Water (H2O)

Water covers about 71 percent of the Earth's surface. Find out more about this colorless and odorless substance found all over Earth.

Water (H2O)
image by Leo Rivas

Water is made up of billions of molecules. Each molecule is made of one oxygen and two hydrogen atoms held together by strong covalent bonds. (H2O is the chemical formula for water.) It is found in three different forms on Earth – gas, solid, and liquid. The form water takes depends on the temperature. Water on our planet flows as liquid in rivers, streams, and oceans; is solid as ice at the North and South Poles; and is gas (vapor) in the atmosphere. Water is also underground and inside plants and animals.

Water is an essential nutrient and all living things need water in some form to survive on Earth. People can survive up to several weeks without food, but only a few days without water. Every system in the body, from cells and tissues, to vital organs requires water to function. It makes up on average 60% of an adult’s body weight, from 31% in bones to 83% in lungs. However, the human body cannot store water. Every day we are constantly losing water though breathing, sweating and through the release of urine and faeces. Ensuring that lost fluids are replenished in a timely manner and that our bodies are correctly hydrated is essential for good health and for the body to operate.