How does dry ice go from gas to solid without becoming a liquid? asks Deidre Hocevar, a student in Brookville, NY.
Dry ice is frozen carbon dioxide, the gas that make soda pop bubbly. Like other substances, carbon dioxide (CO2) can also be a liquid or a solid. At normal air pressure, water is a liquid at room temperature, turning to a gas (steam) when heated to 212 degrees F and into a solid (ice) when its temperature drops to 32 F. But to at 32 F, carbon dioxide remains a floaty gas. A block of dry ice has a temperature of about -109 F.
Dry ice gets its name from its peculiar behavior: Instead of melting into a puddle on the floor, it disappears into a cloud of gas. The scientific term for this process is “sublimation.”
But carbon dioxide can and does exist in liquid form. To turn carbon dioxide into a liquid requires a big squeeze–at room temperature, a pressure of about 30 atmospheres. Many fire extinguishers contain liquid carbon dioxide, held under just such high pressure. And in 2006, scientists discovered a shallow lake of carbon dioxide hidden under the floor of the East China Sea.
To make dry ice, manufacturers first lower the temperature of CO2 to -71 F. or lower, and compress the carbon dioxide into liquid in a containment vessel. Then they release some of the liquid CO2. The rapid expansion and speedy evaporation of the liquid chills the rest of the carbon dioxide in the container, turning it to snow. (Which is also why carbon dioxide snow forms on the nozzle of a spraying fire extinguisher.)
The CO2 snow is pressed into pellets or blocks of dry ice. Among other uses, the CO2 ice keeps foods solidly frozen during shipping. Carbon dioxide changes from ice to vapor at ordinary air pressure, avoiding the potential mess of a melting block of water ice.
The gas that rises from a block of frozen CO2 is actually invisible. So what’s that white vapor, so handy for spooky or spectacular fog effects? When cold carbon dioxide gas hits the air, it causes water vapor to condense into tiny droplets, creating instant (real) fog.
But frozen carbon dioxide isn’t the only kind of solid that sublimates. In your own cold, dry freezer, some of the water in ice cubes will go from solid to vapor without passing liquid. Which is why left-behind ice cubes shrink as weeks pass. Even moth balls sublimate when heated, turning directly from solid to gas.
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. Dry ice must also be handled with tongs or heavy gloves, since even brief contact can cause severe frostbite. For a list of dry ice safety tips, visit the website www.wrh.noaa.gov/vef/kids/dryice.php. To see dry ice in action, view the video at http://wm.kusa.gannett.edgestreams.net/news/1098140376903-10-18-04-spangler4p.wmv.