Water (H
Ball-and-stick 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)
3D model (JSmol)
Beilstein Reference 3587155
ECHA InfoCard 100.028.902
Gmelin Reference 117
PubChem {{{value}}}
RTECS number
  • ZC0110000
SMILES {{{value}}}
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

NFPA 704.svg

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, which is invisible)

Water (H
) is a transparent, tasteless, odorless, and almost colorless chemical substance and covers over 70% of Earth's surface. No known life can live without it.

Lakes, oceans, seas, and rivers are made of water. Precipitation is water that falls from clouds in the sky. It may be rain (liquid) if it is warm, or it may be frozen if it is cold. If water gets very cold (below 0 °C (32 °F)), it freezes and becomes ice, the frozen variant of water. If water gets very hot (above 100 °C (212 °F)), it boils and becomes steam or water vapor. It is moved around in the water cycle.[17] Water is essential for life.[18]

Some studies suggest that by 2025 more than half the people around the world will not have enough fresh water.[19]

Physical chemistry of water[change | change source]

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.[20] Unlike most other liquids such as alcohol or oil, when water freezes, it expands by about 9%.[21][22][23] 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.[24]

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[change | change source]

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.[25][26]

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[change | change source]

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[change | change source]

The weirdness of water[change | change source]

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. Why is it not a gas? It is made of two very light elements. Ice floating on water is also an oddity. Also, hot water freezes faster than cold, and no-one knows why this is. Molecules of water can move up against the force of gravity (that is due to surface adhesion).[27]

Water in the universe[change | change source]

Band 5 ALMA receiver is an instrument specifically designed to detect water in the universe.[28]

Much of the universe's water is produced as a by-product of star formation.[29] On 22 July 2011, a report described the discovery of a gigantic cloud of water vapor containing "140 trillion times more water than all of Earth's oceans combined" around a quasar located 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".[30][31]

Water has been detected in interstellar clouds in our galaxy, the Milky Way.[32] 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 are likely to have similar ingredients.

Origin of water on Earth: hypotheses[change | change source]

We do not know exactly how the Earth came to have so much water. It is everywhere in the Universe, but rarely in such quantity. Reasoning from first principles, every element except hydrogen 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 how the Earth has so much in comparison with, for example, Mars, is not easy. It is an undecided problem in planetary geology.

It was long thought that 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.[33] 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.[34] How much is not known.

Water vapor[change | change source]

Water vapor is found in:

Liquid water[change | change source]

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 Enceladus, Titan, Europa and Ganymede.[49][50][51][52][53]

Water ice[change | change source]

Water ice is found in:

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

Related pages[change | change source]

References[change | change source]

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  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]

Other websites[change | change source]