How Does Dry Ice Work?

What is Dry Ice?

Dry ice isfrozen 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 feature ofsublimation—as it breaks down, it turns directly into carbon dioxide gas rather than 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 will be no messy liquid left over like you would have with normal ice.

Many people are familiar with liquid nitrogen, which boils at -320 degrees F (-196 degrees C). Liquid nitrogen is fairly messy and difficult to handle. So why is nitrogen a liquid while carbon dioxide is a solid? This 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 atsea level, water freezes at 32 degrees F (0 degrees C) and boils at 212 degrees F (100 degrees C). 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 boil at room temperature. If you plot out the solid-liquid-gas behavior of a substance like water on a graph showing both temperature and pressure, you create what's called aphase diagram的物质。相图显示了temperatures and pressures at which a substance changes between solid, liquid and gas.

At normal pressures, carbon dioxide moves straight between gas and 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-dioxidefire extinguishercontains liquid carbon dioxide.

The Temperature of Dry Ice

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 gas cools the remainder of the liquid down to the freezing point, where it turns directly into a solid.

If you have ever seen a carbon dioxide fire extinguisher in action, you have seen this carbon dioxide snow form in the nozzle. You compress the carbon dioxide snow to create a block of dry ice. Dry icesublimates at temperatures higher than −109.2 °Fso you will need to use it quick or store it at temperatures lower than -109.2 °F as unlike regular ice it turns into a gas rather than a liquid.

How to Store Dry Ice

Dry ice should be stored in a well-insulated container outside, like a cooler, with the lid only slightly closed. The thicker the insulation the better, as it will turn back into a gas at a slower rate. Never store it in an sealed container as the sublimated CO2 gas will sink and can be dangerous. If airtight it can make a dry ice bomb causing the container to explode. You might think the freezer is a great place to keep it cool but it actually too warm. Due to the low temperature of dry ice it could cause your fridge to turn off.

The Many Uses of Dry Ice