The details of the chemical nature of liquid water are not well understood; some theories suggest that its unusual behaviour is due to the existence of two liquid states. [54] [81] [82] [83] Taste and odor evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into the air. absorbed from air or water) to form glucose and release oxygen. [107] All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO

One factor in estimating when water appeared on Earth is that water is continually being lost to space. H 2O molecules in the atmosphere are broken up by photolysis, and the resulting free hydrogen atoms can sometimes escape Earth's gravitational pull (see: Atmospheric escape). When the Earth was younger and less massive, water would have been lost to space more easily. Lighter elements like hydrogen and helium are expected to leak from the atmosphere continually, but isotopic ratios of heavier noble gases in the modern atmosphere suggest that even the heavier elements in the early atmosphere were subject to significant losses. [29] In particular, xenon is useful for calculations of water loss over time. Not only is it a noble gas (and therefore is not removed from the atmosphere through chemical reactions with other elements), but comparisons between the abundances of its nine stable isotopes in the modern atmosphere reveal that the Earth lost at least one ocean of water early in its history, between the Hadean and Archean eons. [30] [ clarification needed]

Water: How much should you drink every day?

Water runoff often collects over watersheds flowing into rivers. Through erosion, runoff shapes the environment creating river valleys and deltas which provide rich soil and level ground for the establishment of population centers. A flood occurs when an area of land, usually low-lying, is covered with water which occurs when a river overflows its banks or a storm surge happens. On the other hand, drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation either due to its topography or due to its location in terms of latitude. Many organic substances (such as fats and oils and alkanes) are hydrophobic, that is, insoluble in water. Many inorganic substances are insoluble too, including most metal oxides, sulfides, and silicates. The collective mass of water found on, under, and over the surface of a planet is called the hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1.386billion cubic kilometres (333million cubic miles). [24]

Water is a good polar solvent, dissolving many salts and hydrophilic organic molecules such as sugars and simple alcohols such as ethanol. Water also dissolves many gases, such as oxygen and carbon dioxide—the latter giving the fizz of carbonated beverages, sparkling wines and beers. In addition, many substances in living organisms, such as proteins, DNA and polysaccharides, are dissolved in water. The interactions between water and the subunits of these biomacromolecules shape protein folding, DNA base pairing, and other phenomena crucial to life ( hydrophobic effect). In nature, the color may also be modified from blue to green due to the presence of suspended solids or algae. Liquid water can be split into the elements hydrogen and oxygen by passing an electric current through it—a process called electrolysis. The decomposition requires more energy input than the heat released by the inverse process (285.8 kJ/ mol, or 15.9 MJ/kg). [97] Mechanical properties Pure water has a low electrical conductivity, which increases with the dissolution of a small amount of ionic material such as common salt. Because Earth's environment is relatively close to water's triple point, water exists on Earth as a solid, liquid, and gas. [22] It forms precipitation in the form of rain and aerosols in the form of fog. Clouds consist of suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor.

Watch: Measuring length, weight and volume

In industry, near-infrared spectroscopy is used with aqueous solutions as the greater intensity of the lower overtones of water means that glass cuvettes with short path-length may be employed. To observe the fundamental stretching absorption spectrum of water or of an aqueous solution in the region around 3,500cm −1 (2.85μm) [89] a path length of about 25μm is needed. Also, the cuvette must be both transparent around 3500cm −1 and insoluble in water; calcium fluoride is one material that is in common use for the cuvette windows with aqueous solutions. Aquatic plants, algae, and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water is a diamagnetic material. [59] Though interaction is weak, with superconducting magnets it can attain a notable interaction. [59] Phase transitions Water fit for human consumption is called drinking water or potable water. Water that is not potable may be made potable by filtration or distillation, or by a range of other methods. More than 660 million people do not have access to safe drinking water. [108] [109]