How Come? Properties of dry ice

What makes dry ice dry? And why does dry ice sometimes smoke? -- Nikki DellaSperanza, a student in Brookville

Pour a carbonated drink into a glass and watch as tiny bubbles spring from the surface into the air. If those bubbles changed into snow, you'd have dry ice.

Soda bubbles are made of carbon dioxide gas, trapped under pressure in a container. When you open the soda, the bubbles rush up and out, escaping into the air. There they blend invisibly into the collection of gases, like nitrogen and oxygen, that make up Earth's atmosphere.

Carbon dioxide is a gas at ordinary temperatures and air pressure. But like other substances, such as water, carbon dioxide can also be a sloshy liquid or a solid block. Water is a liquid at room temperature, turning into a steamy gas when heated to 212 Fahrenheit. When water's temperature drops to 32, it freezes to solid ice.

But no matter how cold it gets outside on Earth, carbon dioxide gas in the air doesn't condense into liquid droplets and form clouds, or freeze into ice on the sidewalk in winter. For carbon dioxide to change from a gas to a liquid or solid, the temperature and pressure must be just right.

At ordinary pressures, water transforms from gas to liquid to solid as the temperature drops. (Imagine steam condensing into liquid droplets in a cup -- and then turning to frost when the cup is put in the freezer.) But as it's cooled to -109.3 degrees at ordinary pressures, carbon dioxide goes straight from gas to frozen solid, skipping the liquid stage entirely. In fact, to turn carbon dioxide into a liquid requires a big squeeze: At room temperature that means a pressure equal to 30 Earth atmospheres.

A frozen block of carbon dioxide is called "dry" ice because, unlike water ice, it doesn't melt to liquid. Instead, a cloud of gas continuously escapes from the shrinking carbon dioxide block, in a process known as sublimation.

(If carbon dioxide gas itself is invisible, how can it be used to make spooky fog effects? When frigid carbon dioxide gas hits the air, it causes water vapor to condense into tiny droplets -- creating instant real fog.)

Since dry ice sublimates as its temperature rises, it can't be kept in an ordinary sealed container. Keep the lid on and CO2 gas will build up inside, eventually causing the container to explode. (Put a chunk of dry ice into a balloon, tie it shut and throw it into a swimming pool, and the balloon will sink and disappear. But as the ice warms and turns to gas, the balloon will swiftly rise to the surface, making a startling explosion.)

Dry ice must be handled with tongs or heavy gloves, since even brief contact can cause severe frostbite. Vapors can be a breathing hazard, so always use in a well-ventilated space.