitation, from water vapor condensing from the air and falling to earth or ocean.
• runoff from the land usually reaching the sea.

Most water vapor over the oceans returns to the oceans, but winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year. Precipitation, at a rate of 107 Tt per year over land, has several forms: most commonly rain, snow, and hail, with some contribution from fog and dew. Condensed water in the air may also refract sunlight to produce rainbows.

Water runoff often collects over watersheds flowing into rivers. A mathematical model used to simulate river or stream flow and calculate water quality parameters is hydrological transport model. Some of water is diverted to irrigation for agriculture. Rivers and seas offer opportunity for travel and commerce. Through erosion, runoff shapes the environment creating river valleys and deltas which provide rich soil and level ground for the establishment of population centers. A flood occurs when an area of land, usually low-lying, is covered with water. It is when a river overflows its banks or flood from the sea. A drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation.

Uses of Water

By far the largest share of water in the region goes to agriculture - as much as 90 percent of total water use in some countries, and 60 percent in the more industrialized countries.

Drinking requires only a relatively small volume of water, but it must meet higher standards than that used for irrigation. Thirty liters of potable water per person-day is generally regarded as the minimum for drinking, cooking, and washing.

Industrial water use is low. Food and beverage processing are the largest industrial consumers. More is withdrawn for cooling but most of this water is recycled or returned to the watercourse.

A hidden but critical amount of water must be left in place to support fisheries and hydropower, as well as to protect habitat. This use is typically neglected by governments when they drain swamps, canalize rivers, or extend land. As a result, not only has the environment been degraded, but fish catches have declined and the salinity of groundwater has increased.

Adapted from Answers.com's Water entry.