Water

Water is a chemical substance with the formula H₂O, meaning each molecule consists of two hydrogen atoms and one oxygen atom. Under normal conditions on Earth, it exists in liquid form, and when pure it is colorless, odorless, and tasteless.

Water covers a large portion of Earth's surface, found mainly in oceans and seas. All life depends on water: living organisms contain a significant amount of water, and it is the medium in which the chemical processes of life occur.

Physical Properties

Fresh water under normal pressure freezes at approximately 0 degrees Celsius and boils at approximately 100 degrees Celsius. When water freezes, it expands and its density decreases—therefore ice floats on water. Water molecules have a polar structure (electrically asymmetrical), which makes water an excellent solvent in which many substances dissolve.

States of Matter

Water exists in three common states: ice (solid), liquid water, and water vapor (gas). Water vapor is part of the air in Earth's atmosphere and plays an important role in climate and Earth's heat balance. The change between these states drives the water cycle: evaporation, cloud formation, precipitation, and the flow of rivers and groundwater.

Oceans and Fresh Water

Most of Earth's water is saltwater found in oceans. Fresh water comprises a small portion; a large part of it is locked in ice and glaciers or exists as groundwater. Consequently, fresh water available for direct use—rivers, lakes, and wells—is limited.

Human Uses

People use water for drinking, cooking, sanitation, irrigation, livestock, and industry. Agriculture is the largest user of fresh water worldwide. Water management—the construction of reservoirs, wells and pipes, and treatment of wastewater—is an important matter of policy and public health, especially in regions prone to drought.