Christine Green, age 10, Peterborough, Ontario, for her question:

Why doesn't ice sink?

Can you imagine what our world would be like if ice did not float in water? When the surface of a pond froze in the chilly air, the ice would sink to the bottom. The floors of northern rivers, lakes and Arctic seas would be lined with ice. Down there, below the chilly water, the summer sun could, never reach and melt it. Year by year, more and more ice would pile up under the water. The waters of most seas, lakes and rivers would soon be locked in solid ice. Only a little water in their topics, perhaps, would remain liquid.

This situation would be worse than any Ice Age. But it cannot happen because ice floats in water. On the surface of the water ice is exposed to the air. And, in most places, sooner or later the sun is going to smile and turn the ice back again into water. Even the edges of the icy glaciers must melt and become water again.

 Ice floats because it is lighter than water. When water freezes, it swells a little. It takes up a little more room without gaining any more weight. Let's imagine two pots of equal size, One is filled with water, the other with ice. If the water weighs ten pounds, the ice weighs nine pounds plus about an ounce. We say that ice is less dense than water, which means that in equal amounts it weighs less.

Anything floats in water when it is less dense than water. The lesser its density, the higher it floats in the water. Since ice is about nine tenths as dense as water, then only one tenth floats above the water level. Nine tenths of a great iceberg is under water. When a sailor sees a jagged mountain of ice afloat in the shipping lane, he knows that he is looking at only one tenth of the total iceberg.

The reason why ice is lighter than water is in the chemistry of water itself. Water, like everything else, is composed of atoms„ It is, as we all know, composed all know, composed of hydrogen and oxygen atoms, two atoms of hydrogen for every atom of oxygen. These atoms are bound together in a special fashion. Two atoms of oxygen and four of hydrogen form a water molecule by a sharing of atomic particles.

At normal temperatures, countless numbers of these water molecules cling and flow together. The water is liquid. Its molecules move about fast and freely. As the temperature cools, they lose their energy and speed. As they slow down, the molecules begin to pack together.. When the water reaches four degrees centigrade, it is as densely packed as it will get. As it chills further, it expands.

At about 0 degrees centigrade, the water molecules become quite still. Because of their shapes, they fit together, not in solid form, but in a sort of lattice work. The solid ice is actually a fine lattice of atoms and spaces. It is these spaces which make the ice tike up more room than the liquid water: Too small for our eyes to see, in vast quantities they make the ice lighter than water and therefore able to float.