A Staff Report from the Straight Dope Science Advisory Board

What exactly is dry ice?

August 19, 2003

Dear Straight Dope:

So what's the deal with dry ice? I know it's used to make a cool fog effect for movies, but fill me in a little more. Who invented it and why, and what exactly is it?

I don't know who "invented" dry ice. But the Dry Ice Corporation trademarked the term "Dry Ice" in 1925. Dry Ice was made available to the general public circa 1930 for the purpose of keeping foods like ice cream cold. My dictionary says "dry ice" is now a generic term, but my search for the Dry Ice Corporation resulted in lots of litigation documents, so I'm sticking with "Dry Ice." In this business you can't be too careful.

Dry Ice is frozen carbon dioxide. It's not totally harmless as we'll see below but not especially exotic, as substances go. The cool thing (heh) about Dry Ice is that it passes from a solid directly to a gas, without going through an intervening liquid phase. The fancy physics word for this process is "sublimation."

You may already know that lowering the atmospheric pressure lowers the boiling point of a material. That's why brownie mix has different baking instructions for people who live at high altitude. Now imagine lowering the pressure so low that the boiling temperature is the same as the freezing temperature. (Truth be told, lowering the pressure also changes the freezing point, but eventually the boiling point and freezing point do meet.) This point is called the "triple point"--it's the place where the solid, liquid, and gaseous forms of a substance can coexist. Now lower the pressure still more. Boom, there you go! As soon as the material "melts" it also boils, i.e., it sublimates. No liquid state is possible.

For carbon dioxide, the triple point occurs at a pressure of 5.14 atmospheres. Therefore at regular atmospheric pressure, there is no liquid phase. For H20, you have to lower the pressure to 0.006 atmospheres before you eliminate the liquid phase. Zinc has a triple point at pressure = 0.05 atm and temperature = 419° C.

"Fog" produced by Dry Ice is actually water vapor, either condensing from the air around the Dry Ice, or from warm water the Dry Ice is immersed in.

Dry Ice has some dangers. It's colder than -79° C. Don't touch it with your bare hands! (Use oven mitts.) Don't eat it! Don't try to chip or break it--shards could hit your eye. Also use care when evaporating large quantities of Dry Ice. CO2 is heavier than air and will pile up near the ground, so if you have any babies or pets crawling around down there, they'll suffocate. Don't put small pieces of Dry Ice into a plastic soda bottle half full of water, then put the cap on the soda bottle and wait several seconds or minutes until it explodes. Especially in Utah, where this activity is a felony.

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