chemical compound with raw molecular formula H₂O; main constituent of the fluids of most living organisms

Water (H
) is a transparent, tasteless, odourless, and almost always colourless chemical substance and covers about 71% of Earth's surface.

Water (H
Ball-and-stick model of a water molecule
Ball-and-stick model of a water molecule
Space filling model of a water molecule
Space filling model of a water molecule
IUPAC name
water, oxidane
Other names
Hydrogen hydroxide (HH or HOH), hydrogen oxide, dihydrogen monoxide (DHMO) (systematic name[1]), hydrogen monoxide, dihydrogen oxide, hydric acid, hydrohydroxic acid, hydroxic acid, hydrol,[2] μ-oxido dihydrogen, κ1-hydroxyl hydrogen(0)
  • 7732-18-5 checkY
3D model (JSmol)
Beilstein Reference 3587155
ECHA InfoCard 100.028.902
Gmelin Reference 117
RTECS number
  • ZC0110000
  • O
Molar mass 18.01528(33) g/mol
Appearance White crystal-like solid, almost colorless liquid with a hint of blue, colorless gas
Odor None
Density Liquid:[3]
0.9998396 g/mL at 0 °C
0.9970474 g/mL at 25 °C
0.961893 g/mL at 95 °C
0.9167 g/ml at 0 °C
Melting point 0.00 °C (32.00 °F; 273.15 K) [a]
Boiling point 99.98 °C (211.96 °F; 373.13 K) [5][a]
Solubility Poorly soluble in haloalkanes, aliphatic and aromatic hydrocarbons, ethers.[6] 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.
Vapor pressure 3.1690 kilopascals or 0.031276 atm[7]
Acidity (pKa) 13.995[8][9][b]
Basicity (pKb) 13.995
Conjugate acid Hydronium
Conjugate base Hydroxide
Thermal conductivity 0.6065 W/(m·K)[12]
1.3330 (20 °C)[13]
Viscosity 0.890 cP[14]
1.8546 D[15]
Std enthalpy of
−285.83 ± 0.04 kJ/mol[6][16]
Standard molar
69.95 ± 0.03 J/(mol·K)[16]
Specific heat capacity, C 75.385 ± 0.05 J/(mol·K)[16]
Main hazards Drowning
Avalanche (as snow)

Water intoxication
(see also Dihydrogen monoxide parody)

NFPA 704

Flash point Non-flammable
Related compounds
Other cations Hydrogen sulfide
Hydrogen selenide
Hydrogen telluride
Hydrogen polonide
Hydrogen peroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references
Water in three states: liquid (including the clouds, which are aerosols), solid (ice) and gas (water vapour)

No known life can live without it. Water is essential for life.[17] This has to be qualified a bit. There are some forms of life which can survive without it, but cannot reproduce without it. Since reproduction is a central part of life, it is clear that water is essential for an organism to survive and reproduce.

Lakes, oceans, seas, and rivers are made of water. Precipitation is water that falls from clouds in the sky. It may be rain if it is liquid, or it may be snow or ice frozen if it is cold. When water gets below 0 °C (32 °F), it freezes and becomes ice, the frozen kind of water. If water gets very hot (above 100 °C (212 °F), it boils and becomes steam or water vapor.

There is a water cycle.

Physical chemistry of water


Water is a fluid. Water is the only chemical substance on Earth that exists naturally in three states. There are over 40 anomalies (strange things) about water.[18] Unlike most other liquids such as alcohol or oil, when water freezes, it expands by about 9%.[19][20][21] This expansion can cause pipes to break if the water inside them freezes.

Water is a molecule made of two hydrogen atoms and one oxygen atom. Its chemical formula is H2O. Like other liquids, water has a surface tension, so a little water can make drops on a surface, rather than always spreading out to wet the surface.[22]

Things having something to do with water may have "hydro" or "aqua" in their name, such as hydropower or aquarium, from the Greek and Latin names for water. It is also called the "universal solvent", because it dissolves many other compounds.

In small amounts, water appears to have no colour but in large amounts (such as seas or lakes), it has a very light blue color.

Uses of water


Plants and animals (including people) are mostly water inside, and must drink water to live. It gives a medium for chemical reactions to take place, and is the main part of blood. It keeps the body temperature the same by sweating from the skin. Water helps blood carry nutrients from the stomach to all parts of the body to keep the body alive. Water also helps the blood carry oxygen from the lungs to the body. Saliva, which helps animals and people digest food, is mostly water. Water helps make urine. Urine helps remove bad chemicals from the body. The human body is between 60% and 70% water, but this value differs with age; i.e. a foetus is 95% water inside.

Water is the main component of drinks like milk, juice, and wine. Each type of drink also has other things that add flavor or nutrients, things like sugar, fruit, and sometimes alcohol. Water that a person can drink is called "potable water" (or "drinking water"). The water in oceans is salt water, but lakes and rivers usually have unsalted water. Only about 3% of all the water on earth is fresh water. The rest is salt water.[23][24]

Drop of water falling from a faucet.

Many places, including cities and deserts, don't have as much water as people want. They build aqueducts to bring water there.

Though people can survive a few months without food, they can only survive for a day or two without water. A few desert animals can get enough water from their food, but the others must drink. Water has no smell, taste, or color.

Water is also used for recreational purposes, see list of water sports.

Water is used as both the coolant and the neutron moderator in most nuclear reactors. This may be ordinary water (called light water in the nuclear industry) or heavy water.

Water is also used for washing a lot of objects. Goods, services and people are transported to other countries in watercrafts on bodies of water.

Water is used in chemical reactions as a solvent or reactant. Water is also used in fire fighting. Water is also used for cooking.

Dihydrogen monoxide parody


The dihydrogen monoxide parody involves calling water by the unfamiliar chemical name "dihydrogen monoxide" (DHMO) and listing some of its harmful effects in an alarming way. Some examples include talking about how "it causes burning, suffocation and corrosion," when it is actually just talking about hot water, drowning and rust. Sometimes the parody calls for it to be banned and/or labelled as dangerous.

The prank works because it takes advantage of people's misunderstanding. Calling water by an unfamiliar name and making it sound like a harmful chemical can make people think it is dangerous.

"Dihydrogen monoxide" is an alternative chemical name for water, but nobody uses it. The word "dihydrogen" means two hydrogens, and "monoxide" means one oxygen. The chemical formula of water has two hydrogens and one oxygen.

The parody gained most of its popularity in the 1990s, when a 14-year-old named Nathan Zohner collected anti-DHMO petitions for a science project about gullibility. Zohner fooled a lot of people, which has led to his project being used in lessons about critical thinking and the scientific method.

The website DHMO.org is a joke website which lists the harmful effects of water (DHMO), answers questions, and calls for it to be banned, among other things.

Origin of the Earth's water


The weirdness of water


A BBC short item explains that every molecule on Earth has existed for billions of years, and all of them came from elsewhere. Water is alien because it arrived on asteroids and comets. It is the second most common molecule in the universe. It is made of two very light elements. Ice floating on water is also an oddity, caused by the nature of water to expand and drop in density when it freezes. Also, hot water can freeze faster than cold, and both this effect and it's causes are still the source of scientific debate and study today. Molecules of water can move up against the force of gravity (that is due to surface adhesion).[25]

Water in the universe

Band 5 ALMA receiver is an instrument specifically designed to detect water in the universe.[26]

Much of the universe's water is produced as a by-product of star formation.[27] On 22 July 2011, a report described the discovery of a gigantic cloud of water vapor containing "140 trillion times more water than the Earth's oceans combined" around a quasar 12 billion light years from Earth. According to the researchers, the "discovery shows that water has been prevalent in the universe for nearly its entire existence".[28][29]

Water has been detected in interstellar clouds in our galaxy, the Milky Way.[30] Water probably exists in abundance in other galaxies, too. Its components, hydrogen and oxygen, are among the most abundant elements in the universe. Most other planetary systems may have similar ingredients.

Origin of water on Earth: possibilities


We do not know exactly how the Earth came to have so much water. It is everywhere in the Universe, but it is uncommon for a place to have so much. The reasoning is like this: every element (except hydrogen and some helium) has been formed in stars. Therefore, oxygen was originally formed in stars. The formation of water is not a problem: it is exothermic, so forming the molecule from its atoms does not need outside energy. But to explain why the Earth has so much compared to, for example, Mars, is not easy. It is an undecided problem in planetary geology.

For a while, people thought Earth’s water did not come from the planet’s region of the protoplanetary disk. Instead, it was thought that water and other volatiles must have been delivered to Earth from the outer Solar System later in its history. But hydrogen inside the Earth did play some role in the formation of the ocean.[31] The two ideas may each be partly right. Water was delivered to Earth by impacts from icy planetesimals (asteroids) in the outer edges of the asteroid belt.[32] How much is not known.

Water vapor


Water vapor (or water vapour) is the gas form of water. It is found in:

Liquid water


Liquid water is found on Earth. It covers about 71% of the surface of the Earth. Liquid water is sometimes found in small amounts on Mars. Scientists believe that liquid water is in the moons Enceladus, Titan, Europa and Ganymede.[47][48][49][50][51]

Frozen water


The frozen form of water (ice) is found in:

South polar ice cap of Mars during Martian summer in the year 2000


  1. "naming molecular compounds". www.iun.edu. Archived from the original on 24 September 2018. Retrieved 1 October 2018. Sometimes these compounds have generic or common names (e.g., H2O is "water") and they also have systematic names (e.g., H2O, dihydrogen monoxide).
  2. "Definition of Hydrol". Merriam-Webster.
  3. Riddick 1970, Table of Physical Properties, Water 0b. pg 67-8.
  4. Lide 2003, Properties of Ice and Supercooled Water in Section 6.
  5. Water in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD), http://webbook.nist.gov (retrieved 2016-5-27)
  6. 6.0 6.1 Anatolievich, Kiper Ruslan. "Properties of substance: water". Archived from the original on 2014-06-02. Retrieved 2021-02-07.
  7. Lide 2003, Vapor Pressure of Water From 0 to 370° C in Sec. 6.
  8. Lide 2003, Chapter 8: Dissociation Constants of Inorganic Acids and Bases.
  9. Weingärtner et al. 2016, p. 13.
  10. "What is the pKa of Water". University of California, Davis. 2015-08-09.
  11. Silverstein, Todd P.; Heller, Stephen T. (17 April 2017). "pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?". Journal of Chemical Education. 94 (6): 690–695. Bibcode:2017JChEd..94..690S. doi:10.1021/acs.jchemed.6b00623.
  12. Ramires, Maria L. V.; Castro, Carlos A. Nieto de; Nagasaka, Yuchi; Nagashima, Akira; Assael, Marc J.; Wakeham, William A. (1995-05-01). "Standard Reference Data for the Thermal Conductivity of Water". Journal of Physical and Chemical Reference Data. 24 (3): 1377–1381. Bibcode:1995JPCRD..24.1377R. doi:10.1063/1.555963. ISSN 0047-2689.
  13. Lide 2003, 8—Concentrative Properties of Aqueous Solutions: Density, Refractive Index, Freezing Point Depression, and Viscosity.
  14. Lide 2003, 6.186.
  15. Lide 2003, 9—Dipole Moments.
  16. 16.0 16.1 16.2 Water in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD), http://webbook.nist.gov (retrieved 2014-06-01)
  17. "United Nations". Un.org. 2005-03-22. Retrieved 2010-07-25.
  18. "Forty-one anomalies of water « Fairy LoRe". fathersergio.wordpress.com. 2011. Retrieved September 1, 2011.
  19. "8(a) Physical Properties of Water". physicalgeography.net. 2011. Retrieved August 31, 2011. pan
  20. "Understanding the processes of erosion". mountainnature.com. 2009. Archived from the original on August 28, 2011. Retrieved August 31, 2011.
  21. "iapws.org". 2000. Archived from the original on August 8, 2011. Retrieved August 31, 2011.
  22. "Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force". Boundless. Archived from the original on June 3, 2016. Retrieved December 25, 2016.
  23. "Percentage of water". Archived from the original on 2013-12-14. Retrieved 2008-12-11.
  24. "Fresh water percentage (2)". Archived from the original on 2007-07-15. Retrieved 2008-12-11.
  25. Jha, Alok 2019. Why water is one of the weirdest things in the universe. BBC News Ideas. [1]
  26. "ALMA greatly improves capacity to search for water in universe". Archived from the original on 23 July 2015. Retrieved 20 July 2015.
  27. Melnick, Gary, Harvard-Smithsonian Center for Astrophysics and Neufeld, David, Johns Hopkins University quoted in: "Discover of water vapor near Orion nebula suggests possible origin of H20 in Solar System". The Harvard University Gazette. 1998. Archived from the original on 16 January 2000. "Space cloud holds enough water to fill Earth's oceans 1 million times". Headlines@Hopkins, JHU. 9 April 1998. Archived from the original on 9 November 2007. Retrieved 21 April 2007.
  28. Clavin, Whitney; Buis, Alan (22 July 2011). "Astronomers find largest, most distant reservoir of water". NASA. Archived from the original on 24 July 2011. Retrieved 25 July 2011.
  29. Staff (22 July 2011). "Astronomers find largest, oldest mass of water in Universe". Space.com. Archived from the original on 29 October 2011. Retrieved 23 July 2011.
  30. Bova, Ben (13 October 2009). Faint Echoes, Distant Stars: the science and politics of finding life beyond Earth. Zondervan. ISBN 978-0-06-185448-4.
  31. Monday, Nola Taylor Redd | Published; April 1; 2019. "Where did Earths water come from". Astronomy.com. Retrieved 2020-07-16. {{cite web}}: |last3= has numeric name (help)CS1 maint: numeric names: authors list (link)
  32. Pepin, Robert O. 1991. On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus 92 (1): 2–79. [2]
  33. Solanki, S. K.; Livingston, W.; Ayres, T. (1994-01-07). "New light on the heart of darkness of the Solar chromosphere". Science. 263 (5143): 64–66. Bibcode:1994Sci...263...64S. doi:10.1126/science.263.5143.64. ISSN 0036-8075. PMID 17748350. S2CID 27696504.
  34. "MESSENGER Scientists 'astonished' to find water in Mercury's thin atmosphere - Planetary News | The Planetary Society". 2010-04-06. Archived from the original on 2010-04-06. Retrieved 2020-09-11.
  35. the SPICAV/SOIR team; Bertaux, Jean-Loup; Vandaele, Ann-Carine; Korablev, Oleg; Villard, E.; Fedorova, A.; Fussen, D.; Quémerais, E.; Belyaev, D. (November 2007). "A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO". Nature. 450 (7170): 646–649. Bibcode:2007Natur.450..646B. doi:10.1038/nature05974. ISSN 0028-0836. PMID 18046397. S2CID 4421875.
  36. Sridharan, R.; Ahmed, S.M.; Pratim Das, Tirtha; Sreelatha, P.; Pradeepkumar, P.; Naik, Neha; Supriya, Gogulapati (2010). "'Direct' evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I". Planetary and Space Science. 58 (6): 947–950. Bibcode:2010P&SS...58..947S. doi:10.1016/j.pss.2010.02.013.
  37. Rapp, Donald, 1934- (2013). Use of extraterrestrial resources for human space missions to Moon or Mars. Berlin: Springer. ISBN 978-3-642-32762-9. OCLC 819571505.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  38. Küppers, Michael; O’Rourke, Laurence; Bockelée-Morvan, Dominique; Zakharov, Vladimir; Lee, Seungwon; von Allmen, Paul; Carry, Benoît; Teyssier, David; Marston, Anthony (January 2014). "Localized sources of water vapour on the dwarf planet (1) Ceres". Nature. 505 (7484): 525–527. Bibcode:2014Natur.505..525K. doi:10.1038/nature12918. ISSN 0028-0836. PMID 24451541. S2CID 4448395.
  39. Atreya, Sushil K.; Wong, Ah-San (January 2005). "Coupled clouds and chemistry of the giant planets— a case for multiprobes". Space Science Reviews. 116 (1–2): 121–136. Bibcode:2005SSRv..116..121A. doi:10.1007/s11214-005-1951-5. hdl:2027.42/43766. ISSN 0038-6308. S2CID 31037195.
  40. "Hubble sees evidence of water vapor at Jupiter moon". NASA/JPL. Archived from the original on 2020-11-07. Retrieved 2020-09-11.
  41. Hansen, C. J. (2006-03-10). "Enceladus' Water Vapor Plume". Science. 311 (5766): 1422–1425. Bibcode:2006Sci...311.1422H. doi:10.1126/science.1121254. ISSN 0036-8075. PMID 16527971. S2CID 2954801.
  42. Hubbard, W. B. (1997-02-28). "Neptune's Deep Chemistry". Science. 275 (5304): 1279–1280. doi:10.1126/science.275.5304.1279. PMID 9064785. S2CID 36248590.
  43. Lockwood, Alexandra C.; Johnson, John A.; Bender, Chad F.; Carr, John S.; Barman, Travis; Richert, Alexander J. W.; Blake, Geoffrey A. (2014-02-24). "Near-IR direct detection of water vapor in Tau Bootis b". The Astrophysical Journal. 783 (2): L29. arXiv:1402.0846. Bibcode:2014ApJ...783L..29L. doi:10.1088/2041-8205/783/2/L29. ISSN 2041-8205. S2CID 8463125.
  44. "NASA telescopes find clear skies and water vapor on exoplanet". NASA/JPL. Archived from the original on 2020-12-07. Retrieved 2020-09-11.
  45. 45.0 45.1 Hanslmeier, Arnold. (2011). Water in the universe. Dordrecht: Springer. ISBN 978-90-481-9984-6. OCLC 670074794.
  46. Garner, Rob (2015-05-06). "Hubble Traces Subtle Signals of Water on Hazy Worlds". NASA. Retrieved 2020-09-11.
  47. Anderson, Gina (2015-09-28). "NASA Confirms Evidence That Liquid Water Flows on Today's Mars". NASA. Retrieved 2020-09-12.
  48. "NASA Space Assets Detect Ocean inside Saturn Moon". NASA/JPL. Retrieved 2020-09-12.
  49. Iess, L.; Stevenson, D. J.; Parisi, M.; Hemingway, D.; Jacobson, R. A.; Lunine, J. I.; Nimmo, F.; Armstrong, J. W.; Asmar, S. W. (2014-04-04). "The Gravity Field and Interior Structure of Enceladus". Science. 344 (6179): 78–80. Bibcode:2014Sci...344...78I. doi:10.1126/science.1250551. ISSN 0036-8075. PMID 24700854. S2CID 28990283.
  50. Dunaeva, A. N.; Kronrod, V. A.; Kuskov, O. L. (2016). "Physico-chemical models of the internal structure of partially differentiated Titan". Geochemistry International. 54 (1): 27–47. doi:10.1134/s0016702916010043. ISSN 0016-7029. S2CID 130371184.
  51. "Possibility of Life on Europa". 2007-06-09. Archived from the original on 2007-06-09. Retrieved 2020-09-12.
  52. Bibring, Jean-Pierre; Langevin, Yves; Poulet, François; Gendrin, Aline; Gondet, Brigitte; Berthé, Michel; Soufflot, Alain; Drossart, Pierre; Combes, Michel (April 2004). "Perennial water ice identified in the south polar cap of Mars". Nature. 428 (6983): 627–630. Bibcode:2004Natur.428..627B. doi:10.1038/nature02461. ISSN 0028-0836. PMID 15024393. S2CID 4373206.
  53. SPIEGEL, DER (9 July 2008). "Versteckt in Glasperlen: Auf dem Mond gibt es Wasser - DER SPIEGEL - Wissenschaft". Der Spiegel (in German). Retrieved 2020-09-12.
  54. McCord, Thomas B. (2005). "Ceres: Evolution and current state". Journal of Geophysical Research. 110 (E5): E05009. Bibcode:2005JGRE..110.5009M. doi:10.1029/2004JE002244. ISSN 0148-0227. S2CID 129787104.
  55. Thomas, P. C.; Parker, J. Wm.; McFadden, L. A.; Russell, C. T.; Stern, S. A.; Sykes, M. V.; Young, E. F. (2005). "Differentiation of the asteroid Ceres as revealed by its shape". Nature. 437 (7056): 224–226. Bibcode:2005Natur.437..224T. doi:10.1038/nature03938. ISSN 0028-0836. PMID 16148926. S2CID 17758979.
  56. September 2005, Bjorn Carey 07 (7 September 2005). "Largest asteroid might contain more fresh water than Earth". Space.com. Retrieved 2020-09-12.{{cite web}}: CS1 maint: numeric names: authors list (link)
  57. Chang, Kenneth (2015-03-12). "Suddenly, it seems, water is everywhere in Solar System". The New York Times. ISSN 0362-4331. Retrieved 2020-09-12.
  58. Kuskov, O.L.; Kronrod, V.A. (2005). "Internal structure of Europa and Callisto". Icarus. 177 (2): 550–569. Bibcode:2005Icar..177..550K. doi:10.1016/j.icarus.2005.04.014.
  59. Showman, A. P. (1999-10-01). "The Galilean Satellites". Science. 286 (5437): 77–84. doi:10.1126/science.286.5437.77. PMID 10506564.
  60. 60.0 60.1 The solar system : exploring the planets and their moons from Mercury to Pluto and beyond. Sparrow, Giles, 1970-. San Diego, Calif.: Thunder Bay Press. 2006. ISBN 1-59223-579-4. OCLC 70170307.{{cite book}}: CS1 maint: others (link)
  61. Tobie, Gabriel; Grasset, Olivier; Lunine, Jonathan I.; Mocquet, Antoine; Sotin, Christophe (2005). "Titan's internal structure inferred from a coupled thermal-orbital model". Icarus. 175 (2): 496–502. Bibcode:2005Icar..175..496T. doi:10.1016/j.icarus.2004.12.007.
  62. Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (2007-02-09). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. ISSN 0036-8075. PMID 17289992. S2CID 21932253.
  63. Gibb, E.L.; Mumma, M.J.; Dello Russo, N.; DiSanti, M.A.; Magee-Sauer, K. (2003). "Methane in Oort cloud comets". Icarus. 165 (2): 391–406. Bibcode:2003Icar..165..391G. doi:10.1016/S0019-1035(03)00201-X.
  1. 1.0 1.1 Vienna Standard Mean Ocean Water (VSMOW), used for calibration, melts at 273.1500089(10) K (0.000089(10) °C, and boils at 373.1339 K (99.9839 °C). Other isotopic compositions melt or boil at slightly different temperatures.
  2. A commonly quoted value of 15.7 used mainly in organic chemistry for the pKa of water is incorrect.[10][11]

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