Notes

Water is the most common liquid on Earth. It covers about 71.4% of the Earth.[1] Pure water has no smell, taste, or color. Lakes, oceans, and rivers are made of water. Precipitation is water that falls from clouds in the sky. It may be rain (liquid) if warm, or it may be frozen if cold. If water gets very cold (below 0 degrees Celsius), it freezes and becomes ice. If water gets very hot (above 100 degrees Celsius), it boils and becomes steam. Water is very important for life.[2] However, some studies suggest that by 2025 more than half the people around the world will not have enough water.[3]

Water is a fluid. Water is the only chemical substance on earth that exists naturally in three states. People know of over 40 anomalies about water.[4][5] Unlike most other liquids such as alcohol or oil, when water freezes, it expands by about 9%.[6][7][8] This expansion can cause pipes to break if the water inside them freezes.


Drops of water flowing from a tap
Water is a molecule made of 2 hydrogen atoms and 1 oxygen atom. Its chemical formula is H2O. Water has a surface tension, so a little water can make drops on a surface, rather than always spreading out to wet the surface. Water can also be called 'aqua', which is the Roman word for water.