Jim McClanahan, age 13, of Enid, Oklahoma, for his quest on:    

What makes dry ice steam?

Ordinary ice, of course, is water frozen to its solid state. Dry ice is carbon dioxide has frozen to its solid state. The two are different substances and each obeys its own rules when changing from state to state. At its own boiling point, liquid water turns to gaseous vapor and at its own freezing point the liquid turns to solid ice. Most other substances follow this general rule. But carbon dioxide is an excep¬tion to the rule. True, at a certain temperature carbon dioxide gas must become solid dry ice. But under normal conditions, the dry solid does not melt into a liquid state. At room temperature, it changes directly into gas. Gaseous carbon dioxide is invisible. What makes the "steam" is the condensation of water vapor in the air around the dry ice and the chilly carbon dioxide gas.

The freezing point of carbon dioxide is much lower than that of water and sometimes lumps of dry ice are as cold as minus 80 degrees centigrade. All dry ice is cold enough to freeze the flesh and it should be treated with care. Its special trick of changing from a solid directly into steamy vapor is called "sublimation."