Water

Water is an inorganic compound with the chemical formula H₂O. It is clear, has no taste, has no odorless smell, and is nearly colorless. It is a chemical substance and the main part of Earth's streams, lakes, and oceans.

Water is also the fluid inside all known living organisms, where it acts as a solvent, which means it can dissolve other materials. Water is a polar molecule, which means it has a positive side and a negative side. This makes the molecules stick together through strong hydrogen bonding. This helps explain many of water's physical and chemical properties. Water does not give living things food energy, and it is not an organic micronutrient, but all known forms of life need it. Water is often called the "universal solvent" because it is found in all living things, it is chemically stable, it is found all over the world, and it is strongly polar for such a small molecule.

Earth's surface temperature and pressure are close to water's triple point, so water can exist on Earth as a solid, a liquid, and a gas. Water falls as precipitation, such as rain, and it floats in the air as aerosols, such as fog. Clouds are made of tiny droplets of water and ice. Ice is the solid form of water. When crystalline ice is broken into small pieces, it may fall as snow. The gas form of water is steam or water vapor.

Water covers about 71% of Earth's surface. Seas and oceans hold most of it, about 96.5%. Smaller amounts are found as groundwater (1.7%), in the glaciers and ice caps of Antarctica and Greenland (1.7%), and in the air as vapor, clouds, and precipitation (0.001%). Water keeps moving through the water cycle. This cycle includes evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff. Water usually ends up flowing back to the sea.

Water is very important to the world economy. About 70% of the fresh water people use goes to agriculture, which means farming. Fishing in salt and fresh water has long been a major source of food for many parts of the world. It provides 6.5% of the world's protein. Many goods, such as oil, natural gas, and manufactured products, are transported by boats across seas, rivers, lakes, and canals. Water is also important for many sports and fun activities, such as boat racing, surfing, sport fishing, diving, ice skating, snowboarding, and skiing.

Other uses of the name are covered at H2O (disambiguation) and Water (disambiguation).

Etymology

The word water comes from Old English wæter. That word came from Proto-Germanic *watar. This old word also gave rise to similar words in Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, and vatn, and to the Gothic 𐍅𐌰𐍄𐉍 (wato).

The word also comes from the Proto-Indo-European root *wed-, which means both "water" and "wet." Words for water in other languages, such as Greek ύδωρ (ýdor), Russian вода́ (vodá), Irish uisce, and Albanian ujë, are related to it. The Greek word ὕδωρ (hýdōr) is where the English prefix 'hydro-' comes from.

History

Main articles: Origin of water on Earth § History of water on Earth, and Properties of water § History

Main article: Properties of water

See also: Water (data page) and Water model

Water (H₂O) is a polar inorganic compound. At room temperature, it is a tasteless and odorless liquid that is nearly colorless, with a hint of blue. It is the most studied chemical compound. It can dissolve more substances than almost any other liquid, so it is sometimes called the "universal solvent." It is also called the "solvent of life," because water in nature almost always has dissolved substances in it.

Water is the only common substance that can exist as a solid, a liquid, and a gas under normal conditions on Earth. Its three common states of matter are ice (solid), liquid water, and water vapor or steam (gas). Adding or removing heat causes phase transitions, such as freezing, melting, vaporization, condensation, sublimation, and deposition.

Water is unusual because it becomes less dense as it freezes. Between 0 °C and 3.98 °C, it expands as it cools. This helps life in lakes survive winter, because ice floats and insulates the water below. Water is diamagnetic. It boils at 100 °C at normal atmosphere pressure. Water can evaporate at lower temperatures, and frost and snowflakes form on cold surfaces. Melting and boiling points change with pressure. This is why pressure cookers cook food faster, and why water can boil at room temperature in a vacuum. The triple point is where all three phases can exist together. The critical point is where water becomes a supercritical fluid. Ice has many crystal forms, but the normal form on Earth's surface is ice.

Water's molecules are shaped so that they form hydrogen bonds with each other. These bonds are much stronger than the Van der Waals force found in most liquids. This explains why water has high melting and boiling points and high specific heat capacity. It helps water store and move heat around Earth's climate. The bonds also cause surface tension and capillary action, which lets water move up narrow tubes. Vascular plants such as trees use this ability. Water dissolves many salts, sugars, and ethanol, as well as gases such as oxygen and carbon dioxide. The carbon dioxide gives fizz to carbonated drinks. Many substances in living things, such as proteins and DNA, are dissolved in water. Fats, oils, and many metal oxides do not dissolve in water. Pure water has low electrical conductivity, which increases when a small amount of common salt dissolves in it. An electric current can split liquid water into hydrogen and oxygen through electrolysis. Sound travels far in water, a property used by sonar. Water can also react with alkali metals, which release hydrogen and form hydroxides.

On Earth

Main articles: Seawater and Tides

Hydrology is the study of how water moves, where it is found, and how clean it is across the Earth. Other fields study water in different places. Hydrography studies how water is spread out. Hydrogeology studies groundwater. Glaciology studies glaciers. Limnology studies inland waters. Oceanography studies the oceans. Ecohydrology focuses on living things and how water affects them. All the water on, under, and over a planet's surface is called the hydrosphere. Earth has about 1.386 billion cubic kilometres (333 million cubic miles) of water. Liquid water is found in bodies of water such as oceans, lakes, rivers, canals, ponds, and puddles. Most of it is seawater. Water is also in the air as solid, liquid, and vapor. It also flows underground as groundwater in aquifers. Water plays a big part in geology too. Groundwater affects faulting. Water in the mantle helps form volcanoes at subduction zones. Water and ice carry sediment, which is small bits of earth and rock. When the sediment settles, it forms sedimentary rocks. These rocks make up the geologic record of Earth history.

The water cycle, also called the hydrologic cycle, is the constant movement of water between the atmosphere, soil water, surface water, groundwater, and plants. Water moves through this cycle by evaporation, transpiration, and precipitation. Precipitation is mostly rain, snow, and hail, with some fog and dew. Water also moves by runoff, which usually reaches the sea. Runoff collects in watersheds. Over time, it can shape valleys and deltas through erosion. Water resources are natural supplies that people can use, such as drinking water or water for irrigation. Most of the world's freshwater is stored in glaciers and snow. Seawater contains about 3.5% sodium chloride, which is salt. It freezes at a lower temperature than fresh water. Tides are the rising and falling of sea levels, caused by the Moon and Sun. The intertidal zone is an important area that results from ocean tides.

Effects on life

Water has special properties that help life grow. All known forms of life need it. Water is a solvent, which means many of the body's substances dissolve in it. It is also an important part of many metabolic processes. In anabolism, water is removed to build larger molecules. This is called a dehydration reaction. In catabolism, water is used to break bonds and make smaller molecules. This is called hydrolysis. Water also plays a part in photosynthesis and respiration, where plant cells use sunlight to split water's hydrogen from oxygen. Water is neutral, with a pH of 7. Acids have lower values and bases have higher values. Water helps organic compounds react in ways that allow replication.

Aquatic life forms

Further information: Hydrobiology, Marine life, and Aquatic plant

Earth's surface waters are full of life, and the first life forms appeared in water. Most fish live only in water, and marine mammals such as dolphins and whales also live there. Some amphibians spend part of their lives in water and part on land. Plants like kelp and algae grow in water. Plankton is generally the base of the ocean food chain. Fish breathe with gills instead of lungs, though the lungfish has both. Marine mammals such as otters and seals must come up to breathe air. Many small water animals have ways to survive where there is little oxygen. Some have breathing tubes (see insect and mollusc siphons), and others have gills (Carcinus).

Effects on human civilization

Water has shaped where people have lived for a very long time. Many early civilizations grew up along rivers and major waterways. Mesopotamia, one of the so-called cradles of civilization, lay between the Tigris and Euphrates rivers, and the ancient Egyptians depended completely on the Nile. The early Indus Valley civilization grew along the Indus River, and Rome was founded on the banks of the Tiber. Big cities such as Rotterdam, London, Paris, and Buenos Aires grew up near water, which made travel and trade easier. In dry places, such as North Africa and the Middle East, having clean drinking water has been very important to how people have lived.

Water that is safe for people to drink is called drinking water or potable water. Water that is not safe to drink can be cleaned through filtration or distillation, or by other methods. Many people still do not have safe drinking water. Most of the water people use goes to farming. Water is also needed to cool power plants and for many factories. Because water is not spread evenly around the world, and because more people are living on Earth, people are worried about having enough water.

Main article: Drinking water

Distribution in nature

Much of the water in the universe forms as a side effect of star formation. When stars form, strong winds of gas and dust blow outward. When this material hits the gas around it, the gas is squeezed and heated, and water forms quickly in the warm, dense gas. In July 2011, scientists reported a huge cloud of water vapor near a quasar about 12 billion light years from Earth. They said it held "140 trillion times more water than all of Earth's oceans combined." Water has been found in interstellar clouds within the Milky Way. It probably exists in many other galaxies too, because hydrogen and oxygen are among the most common elements in the universe. Water is also found as vapor in places such as the Atmosphere of the Sun, Earth's atmosphere, the Atmosphere of Mars, the Atmosphere of Jupiter, the Atmosphere of Saturn, and the Atmosphere of Neptune.

Further information: List of largest lakes and seas in the Solar System and Extraterrestrial liquid water

Liquid water covers 71% of Earth's surface. It is sometimes present in small amounts on Mars. Scientists believe liquid water exists in the ocean under the surface of Enceladus, and as a layer below the surface of Titan. Water is present as ice on many worlds. On Earth and the Moon, it is mainly found as ice sheets on Earth. It is also found on Callisto, Pluto, comets, and objects in the Kuiper belt and Oort cloud. Mars has water ice under its regolith and at its poles.

Earth's location in the habitable zone of the Solar System matters greatly for life on Earth. Earth's gravity lets it hold an atmosphere. Water vapor and other greenhouse gases help keep its surface temperature fairly steady (greenhouse effect). Smaller planets like Mars have very thin atmospheres or none at all.

Law, politics, and crisis

Water politics is politics that is shaped by water and water resources. Fresh water is valuable all over the world, and it is part of many political disagreements. Water problems can also make people sick and harm nature. Over the last few decades, more people have gained access to safe drinking water in almost every part of the world. Even so, about one billion people still do not have safe water, and more than 2.5 billion do not have adequate sanitation. Some observers have estimated that by 2025, more than half of the world population will face water-based vulnerability. A report from November 2009 suggested that by 2030, some developing regions will need much more water than they have. Since 1990, 1.6 billion people have gained access to a safe water source. In developing countries, more people have access to safe water each year. A 2006 United Nations report said that "there is enough water for everyone," but that good water is kept from people by poor management and corruption. Global efforts to improve aid, such as the Paris Declaration on Aid Effectiveness, have not been taken up by water donors as well as they have in education and health. The authors of the 2007 Comprehensive Assessment of Water Management in Agriculture said that poor governance is one cause of water shortages. Good water governance means knowing which methods work best in a particular place. Mistakes have happened when plans that worked in wealthy countries were used in developing places, as seen with the Mekong river. A review by the International Water Management Institute found that careful cost-benefit analyses and environmental impact assessments were rarely done.

In 2004, the UK charity WaterAid reported that many children become ill or die from water-related diseases that could be prevented. These diseases are often linked to a lack of adequate sanitation. Since 2003, the UN World Water Development Report, produced by the UNESCO World Water Assessment Programme, has given decision-makers tools for sustainable water policies. The 2023 report says that about one in four people lack access to drinking water, and almost half lack safely managed sanitation. People in cities are expected to face water scarcity by 2050. Water scarcity has been described as widespread, due to overconsumption and pollution. Organizations working on water protection include the International Water Association (IWA), WaterAid, Water 1st, and the American Water Resources Association. Water-related agreements include the United Nations Convention to Combat Desertification (UNCCD), the International Convention for the Prevention of Pollution from Ships, the United Nations Convention on the Law of the Sea, and the Ramsar Convention. World Day for Water takes place on 22 March, and World Oceans Day on 8 June. In 2026, the United Nations said that the world had entered an era of "Water bankruptcy," because many large lakes had shrunk since the 1990s and most of the world's people lived in countries where water was not secure. It called for policies that match today's reality rather than past norms.

In culture

See also: Sacred waters

Many religions see water as a way to make things clean and pure. Some faiths use washing rituals (ablution), including Christianity, Hinduism, Islam, Judaism, the Rastafari movement, Shinto, Taoism, and Wicca. In Christianity, holy water is used to blessing people, places, and objects. In Zoroastrianism, water (āb) is respected as the source of life. The Ancient Greek thinker Empedocles saw water as one of the four classical elements. Plato believed that the shape of water is an icosahedron. Some folktales tell of "living water" that can bring the dead back to life. The Fountain of Youth is a similar idea about magical waters.

The Irish writer James Joyce included a "water hymn" in his novel Ulysses (1922). The painter and activist Fredericka Foster organized an art show called The Value of Water at the Cathedral of St. John the Divine in New York City. More than forty artists took part. Since 1983, when an April Fools' Day article appeared in a newspaper in Durand, Michigan, the name 'dihydrogen monoxide' has been used in jokes and pranks. It is a chemical name for water. Hoaxes have also used it. Florida-based rappers have used the word "Water" as a catchphrase.

Main article: Dihydrogen monoxide parody