Because of its polarity, a molecule of water in the liquid or solid state can form up to four hydrogen bonds with neighboring molecules. Hydrogen bonds are about ten times as strong as the Van der Waals force that attracts molecules to each other in most liquids. This is the reason why the melting and boiling points of water are much higher than those of other analogous compounds like hydrogen sulfide. They also explain its exceptionally high specific heat capacity (about 4.2 J/g/K), heat of fusion (about 333 J/g), heat of vaporization ( 2257 J/g), and thermal conductivity (between 0.561 and 0.679 W/m/K). These properties make water more effective at moderating Earth's climate, by storing heat and transporting it between the oceans and the atmosphere. The hydrogen bonds of water are around 23 kJ/mol (compared to a covalent O-H bond at 492 kJ/mol). Of this, it is estimated that 90% is attributable to electrostatics, while the remaining 10% is partially covalent. [94]

Earth's surface waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain. The viscosity of water is about 10 −3 Pa· s or 0.01 poise at 20°C (68°F), and the speed of sound in liquid water ranges between 1,400 and 1,540 meters per second (4,600 and 5,100ft/s) depending on temperature. Sound travels long distances in water with little attenuation, especially at low frequencies (roughly 0.03 dB/km for 1 k Hz), a property that is exploited by cetaceans and humans for communication and environment sensing ( sonar). [100] ReactivityWater covers about 71% of the Earth's surface, with seas and oceans making up most of the water volume (about 96.5%). [23] Small portions of water occur as groundwater (1.7%), in the glaciers and the ice caps of Antarctica and Greenland (1.7%), and in the air as vapor, clouds (consisting of ice and liquid water suspended in air), and precipitation (0.001%). [24] [25] Water moves continually through the water cycle of evaporation, transpiration ( evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. Water is a diamagnetic material. [59] Though interaction is weak, with superconducting magnets it can attain a notable interaction. [59] Phase transitions In a water molecule, the hydrogen atoms form a 104.5° angle with the oxygen atom. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. At the other two corners are lone pairs of valence electrons that do not participate in the bonding. In a perfect tetrahedron, the atoms would form a 109.5° angle, but the repulsion between the lone pairs is greater than the repulsion between the hydrogen atoms. [90] [91] The O–H bond length is about 0.096nm. [92] Aquatic plants, algae, and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them.

Poorly soluble in haloalkanes, aliphatic and aromatic hydrocarbons, ethers. [7] Improved solubility in carboxylates, alcohols, ketones, amines. Miscible with methanol, ethanol, propanol, isopropanol, acetone, glycerol, 1,4-dioxane, tetrahydrofuran, sulfolane, acetaldehyde, dimethylformamide, dimethoxyethane, dimethyl sulfoxide, acetonitrile. Partially miscible with diethyl ether, methyl ethyl ketone, dichloromethane, ethyl acetate, bromine. 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. d T d P = T ( v L − v S ) L f {\displaystyle {\frac {dT}{dP}}={\frac {T\left(v_{\text{L}}-v_{\text{S}}\right)}{L_{\text{f}}}}} The refractive index of liquid water (1.333 at 20°C (68°F)) is much higher than that of air (1.0), similar to those of alkanes and ethanol, but lower than those of glycerol (1.473), benzene (1.501), carbon disulfide (1.627), and common types of glass (1.4 to 1.6). The refraction index of ice (1.31) is lower than that of liquid water.

Water is an inorganic compound with the chemical formula H 2O. It is a transparent, tasteless, odorless, [c] and nearly colorless chemical substance, and it is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent [20]). It is vital for all known forms of life, despite not providing food energy or organic micronutrients. Its chemical formula, H 2O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°. [21] "Water" is also the name of the liquid state of H 2O at standard temperature and pressure. evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into the air. 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]

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 water cycle (known scientifically as the hydrologic cycle) is the continuous exchange of water within the hydrosphere, between the atmosphere, soil water, surface water, groundwater, and plants.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. 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 In a vacuum, water will boil at room temperature. [69] Triple and critical points Phase diagram of water (simplified) Water reclamation is the process of converting wastewater (most commonly sewage, also called municipal wastewater) into water that can be reused for other purposes. There are 2.3 billion people who reside in nations with water scarcities, which means that each individual receives less than 1,700 cubic metres (60,000cuft) of water annually. 380billion cubic metres (13 × 10

Health and pollution An environmental science program – a student from Iowa State University sampling water The Clausius-Clapeyron relation also applies to the boiling point, but with the liquid/gas transition the vapor phase has a much lower density than the liquid phase, so the boiling point increases with pressure. [64] Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed 205°C (401°F) in Old Faithful, a geyser in Yellowstone National Park. [65] In hydrothermal vents, the temperature can exceed 400°C (752°F). [66] In a lake or ocean, water at 4°C (39°F) sinks to the bottom, and ice forms on the surface, floating on the liquid water. This ice insulates the water below, preventing it from freezing solid. Without this protection, most aquatic organisms residing in lakes would perish during the winter. [58] Magnetism to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7.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] On a pressure/temperature phase diagram (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the triple point, where all three phases can coexist. The triple point is at a temperature of 273.16K (0.01°C; 32.02°F) and a pressure of 611.657 pascals (0.00604atm; 0.0887psi); [70] it is the lowest pressure at which liquid water can exist. Until 2019, the triple point was used to define the Kelvin temperature scale. [71] [72] At sea level, the boiling point of water is 100°C (212°F). As atmospheric pressure decreases with altitude, the boiling point decreases by 1°C every 274meters. High-altitude cooking takes longer than sea-level cooking. For example, at 1,524 metres (5,000ft), cooking time must be increased by a fourth to achieve the desired result. [67] (Conversely, a pressure cooker can be used to decrease cooking times by raising the boiling temperature. [68]) 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