what is dry ice?
dry ice is frozen carbon dioxide. A block of dry ice has a surface temperature of -109.3 degrees Fahrenheit (-78.5 degrees C). dry ice also has the very nice characteristic of sublimation: as it breaks down, it turns directly into carbon dioxide gas instead of a liquid. super cold temperature and sublimation function make dry ice great for cooling. For example, if you want to ship something frozen across the country, you can pack it in dry ice. it will freeze when it reaches its destination and there won’t be any messy liquid left behind like you would with regular ice.
many people are familiar with liquid nitrogen, which boils at -320 degrees f (-196 degrees c). liquid nitrogen is quite complicated and unwieldy. so why is nitrogen a liquid while carbon dioxide is a solid? this difference is due to the solid-liquid-gas characteristics 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 F (0 degrees C) and boils at 212 degrees F (100 degrees C). however, water behaves differently as pressure changes. as you lower the pressure, the boiling point drops. if you lower the pressure enough, the water will boil at room temperature. If you plot the solid-liquid-gas behavior of a substance like water on a graph that shows 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 solid, liquid, and gas.
At normal pressures, carbon dioxide moves directly between the gas and the solid. only at much higher pressures is liquid carbon dioxide found. for example, a carbon dioxide high-pressure tank or a carbon dioxide fire extinguisher contains liquid carbon dioxide.
the temperature of dry ice
To make dry ice, you start with a high-pressure container filled with liquid carbon dioxide. When it releases the liquid carbon dioxide from the tank, the expansion of the liquid and the high-speed evaporation of the carbon dioxide gas cools the rest of the liquid to the freezing point, where it converts directly to a solid. If you’ve ever seen a carbon dioxide fire extinguisher in action, you’ve seen this carbon dioxide snow form on the nozzle. you compress carbon dioxide snow to create a block of dry ice. Dry ice sublimates at temperatures above -109.2°F, so you will need to use it quickly or store it below -109.2°F because, unlike regular ice, it turns to gas rather than of liquid.
how to store dry ice
Dry ice should be stored in a well-insulated container outside, such as a cooler, with the lid loosely closed. the thicker the insulation the better as it will convert back to gas at a slower rate. never store it in a sealed container as the sublimated co2 gas will sink and can be dangerous. if it’s airtight, you can make a dry ice bomb that makes the container explode. You may think the freezer is a great place to keep it cool, but it’s actually too hot. Due to the low temperature of dry ice, it could cause your cooler to shut down.
the multiple uses of dry ice
originally published: April 1, 2000
