The water molecule has this basic geometric structure
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
A drop of water falling towards water in a glass
IUPAC name
Systematic IUPAC name
Other names
  • Hydrogen oxide
  • Hydrogen hydroxide (HH or HOH)
  • Hydroxylic acid
  • Dihydrogen monoxide (DHMO) (parody name[1])
  • Dihydrogen oxide
  • Hydric acid
  • Hydrohydroxic acid
  • Hydroxic acid
  • Hydroxoic acid
  • Hydrol
  • [2] μ-Oxidodihydrogen
  • Neutral liquid
3D model (JSmol)
ECHA InfoCard 100.028.902 Edit this at Wikidata
RTECS number
  • ZC0110000
  • InChI=1S/H2O/h1H2 checkY
  • O
Molar mass 18.01528(33) g/mol
Appearance Almost colorless or white crystalline solid, almost colorless liquid, with a hint of blue, colorless gas[3]
Odor Odorless
  • Liquid:[4]
  • 0.9998396 g/mL at 0 °C
  • 0.999972 g/mL at 3.98 °C (temperature of maximum density)
  • 0.9970474 g/mL at 25 °C
  • 0.961893 g/mL at 95 °C
  • Solid:[5]
  • 0.9167 g/mL at 0 °C
Melting point 0.00 °C (32.00 °F; 273.15 K) [b]
Boiling point 99.98 °C (211.96 °F; 373.13 K)[15][b]
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 at 25 °C[7]
Acidity (pKa) 13.995[8][9][a]
Basicity (pKb) 13.995
Conjugate acid Hydronium H3O+ (pKa = 0)
Conjugate base Hydroxide OH (pKb = 0)
Thermal conductivity 0.6065 W/(m·K)[12]
1.3330 (20 °C)[13]
Viscosity 0.890 mPa·s (0.890 cP)[14]
1.8546 D[16]
75.385 ± 0.05 J/(mol·K)[17]
69.95 ± 0.03 J/(mol·K)[17]
−285.83 ± 0.04 kJ/mol[6][17]
−237.24 kJ/mol[6]
Occupational safety and health (OHS/OSH):
Main hazards
Avalanche (as snow)
Water intoxication
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Flash point Non-flammable
Safety data sheet (SDS) SDS
Related compounds
Other cations
Related solvents
Supplementary data page
Water (data page)
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 ?)

Water is an inorganic compound with the chemical formula H2O. 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[19]). It is vital for all known forms of life, despite not providing food energy, or organic micronutrients. Its chemical formula, H2O, 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°.[20] "Water" is also the name of the liquid state of H2O at standard temperature and pressure.

Because Earth's environment is relatively close to water's triple point, water exists on Earth as a solid, liquid, and gas.[21] 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.

Water covers about 71% of the Earth's surface, with seas and oceans making up most of the water volume (about 96.5%).[22] 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%).[23][24] Water moves continually through the water cycle of evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea.

Water plays an important role in the world economy. Approximately 70% of the freshwater used by humans goes to agriculture.[25] Fishing in salt and fresh water bodies has been, and continues to be, a major source of food for many parts of the world, providing 6.5% of global protein.[26] Much of the long-distance trade of commodities (such as oil, natural gas, and manufactured products) is transported by boats through seas, rivers, lakes, and canals. Large quantities of water, ice, and steam are used for cooling and heating in industry and homes. Water is an excellent solvent for a wide variety of substances, both mineral and organic; as such, it is widely used in industrial processes and in cooking and washing. Water, ice, and snow are also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, diving, ice skating, snowboarding, and skiing.

  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. Retrieved 21 April 2019.
  3. ^ Cite error: The named reference Braun_1993_612 was invoked but never defined (see the help page).
  4. ^ Riddick 1970, Table of Physical Properties, Water 0b. pg 67-8.
  5. ^ Lide 2003, Properties of Ice and Supercooled Water in Section 6.
  6. ^ a b c Anatolievich, Kiper Ruslan. "Properties of substance: water".
  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. 9 August 2015.
  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. (1 May 1995). "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. ^ 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)
  16. ^ Lide 2003, 9—Dipole Moments.
  17. ^ a b c 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)
  18. ^ GHS: PubChem 962
  19. ^ "Water Q&A: Why is water the "universal solvent"?". Water Science School. United States Geological Survey, U.S. Department of the Interior. 20 June 2019. Archived from the original on 6 February 2021. Retrieved 15 January 2021.
  20. ^ "10.2: Hybrid Orbitals in Water". Chemistry LibreTexts. 18 March 2020. Archived from the original on 30 July 2022. Retrieved 11 April 2021.
  21. ^ Butler, John. "The Earth – Introduction – Weathering". University of Houston. Retrieved 30 January 2023. Note that the Earth environment is close to the triple point and that water, steam and ice can all exist at the surface.
  22. ^ "How Much Water is There on Earth?". Water Science School. United States Geological Survey, U.S. Department of the Interior. 13 November 2019. Archived from the original on 9 June 2022. Retrieved 8 June 2022.
  23. ^ Gleick, P.H., ed. (1993). Water in Crisis: A Guide to the World's Freshwater Resources. Oxford University Press. p. 13, Table 2.1 "Water reserves on the earth". Archived from the original on 8 April 2013.
  24. ^ Water Vapor in the Climate System Archived 20 March 2007 at the Wayback Machine, Special Report, [AGU], December 1995 (linked 4/2007). Vital Water Archived 20 February 2008 at the Wayback Machine UNEP.
  25. ^ Baroni, L.; Cenci, L.; Tettamanti, M.; Berati, M. (2007). "Evaluating the environmental impact of various dietary patterns combined with different food production systems". European Journal of Clinical Nutrition. 61 (2): 279–286. doi:10.1038/sj.ejcn.1602522. PMID 17035955.
  26. ^ Troell, Max; Naylor, Rosamond L.; Metian, Marc; Beveridge, Malcolm; Tyedmers, Peter H.; Folke, Carl; Arrow, Kenneth J.; Barrett, Scott; Crépin, Anne-Sophie; Ehrlich, Paul R.; Gren, Åsa (16 September 2014). "Does aquaculture add resilience to the global food system?". Proceedings of the National Academy of Sciences. 111 (37): 13257–13263. Bibcode:2014PNAS..11113257T. doi:10.1073/pnas.1404067111. ISSN 0027-8424. PMC 4169979. PMID 25136111.

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