Water is the chemical compound with the formula H₂O and covers most of Earth’s surface.
Each water molecule contains two hydrogen atoms bonded to one oxygen atom. Because the molecule is bent and polar, water can dissolve many ionic and polar substances. This unusual solvent ability helps drive biological chemistry and Earth’s geological cycles.
About 71% of Earth’s surface is covered by water, although most of it is saline ocean water. Only a small fraction is readily available freshwater, with much freshwater stored in glaciers, ice caps, groundwater, lakes, and rivers. Water naturally occurs as solid ice, liquid water, and water vapor.
Water is often confused with hydrogen peroxide, whose formula is H₂O₂ and whose extra oxygen atom gives it different chemical properties. Water also expands when it freezes, making ice less dense than liquid water. That behavior allows ice to float and helps insulate aquatic environments.