Water

Water is a chemical compound with the formula H2O. It consists of two hydrogen atoms covalently bonded to one oxygen atom in a bent geometry. Water is polar and forms hydrogen bonds, which give it many distinctive properties. In pure form it is colorless, odorless, and tasteless. At room temperature it is a liquid, but it also occurs as ice (solid) and water vapor (gas) under different conditions. Water is essential for life and serves as a primary solvent in biological and chemical processes.

Key properties include a density near 1 g/cm3 at 4°C, and the unusual fact that it expands when it freezes, so ice is less dense than liquid water and floats. It has a high heat capacity and a high heat of vaporization, helping regulate temperatures in the environment. Water exhibits surface tension and a high dielectric constant. It can dissociate slightly into hydronium and hydroxide ions (H2O ⇌ H+ + OH−), giving neutral pH around 7 under standard conditions. As a solvent, water dissolves many substances, which underpins its role in chemistry, biology, and daily life.

Occurrence and cycles: On Earth, water is found in oceans, rivers, lakes, groundwater, ice caps, and the atmosphere. Much freshwater is not readily accessible, being locked in glaciers or stored underground. The water cycle—evaporation, condensation, precipitation, and collection—drives the movement and distribution of Earth's water.

Uses and management: Water supports life and is vital for drinking, sanitation, agriculture, and industry. Drinking water often requires treatment to remove contaminants. Water management addresses access, quality, infrastructure, and resilience to drought and climate change, with emphasis on conservation and efficient use.