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The water molecule

All substances on earth consist of molecules. One of the molecules on earth is the water molecule. Molecules consist of smaller parts, called atoms. The water molecule contains three atoms: one oxygen atom and two hydrogen atoms. Oxygen, the substance we all breath, consists of two oxygen atoms. The chemical symbol for water is H2O, where H2 stands for two hydrogen atoms and the O for one oxygen atom.

All forms of water use the same water molecule. All substances have three forms: solid, liquid and gas. The same goes for water, but we know the three forms as ice, water and steam/vapour. The form of water depends on the temperature and pressure of the water. At room temperature, water is liquid. Below zero degrees Celsius (the freezing point) it changes into ice. Above 100 degrees it boils and evaporates into steam. The difference is caused by the way in which the molecules are attracted to one another. In ice, the molecules are attached to each other in fixed positions. In water, they are only held together loosely, but with steam/vapour, they are completely free. Because the molecules are not attached to each other with water vapour, we are unable to see them.