Anne Egerton, age 10, of Phoenix, Ariz., for her question:

Why does dry ice burn like fire?

Some people use dry ice instead of ordinary ice to keep their picnic drinks cool and refreshing. It does not melt in the summery heat and douse the sandwiches with water, as ordinary ice tends to do. But dry ice must be handled with care because it tends to burn the fingers.

When you touch a hot stove, as everyone knows, you get burned. Some of the small skin cells are destroyed, and if the burn is bad enough the ce11s in the tender tissue bel0w the skin also are destroyed. Your f1esh is cooked like meat, and it takes the body perhaps weeks to repair the damage. Besides, burns are very painful accidents.

When you handle dry ice, your fingers feel as if they have touched a hot stove. The painful sensation feels like a burn. But the cause of the damage to the fingers by dry ice is the very opposite from the hot stove that causes a burn.

Both wounds are caused by extreme temperatures. But a hot stove is a lot hotter than boiling water, and dry ice is a lot colder than freezing water. Water boils at 100 centigrade degrees and freezes at 100 degrees colder, or 0 centigrade degrees. Frozen water, of course, is ice, and if you handle chunks of ice for too long your fingers become stiff and frozen. They even may be damaged by frostbite, which is somewhat like the damage caused by a burn.  The temperature of dry ice is 78.5 centigrade degrees colder than ordinary ice. It is that much colder and that much more dangerous to handle than ordinary water ice. A mere touch of it with your bare hands can give you a frostbite. The wound feels and looks like a burn, but its damage is done by extreme cold instead of extreme heat.

Ordinary ice is frozen water. Dry ice is frozen carbon dioxide gas, which freezes at minus 78.5 degrees centigrade. However, as the temperature of water rises it melts and becomes a liquid    and dry ice does not do this. As its temperature rises, solid dry ice breaks apart into molecules of gas. As it melts it disappears into the air as ordinary carbon dioxide gas.

Carbon dioxide is the waste gas we breathe out from our lungs. It is made of molecules. Each molecule is a small package of one atom of carbon and two atoms of oxygen. The weather is never cold enough to freeze the carbon dioxide in the air, so we do not find carbon dioxide in the solid state in the world of nature. Our dry ice is manufactured by putting the gas under tremendous pressure and allowing it to escape. This action causes the compressed gas to form chilly flakes of solid dry ice.