Michael Krustiniak, age 10, of Stratford, Connecticut, for his question:
Exactly how does water turn to ice?
The trick, of course, involves temperature and with changing temperatures most liquid substances can become frozen solids. Water, however, has a special trick of its own. Other substances become smaller when they freeze. Water breaks the rule and grows bigger.
Water happens to be one of the world's most plentiful substances. We take it for granted and no one seems astonished that frozen water increases in volume. If water were rare, we might treasure it for this remarkable quality. Iron and other substances shrink when they become frozen solids. Experts delved deeply into the secrets of nature to discover why solid ice occupies more space than an equal amount of liquid water. The puzzling problem helped to reveal the nature of the basic water molecule and the stages by which it passes from the liquid to the solid state of matter.
The basic particle of water is a special molecule called a hydrogen bond. It is a package of three atoms, one of oxygen and two of hydrogen. The package is united by the attraction forces within the charged particles of the different atoms. The nucleus of an atom, as we know, carries a positive electrical charge. This positive charge equals and balances the negative charges of the electrons that orbit the central nucleus. The oxygen atom in the package has a positive nucleus balanced by eight negative electrons. Each of the two hydrogen atoms has but one. electron to balance its positive nucleus.
This basic framework accounts for the structure of the water molecule. The oxygen atom borrows two electrons, one from each of two hydrogen atoms. This ties the hydrogen atoms into the package and the trio of atoms share their total of 10 electrons. The electrons add their negative charge to one end of the molecule, leaving a slight positive charge at the opposite end. The negative end attracts the positive end of the next hydrogen bond and energetic molecules of liquid water string together in chains.
As heat energy is lost, these bonds weaken. Molecules slow down, separate and pack closer together. At 4 degrees centigrade, the water is densest. Then the odd shape of the molecules becomes a factor. As the temperature reaches the 0 degree of freezing point, they must arrange themselves in rigid, crystal formation. But they cannot do so without leaving miniature spaces in the framework. The solid ice is a deli¬cate lattice riddled with tiny spaces. These spaces add bulk and the solid ice expands to occupy about one ninth more volume than the liquid water.
As water chills to its freezing point, it follows the general rule and becomes denser, heavier and smaller. However, this contraction ceases at four degrees above freezing point. The solid ice crystal increases in volume, loses its extra density and becomes lighter than the liquid water. It floats which is a very good thing. Imagine what the world would be like if ice sank to the bottom of our seas and rivers. The warm sun would fail to reach it and melt it. Every winter would add to the choking supply of submerged ice.