How does dry ice work?

by | May 17, 2009 08:00 PM ET

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Hi, I'm Marshall Brain with today's question.

How Does Dry Ice Work?

Dry ice is frozen carbon dioxide. A block of dry ice has a surface temperature of minus 109 degrees Fahrenheit, or minus 78 degrees centigrade. Dry ice also has the very nice feature of sublimation. As it breaks down, it turns directly into carbon dioxide gas rather than ever becoming a liquid. The super cold temperature and the sublimation feature make dry ice great for refrigeration. For example, if you want to send something frozen across the country, you can pack it in dry ice. It will be frozen when it reaches its destination, and there won't be any messy liquid left over like you would have with normal ice.

Many people are familiar with liquid nitrogen, which boils at minus 320 degrees Fahrenheit. Liquid nitrogen is fairly messy, and difficult to handle. So why is nitrogen a liquid, while carbon dioxide is a solid? The difference is caused by the solid, liquid, gas features of nitrogen and carbon dioxide. We are all familiar with the solid liquid gas behavior of water. We know that at sea level water freezes at 32 degrees Fahrenheit, and boils at 212 degrees Fahrenheit. Water behaves differently as you change the pressure, however. As you lower the pressure, the boiling point falls. If you lower the pressure enough, water will actually boil at room temperature. If you plot out the solid liquid gas behavior of a substance like water on a graft, showing both temperature and pressure, you create what is called a phase diagram for the substance.

The phase diagram shows the temperatures and pressures at which a substance changes between a solid, a liquid, and a gas. When you look at the phase diagram for carbon dioxide, what you can see is that at normal pressures, carbon dioxide moves straight between a gas and a solid. It is only at much higher pressures that you find liquid carbon dioxide. For example, a high-pressure tank of carbon dioxide, or a carbon dioxide fire extinguisher contains liquid carbon dioxide.

To make dry ice, you start with a high-pressure container full of liquid carbon dioxide. When you release the liquid carbon dioxide from the tank, the expansion of the liquid, and the high-speed evaporation of carbon dioxide cools the remainder of the liquid down to the freezing point, where it turns directly into a solid. If you've ever seen a carbon dioxide fire extinguisher in action, you've seen this carbon dioxide snow form in the nozzle. You compress the carbon dioxide snow to create a block of dry ice.

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