Water (H₂O)is a fluid compound composed of two hydrogen atoms and one oxygen atom.

Scientists have long known that water is essential to the existence of life on our planet. In fact, the ancient Greeks believed that water, air, fire, and earth, were the four basic elements from which all matter was derived. It was not until the 1600s and 1700s that scientists began to explore the composition of water. Even as late as the 1700s it was generally accepted that solid materials could be made by heating water in a sealed flask. People observed that after several days of heating a solid sediment would appear in the flask, and they assumed that the water in the container had been changed into that sedimentary substance. In 1768 French chemist Antoine-Laurent Lavoisier dispelled this ancient notion in an experiment where he carefully weighed the...