Richard Dudley, age 10, of Toronto, Mt.., Canada, for his question:
Why is ice lighter than water?
Imagine what would happen if ice were heavier than water. Every winter layers of solid ice would sink to the bottom of the seas and rivers. It would stay cool dawn there, and the summer sun would be unable to melt it. Next winter, more ice would sink to the bottom. In time, the oceans and rivers would be frozen solid almost to the top.
The bulk of a drifting iceberg is below the water. When you see the pale Mountain of ice from a ship, it looks like a floating city with sheer cliffs and lofty, Green gray pinnacles. It is massive. But there is always nine times more ice below the surface of the sea. We can say that an ounce of ice is bigger than an ounce of water or that ice is lighter than water. We can also say that water is denser than ice.
Density takes our minds dawn to the miniature world of atoms and molecules. All solids, liquids and gases are made fry these tiny particles of matter. The density of a substanee depends upon how closely its atoms and molecules are packed together. The water molecules in liquid water, in ice and in gaseous vapor are all alike. But the molecules in liquid water are packed c1oser together than the molecules in solid ice.
Each water molecule is a unit of one oxygen atom and two hydrogen atoms a Hydrogen ate in one molecule tends to cling to the oxygen atom in the next molecu1e. This link is lop sided because of the electrical riatllre Of the water mo1ecule. The Links form chains of molecules. The chains are pliable in liquid water, and. The Streaming mo1ecules can pack close together.
As water reaches its freezing point, the streaming molecules slow dovn. When the water becomes ice the particles Of matter are frozen in a stiff latticework. The lop sided molecules cannot fit tightly together in the frozen state, so the solid ice is riddled with tiny holes. Liquid water is denser than solid ice because its Molecules are more crowded., so there are more molecules in a cupful of water than there are in a cupful of ice.
The volume of an object is the amount of space it occupies. Density is the amount of matter packed into a certain volume, and all matter is made from atoms and molecules. The density of ice is .917, or about nine tenths the density of water.
An iceberg floats because it is made of lighter material than the water around it. It is almost, but not quite, as heavy as an equal volume of water. The fact that nine tenths of the ice is underwater tells us that ice is only nine tenths as dense as water. The streaming molecules in liquid water pack c1ose together. In ice., there are spaces throughout the lattice of frozen molecules.