Water is an inorganic chemical compound with the formula H₂O: each molecule contains two hydrogen atoms bonded to one oxygen atom. It occurs naturally as liquid water, solid ice, and gaseous water vapor. Essential to all known life, it is both a medium for biological processes and a major component of Earth’s surface environment. Pure liquid water is nearly colorless, odorless, and tasteless; natural water generally contains dissolved substances or suspended particles. Its molecular properties help explain its importance in organisms, landscapes, and human activities. (openstax.org)
Molecular structure and chemistry
Within a water molecule, hydrogen and oxygen are connected by polar covalent chemical bonds. Oxygen attracts the shared electrons more strongly than hydrogen, giving the oxygen region a partial negative charge and the hydrogen regions partial positive charges. Although the molecule has no overall electrical charge, this uneven distribution makes it polar. Attractions between neighboring molecules produce hydrogen bonds, which continually form and break in liquid water. (openstax.org)
Polarity makes water an effective solvent for many ionic and polar substances. Water molecules surround dissolved ions, helping keep them dispersed. Nonpolar substances, including many oils, dissolve poorly, so the expression “universal solvent” does not mean that water dissolves everything. These selective interactions are fundamental to aqueous chemistry. (openstax.org)
Water can act as either a proton donor or a proton acceptor. A small proportion of its molecules undergo self-ionization:
2H₂O ⇌ H₃O⁺ + OH⁻
At 25°C, pure water contains approximately equal hydronium and hydroxide concentrations of 10⁻⁷ moles per liter, corresponding to a neutral pH of about 7. These concentrations change with temperature, so neutrality does not always mean pH 7. (openstax.org)
Physical properties
At approximately one atmosphere of pressure, pure water freezes near 0°C and boils near 100°C. These temperatures are conditional rather than universal: boiling occurs at lower temperatures under lower atmospheric pressure, such as at high elevations. Water’s density is close to 1,000 kilograms per cubic meter under ordinary conditions, but varies with temperature and dissolved substances. (usgs.gov)
Unlike most liquids, freshwater reaches its greatest density near 4°C and expands as it cools further toward freezing. Ordinary ice is less dense than liquid water and therefore floats. This behavior influences lake circulation and allows an insulating ice cover to form above liquid water, rather than causing ice to accumulate first at the bottom. (usgs.gov)
Water has a high specific heat capacity, meaning that substantial energy is required to raise its temperature. Large bodies of water consequently warm and cool comparatively slowly, moderating nearby climates. Water also has high surface tension, associated with attraction between its molecules. Cohesion between water molecules and adhesion to other materials contribute to capillary movement through narrow spaces. (usgs.gov)
Distribution and circulation on Earth
Earth’s water collectively forms the hydrosphere. A widely used global inventory estimates that oceans contain about 96.5 percent of its total water. Only about 2.5 percent is freshwater; approximately 68.7 percent of that freshwater is stored in ice caps, glaciers, and permanent snow, and about 30.1 percent is groundwater. Lakes and rivers therefore represent only a small fraction of the planet’s water, despite their importance to human supply and aquatic environments. (usgs.gov)
The water cycle continually transfers water among oceans, land, the atmosphere, and underground stores. Evaporation moves liquid water into the atmosphere, while condensation and precipitation return it to the surface. Plants contribute atmospheric moisture through transpiration. Water then moves as surface runoff, infiltrates soil, or flows underground before returning to other reservoirs. Storage and movement occur at very different rates: atmospheric moisture circulates relatively rapidly, whereas some groundwater and glacial stores persist much longer. Human water use and land-use changes also alter these pathways. (usgs.gov)
Biological importance
Water provides the aqueous environment in which much of the chemistry of a cell occurs. Its solvent properties permit dissolved substances to interact and move, while its thermal properties help limit rapid temperature changes. Evaporation requires considerable energy, making evaporative cooling biologically important. Water’s molecular interactions also distinguish substances that readily associate with water from those that tend to avoid it. (openstax.org)
Water is not merely a passive medium: it participates directly in chemical reactions. In oxygen-producing photosynthesis, the light-dependent machinery extracts electrons from water. This process supplies electrons to the photosynthetic system and releases oxygen and protons. The oxygen produced by plants and other oxygenic photosynthetic organisms thus originates from water, rather than from the carbon dioxide incorporated into sugars. (openstax.org)
Human use and water quality
Human uses include domestic supply, irrigation, manufacturing, washing, and industrial cooling. In thermal power generation, water is used in steam-driven generating systems and to remove waste heat. Different uses require different water qualities: water suitable for an industrial process is not necessarily suitable for drinking. (apps.usgs.gov)
Drinking-water quality is assessed through contaminants and treatment performance, not simply through appearance. Conventional treatment may involve coagulation, sedimentation, filtration, and disinfection. Additional technologies target particular dissolved chemicals; activated carbon, for example, can remove various organic compounds and substances responsible for taste and odor. Treatment requirements depend on source-water characteristics and the contaminants present. (archive.epa.gov)
Water quality is a central concern of public health. Drinking-water controls address microorganisms, chemical contaminants, and treatment byproducts. Wastewater treatment serves a different but related function: it processes used water before discharge or reuse, helping limit contamination of receiving waters. Drinking-water production and wastewater treatment therefore form distinct parts of the broader management of water supplies and environmental quality. (epa.gov)