Laurence Lopresti, age 10, of Tempe, Arizona, for his question:
Why doesn't water burn?
This popular question pops into our heads when we learn that water is composed of hydrogen and oxygen. Its two ingredients are gases that burn, yet water is famous for putting out fires. This mysterious situation is one of the miracles of chemistry. It takes us down, down to the invisibly small world of atoms and the extraordinary changes that occur when atoms combine to form molecules.
Chlorine is a yellowish poisonous gas and sodium is a soft poisonous metal. These two elements combine to form ordinary table salt. Hydrogen and oxygen are flammable gases and a mixture of the two is dangerously explosive. When the mixture is ignited with a spark, it shatters a glass container into fast flying fragments and produces a fierce flash of heat. The foolish experiment also produces a drop of water.
The two gases lost an enormous amount of heat energy to combine their separate atoms into liquid molecules. They also lost their separate characters and the water molecules must obey a very different set of rules. In somewhat the same way, atoms of chlorine and sodium take on a different character when they combine in molecules of salt.
The behavior of an atom depends upon its numbers of particles and how they are arranged. In chemical reactions, the outer electrons are rearranged. This rearrangement of electron particles changes the nature of hydrogen and oxygen when they combine into water molecules.
Hydrogen is a restless atom because its lone electron needs another one to complete a shell around its nucleus. Oxygen is restless because it has eight electrons. Two complete an inner shell and two more are needed to complete a second shell. Atoms spend a lot of frantic energy trying to arrange the correct number of electrons in their shells. Neon gas has no such problems because its normal quota of ten electrons complete a first of two and second shell of eight. It is reluctant to burn and refuses to form chemical compounds with other elements.
Hydrogen and oxygen solve their mutual problems by sharing a pair of electrons. Two hydrogen atoms each provide a single electron for the second shell of one oxygen atom. The two restless hydrogen atoms find a home and the frantic oxygen atom takes down its vacancy sign. The three atoms are bonded together in a stable molecule. However, if the temperature soars to 2700 degrees, centigrade, the molecules separate and the ingredients become flammable gases again.
Another dramatic change occurs because the ends of the water molecule bear opposite electrical charges. The hydrogen atoms attached to the oxygen's outer shell bear slight positive charges. The oxygen end bears a slight negative charge. These opposite poles attract each other. A negative pole attracts and clings to the positive pole of another molecule. Four to eight molecules link together in a chain. This structure creates a liquid substance and gives water its wet quality.