Water (H2O)

Water is the most important chemical compound on Earth. Its chemical formula is H2O: each molecule contains 2 hydrogen atoms joined to 1 oxygen atom. Around 60 per cent of your body is water. Around 70 per cent of the Earths surface is covered in water. Every living thing we know of, from the smallest bacteria to the biggest whales, needs water to survive. So far in the universe, scientists have not found another planet with liquid water on its surface (although they keep searching).

FormulaH2O2 hydrogen + 1 oxygen
Molar mass18 g/molA very light molecule
Freezing point0 CAt normal pressure
Boiling point100 CAt normal pressure
In your bodyAround 60 per cent40 litres in an adult
On Earths surfaceAround 70 per cent97 per cent in oceans

The H2O molecule

A water molecule has a very simple structure: two hydrogen atoms attached to one oxygen atom, with the bonds at an angle of about 104.5 degrees. The shape looks a bit like Mickey Mouses head, with the oxygen as the face and the two hydrogens as the ears.

The oxygen atom pulls the shared electrons closer to itself than the hydrogen atoms do. This makes the oxygen end slightly negative and the hydrogen ends slightly positive. The molecule is electrically polar overall, like a tiny magnet. This polar character is responsible for most of waters unusual properties.

Why water is special

Water behaves in ways that are unusual for such a small, simple molecule:

Excellent solvent: more substances dissolve in water than in any other common liquid. It is sometimes called the "universal solvent". This is why blood, sea water and sap can carry so many useful things around.
High boiling point: similar-sized molecules (like methane) are gases at room temperature. Water is a liquid because its molecules stick to each other through hydrogen bonds.
Ice floats: most substances are denser as solids than as liquids. Water is the opposite: ice is less dense than liquid water, so it floats. This is vital for life. Frozen ponds and lakes form a top layer of ice, insulating the water below and letting fish survive winter.
High heat capacity: water can absorb a lot of heat without warming up much. Oceans take in huge amounts of solar energy and release it slowly, smoothing Earths climate.
High surface tension: water molecules pull on each other strongly at the surface, letting small insects walk on water.

Fact Water is the only common substance on Earth that exists naturally in all three states (solid, liquid, gas) at temperatures we experience: ice, liquid water and water vapour. This three-state behaviour drives the water cycle, evaporating from the oceans, condensing into clouds, and falling as rain or snow.

Where Earths water lives

Oceans: 97.5 per cent of all Earths water is salty sea water.
Ice caps and glaciers: about 1.75 per cent, mostly in Antarctica and Greenland.
Groundwater: about 0.75 per cent, hidden in soil and rock.
Rivers and lakes: a tiny fraction (0.02 per cent), but huge in importance for life.
Atmosphere: just 0.001 per cent, but vital for weather and climate.

Only about 1 per cent of all Earths water is fresh and easy for humans to use. That is why fresh water is precious, especially in places where rivers and groundwater are running low.

Water in your body

An adult human body is about 60 per cent water by mass. Some parts contain even more: the brain is around 75 per cent water, blood is around 92 per cent.

Water plays many roles in your body:

Carries nutrients and waste around in the blood
Cools you down through sweating
Cushions joints with synovial fluid
Protects the brain inside its watery cushion
Lubricates eyes with tears
Helps digest food
Provides the watery environment inside every cell where chemistry happens

You need to drink around 2 litres of water a day to replace what you lose through breathing, sweating and going to the toilet. Without water, a person can only survive about 3 to 5 days.

Did you know? Every water molecule on Earth has been here for billions of years. The same molecules that flow through your tap have been recycled through countless oceans, rivers, plants and animals. A single drink of water you have today may contain molecules that once flowed through a dinosaurs blood, were part of an iceberg in the last ice age, or floated as steam from a volcano. Water is one of the great recyclers of the planet.

Hard water and soft water

"Hard" water contains dissolved minerals (mainly calcium and magnesium) picked up from limestone and chalk rocks. "Soft" water has fewer dissolved minerals.

Hard water is harmless to drink but causes practical problems:

Soap does not lather well in it (it reacts with the minerals to form scum)
It leaves white limescale deposits in kettles and pipes
It can make shower screens look streaky

People in hard-water areas often use water softeners or descalers to remove the minerals.

Pure water versus tap water

Drinkable tap water in the UK is not pure H2O. It contains tiny amounts of dissolved minerals, trace chlorine (added to kill germs) and (in many places) added fluoride for healthy teeth. These are all safe and even helpful.

Pure water (called distilled water) is used in laboratories, batteries, steam irons and aquariums where minerals could cause problems. You make it by boiling water and condensing the steam, leaving the impurities behind.

Try this Compare the behaviour of water with other liquids. Drop a needle (very carefully) on the surface of a still glass of water. The surface tension can support it even though steel is denser than water. Try the same with oil or alcohol: they have much lower surface tension and the needle sinks immediately. Then float an ice cube in water and watch about 9/10 of it sit below the surface. Try the same with cooking oil and the ice cube will float lower (oil is less dense than water).

Deeper dive: are we alone in the universe? Looking for water

Astronomers searching for life beyond Earth almost always start by looking for liquid water. Every form of life we know of uses water as the solvent in which its chemistry happens. Without liquid water, life as we understand it cannot exist.

Around each star, there is a "habitable zone" or Goldilocks zone: not too hot, not too cold, but just the right temperature for water to be liquid on a planets surface. Earth is in our suns habitable zone. Venus is too hot (water boils off). Mars is too cold (water freezes), although it once may have had oceans billions of years ago.

The search has turned up surprises. Europa, a moon of Jupiter, has a deep ocean of liquid water hidden under a crust of ice. Enceladus, a tiny moon of Saturn, shoots jets of water from its south pole, hinting at a salty underground ocean. Mars has frozen polar caps and traces of briny water seeping from cliff faces. Some scientists think any of these places could host simple microbial life.

Outside our solar system, telescopes have found thousands of exoplanets orbiting other stars. A handful are roughly Earth-sized, sitting in their stars habitable zones. Whether any of them have liquid water (or actual life) remains unknown. The next generation of space telescopes is being designed to look at the atmospheres of these planets, hoping to detect water vapour, oxygen and other tell-tale signs.

It may be a few decades before we know for sure, but for now, every water molecule we have ever met has been here on the only known living planet in the universe. That makes water even more remarkable.

For more, see carbon dioxide and the water cycle.