Water

:This article focuses on water as it is experienced in everyday life. See water (molecule) for information on the chemical and physical properties of pure water (H2O, hydrogen oxide).

A surprising substance

Changing appearances

Water takes many different shapes on earth: water vapor and clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, aquifers in the ground, to name but a few. Through evaporation, precipitation, and runoff, water is continuously flowing from one form to another, in what is called the water cycle.

Related Topics:
Water vapor - Cloud - Wave - Iceberg - Sea - Glacier - Mountain - Aquifers - Evaporation - Precipitation - Runoff - Water cycle

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Because of the importance of precipitation to agriculture, and to mankind in general, we give different names to its various forms: while rain is common in most countries, other phenomena are quite surprising when seen for the first time: hail, snow, fog or dew for example. When appropriately lit, water drops in the air can refract sunlight to produce rainbows.

Related Topics:
Agriculture - Mankind - Rain - Hail - Snow - Fog - Dew - Refract - Rainbows

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Similarly, water runoffs have played major roles in human history: rivers and irrigation brought the water needed for agriculture. Rivers and the seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping our environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers.

Related Topics:
River - Irrigation - Sea - Erosion - Valley - Delta

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Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells.

Related Topics:
Aquifer - Groundwater - Springs - Hot spring - Geyser - Well

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Because water can contain many different substances, it can taste or smell very differently. In fact, humans and other animals have developed their senses to be able to evaluate the drinkability of water: animals generally dislike the taste of salty sea water and the putrid swamps and favor the purer water of a mountain spring or aquifer.

Related Topics:
Salt - Sea water - Swamp

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Important properties for living organisms

Water has many unusual properties that are critical for life: it is a good solvent and has high surface tension. Fresh water has its greatest density at 4°C. It becomes less dense as it freezes or heats up. As a stable, polar molecule prevalent in the atmosphere, it plays an important atmospheric role as an absorber of infrared radiation, crucial in the atmospheric greenhouse effect (which, contrary to one popular belief, is actually essential to life; with no greenhouse effect, the average surface temperature would be −18° Celsius). Water also has an unusually high specific heat, which plays many roles in regulating global (and regional; see for example the Gulf Stream) climate.

Related Topics:
Solvent - Surface tension - Density - C - Freezes - Heat - Polar molecule - Infrared radiation - Greenhouse effect - Specific heat - Gulf Stream

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Water is a very good solvent, chemically not unlike ammonia, and dissolves many types of substances, such as various salts and sugar, and facilitates their chemical interaction, which aids complex metabolisms.

Related Topics:
Solvent - Ammonia - Sugar - Chemical interaction - Metabolism

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Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water.

Related Topics:
Oil - Hydrophobic - Cell membrane - Lipid - Protein

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Water drops are stable due to the high surface tension of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property plays a key role in plant transpiration.

Related Topics:
Glass - Plant - Transpiration

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A simple but environmentally important and unique property of water is that its common solid form, ice, floats on the liquid. This solid phase is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases of water ice except ice-XI, the solid form is more dense than the liquid form. Fresh water is most dense at 4°C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold.

Related Topics:
Ice - Convection

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While this behavior may seem obvious, even intuitive, it should be noted that almost all other chemicals are denser as solids than they are as liquids, and freeze from the bottom up.

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Life on earth has evolved with and fine tuned itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has no doubt been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions.

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In fact, civilizations have historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, is situated between two major rivers. Large metropolises like London, Paris, New York, and Tokyo owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore and Hong Kong, have flourished for precisely this reason. In places such as North Africa and the Middle East, where water is scarcer, access to clean drinking water was and is a major factor in human development.

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