Dianne Bockus, age 11, of North Burnaby, B; C. for her question:

What exactly is dry ice?

Dry ice looks like flakes and crystals of frozen snow but do not try to make it into a snowball. In fact, do not handle it directly or let it touch your bare skin. For dry ice is so cold that it can burn. It is almost twice as cold as ordinary ice, which is frozen water, and the so called burn it causes is actually frost bite. This colder than ¬cold dry ice is really frozen carbon dioxide, the waste gas we breathe out.

At ordinary temperatures, carbon dioxide is a gas. There are traces of it in the atmosphere and the green plants use it to make their food. To make dry ice, however, we need carbon dioxide gas in large quantities. It must be made from the elements carbon and oxygen. It is formed when coal, which is composed largely of carbon, is burned with the oxygen in the air. Carbon dioxide also forms as a chemical reaction between lime and certain acids.

The temperature of the gas is lowered until the gas becomes a liquid and then lowered again until it becomes a solid. We cannot do this by simply putting the gas into a refrigerator, because it needs a much lower temperature. Step one is done by squeezing the gas under terrific pressure. When carbon dioxide at room temperature is compressed under 75 times the weight of the atmosphere, it becomes a liquid.

In liquid form it is stored inside heavy metal containers. Liquid carbon dioxide may be used as a fire extinguisher. When sprayed into the flames, it cuts off the supply of oxygen, without which the fire cannot burn. Carbon dioxide for making dry ice is compressed and stored in liquid form.

In step two of the process, some of the cold liquid is allowed to evaporate, A little of the pressure is taken off the heavy container. This is rather like taking the cap off a bottle of pop   only more go, The chilly liquid is minus 57 degrees centigrade   which is 57 centigrade degrees colder than frozen water. As the pressure on its prison is released, the chilly liquid boils, expands and cools itself. It.becomes so cold that it freezes solid. In no time at all it reaches a temperature of minus 80 degrees centigrade and issues forth from its tank in fine flakes of frozen snow.

This flying snow is captured by machines and pressed into solid little bricks. This is the dry ice we buy to keep the picnic ice cream cold. Since dry ice is colder than ordinary ice, it lasts longer. What's more, when it disappears it does not leave a watery puddle. Instead of melting, dry ice evaporates into gaseous carbon dioxide and joins the gases in the air.