Freddie Gallihan, age 13, of Vienna, W. Va., for his question:

Why is ice lighter than water?

A lump of coal and a lump of cotton may be the same size. But they are not the same weight. This is a matter of density. Both are composed of tiny particles. But the particles of coal are more densely packed than the particles of cotton. Hence, in bulk, or volume, coal weighs more than cotton.

A lump of ice weighs only nine tenths as much as the same volume of water. This too is a matter of density. Most substances expand when warm and shrink when chilled. The particles are most closely packed when the substance is frozen in the solid state. This is not true for water.

As water chills, it starts out by following the normal pattern. As the temperature falls, the particles become more closely packed, the water becomes more and more dense. This goes on until the water reaches four degrees centigrade ‑ four degrees above the freezing point. Then the process reverses. The chilly water becomes less and less dense until it becomes ice.

The reason for this is in the particles of which water is composed. Each water particle is two oxygen atoms and four hydrogen atoms. It is easy for them to fit tightly together in liquid form. But they cannot fit together tightly to form a dense solid. There are tiny gaps in the structure. This not only makes ice lighter than the same volume of water, it makes it a very fragile mineral. A quantity of ice will weigh the same as it did when it was water. But its volume will be slightly larger ‑it will take up ten per at more space.