Water is a chemical substance that is essential to all known forms of life. It covers 71% of Earth's surface. There are 1.4 billion cubic kilometers (330 million mi³) of it available on Earth. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, airborne vapor and sea ice. Water in these bodies perpetually moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human life. In many parts of the world, it is in short supply. Outside of our planet, a significant quantity of water is thought to exist at the north and south poles of the planet Mars, and on the moons Europa and Enceladus.
Chemical and physical properties
Water is the chemical substance with the chemical formula H2O: one molecule of water is composed of two hydrogen atoms covalently bonded to a single oxygen atom. Water is a colorless, tasteless, and odorless liquid at ambient temperature and pressure. It is a very important solvent, capable of dissolving many other chemical substances, such as salts, sugars, acids, alkalis, some gases and many organic molecules.
Water is unusual in that it is a liquid under normal conditions, when relationships between other analogous hydrides of oxygen's column in the periodic table suggest it should be a gas, as is hydrogen sulfide. If the periodic table is examined, it will be noted that the elements surrounding oxygen are nitrogen, fluorine, phosphorus, sulfur and chlorine. All of these elements combine with hydrogen to produce gases at normal temperature and pressure. The reason that oxygen forms a liquid is that it is more electronegative than all of these elements (other than fluorine). Oxygen pulls on electrons much more strongly than hydrogen, leaving a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom. The presence of a charge on each of these atoms gives each water molecule a net dipole moment. Electrical attraction between water molecules due to this dipole pulls individual molecules closer together, making it more difficult to separate the molecules and therefore raising the boiling point. This attraction is known as hydrogen bonding.
Water has been referred to as the universal solvent, and is the only real pure substance found naturally on Earth in all three states of matter. It is in dynamic equilibrium between the liquid and solid states at standard temperature and pressure. Ionically, water can be described as a hydrogen ion (H+) that is associated with a hydroxide ion (OH-).
High concentrations of dissolved lime make the water of Havasu Falls appear turquoise.
Water is a very strong solvent, dissolving many types of substances. The substances that will mix well and dissolve in water (e.g. salts) are known as "hydrophilic" (water-loving) substances, and those that do not mix well with water (e.g. fats and oils), are known as "hydrophobic" (water-fearing) substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. If a substance has properties that do not allow it to overcome the strong intermolecular forces between water molecules, the molecules are "pushed out" from amongst the water and do not dissolve.
Cohesion and adhesion
Dew drops adhering to a spider web
Water sticks to itself (cohesion) because it is polar. Water has a partial negative charge (σ-) near the oxygen atom due the unshared pairs of electrons, and partial positive charges (σ+) near the hydrogen atoms. In water, this happens because the oxygen atom is more electronegative than the hydrogen atoms—that is, it has a stronger "pulling power" on the molecule's electrons, drawing them closer (along with their negative charge) and making the area around the oxygen atom more negative than the area around both of the hydrogen atoms.
Water also has high adhesion properties because of its polar nature.
This daisy is under the water level, which has risen gently and smoothly. Surface tension prevents the water from submerging the flower.
Water has a high surface tension caused by the strong cohesion between water molecules. This can be seen when small quantities of water are put onto a non-soluble surface such as polythene; the water stays together as drops. On extremely clean/smooth glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces.
In biological cells and organelles, water is in contact with membrane and protein surfaces that are hydrophilic; that is, surfaces that have a strong attraction to water. Irving Langmuir observed a strong repulsive force between hydrophilic surfaces. To dehydrate hydrophilic surfaces—to remove the strongly held layers of water of hydration—requires doing substantial work against these forces, called hydration forces. These forces are very large but decrease rapidly over a nanometer or less. Their importance in biology has been extensively studied by V. Adrian Parsegian of the National Institute of Health. They are particularly important when cells are dehydrated by exposure to dry atmospheres or to extracellular freezing.'
Capillary action refers to the process of water moving up a narrow tube against the force of gravity. It occurs because water adheres to the sides of the tube, and then more water is pulled on top of that water through cohesion, which sticks to the sides of the tube. The process is repeated as the water flows up the tube until there is enough water that gravity can counteract the adhesive force.
Heat capacity and heat of vaporization
Water has the second highest specific heat capacity of any known chemical compound, after ammonia, as well as a high heat of vaporization (40.65 kJ mol-1), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.
A simple but environmentally important and unusual property of water is that its common solid form, ice, floats on its liquid form. This solid phase is not as dense as liquid water because of the geometry of the hydrogen bonds which are formed only at lower temperatures. For almost all other substances the solid form has a greater density than the liquid form. Fresh water at standard atmospheric pressure is most dense at 3.98 °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. The freezing point of water is 0°C (32°F, 273 K).
The triple point of water (the single combination of pressure and temperature at which pure liquid water, ice, and water vapor can coexist in a stable equilibrium) is used to define the kelvin, the SI unit of thermodynamic temperature. As a consequence, water's triple point temperature is an exact value rather than a measured quantity : 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (0.0060373 atm).
A common misconception about water is that it is a good conductor of electricity, with risks of electrocution explaining this popular belief. Any electrical properties observable in water are from the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize where two water molecules become one hydroxide anion and one hydronium cation, but not enough to carry enough electric current to do any work or harm for most operations. In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.055 µS/cm at 25°C. Pure water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process and thus very little current is conducted.
Water takes many different forms on Earth: water vapor and clouds in the sky; seawater and icebergs in the ocean; glaciers and rivers in the mountains; and 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.
Rainbows like this one are formed by rain drops acting as a natural prism.
Because of the importance of precipitation to agriculture, and to mankind in general, different names are given to its various forms: rain is common in most countries, and hail, snow, fog and dew are other examples. When appropriately lit, water drops in the air can refract sunlight to produce rainbows.
Similarly, water runoffs have played major roles in human history as rivers and irrigation brought the water needed for agriculture. Rivers and seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping the environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers.
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.
Water can dissolve many different substances imparting upon it different tastes and odors. In fact, humans and other animals have developed senses to be able to evaluate the drink-ability 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. The taste advertised in spring water or mineral water derives from the minerals dissolved, while pure H2O is tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microbes.
Position of the Earth relating to water
Over two thirds of the earth's surface is covered with water, 97.2% of which is contained in the five oceans. The Antarctic ice sheet, containing 90% of all fresh water on the planet, is visible at the bottom. Atmospheric water vapor can be seen as clouds, contributing to the earth's albedo.
Scientists theorize that most of the universe's water is produced as a byproduct of star formation. Gary Melnick, a scientist at the Harvard-Smithsonian Center for Astrophysics, explains: "For reasons that aren't entirely understood, when stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflowing material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water we observe is quickly produced in this warm dense gas." 
The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to the existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer to or further from the Sun (a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.
Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars).
It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.
Effects on life
A captive lion drinking water
From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Water is thus essential and central to these metabolic processes.
Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO2 (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO2 in the process (cellular respiration).
Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H+, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH−) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7. Stomach acid (HCl) is useful to digestion. However, its corrosive effect on the esophagus during reflux can temporarily be neutralized by ingestion of a base such as aluminum hydroxide to produce the neutral molecules water and the salt aluminum chloride. Human biochemistry that involves enzymes usually performs optimally around a biologically neutral pH of 7.4.
Aquatic life forms
Some of the biodiversity of a coral reef
Earth's waters are filled with life. Nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales that also live in the water. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.
Some marine diatoms - a key phytoplankton group
Different water creatures have found different solutions to obtaining oxygen in the water. Fish have gills instead of lungs, though some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to surface periodically to breathe air.
Effects on human civilization
A manual water pump in China
Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates; the ancient society of the Egyptians depended entirely upon the Nile. Large metropolises like Rotterdam, London, Montreal, Paris, New York, Shanghai, Tokyo, and Chicago 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 the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.
Health and pollution
Water fit for human consumption is called drinking water or "potable water". Water that is not fit for drinking but is not harmful for humans when used for food preparation is called safe water.
This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about 1 billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitation during the 2003 G8 Evian summit. Even if this difficult goal is met, it will still leave more than an estimated half a billion people without access to safe drinking water supplies and over 1 billion without access to adequate sanitation facilities. Poor water quality and bad sanitation are deadly; some 5 million deaths a year are caused by polluted drinking water.
In the developing world, 90% of all wastewater still goes untreated into local rivers and streams. Some 50 countries, with roughly a third of the world’s population, also suffer from medium or high water stress, and 17 of these extract more water annually than is recharged through their natural water cycles. The strain affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.
Main article: Drinking water
About 70% of the fat free mass of the human body is made of water. To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though most experts agree that 8-10 glasses of water (approximately 2 liters) daily is the minimum to maintain proper hydration. For those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which can be fatal. The "fact" that a person should consume eight glasses of water per day cannot be traced back to a scientific source. There are other myths such as the effect of water on weight loss and constipation that have been dispelled.
Original recommendation for water intake in 1945 by the Food and Nutrition Board of the National Research Council read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men. Also noted is that normally, about 20 percent of water intake comes from food, while the rest comes from drinking water and beverages (caffeinated included). Water is lost from the body in urine and feces, through sweating, and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well.
Humans require water that does not contain too many impurities. Common impurities include metal salts and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes. The single largest freshwater resource suitable for drinking is the Lake Baikal in Siberia, which has a very low salt and calcium content and is very clean.