Tuesday, January 30, 2007

What is dry ice, and why does it smoke?

Who's asking: Jen Lechner, Freeport, ME

Dry ice is frozen carbon dioxide. It's made by compressing the gas into a liquid, then freezing it at temperatures below -109.3°F (-78.5°C). It doesn't smoke, it steams; the steam is pure CO2, released into the atmosphere whenever the dry ice is stored at temperatures above -109.3°F.

Because CO2 isn't water, dry ice doesn't "melt" -- it sublimates, passing directly from a solid state into a gas. If stored in a regular ice chest, it sublimates at a rate of 5-10 pounds every 24 hours, and has 2-3 times the cooling power of water-based ice.

DryIceInfo.com advises against storing dry ice in a regular freezer, because it's so cold that it shuts off the machine's thermostat.

Dry ice is popular for school science projects because it's fairly safe, as long as you use common sense when handling it: don't let it touch your bare skin, and make sure your work area is well-ventilated. Inhaling concentrated steam from dry ice can kill you, and anoxia (death from lack of oxygen in the bloodstream) is a nasty way to die.

We don't need any dry ice here in Gardiner; we have plenty of the regular kind. Yesterday's official high temperature in Gardiner was 22°F, but it felt much warmer in the sunshine. Amazing how fast one acclimates to the cold.

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