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# Dry Ice is the Solid State of Carbon Dioxide

By Ron Kurtus

Frozen carbon dioxide (CO2) is called dry ice. It is extremely cold and has the unusual feature that when dry ice melts under normal conditions, it goes directly from the solid form to the CO2 gas.

Dry ice will melt into a liquid under high pressure. Dry ice is often used for handy refrigeration and for special-effects demonstrations.

Questions you may have include:

• How cold is dry ice?
• Why does it go from a solid to a gas?
• How is it used?

This lesson will answer those questions. Useful tool: Units Conversion

## Temperature

Dry ice is frozen CO2 gas. The freezing temperature at normal sea level air pressure is -78.5 degrees C (-109.3 degrees F).

Although a block of dry ice looks relatively harmless, a person should use caution in handling the material because of its extremely low temperature. It could easily cause frost burns on the skin.

Using safety gloves when handling dry ice blocks

Another hazard is the fact that when dry ice melts, the result is carbon dioxide gas. Since CO2 gas is heavier than air, it settles to the bottom. In some rare cases, it could cause suffocation.

## Solid to gas

At standard pressures and common temperatures, a material goes from a solid to liquid to gas. Water is a good example, where it goes from steam to water to ice, as the temperature drops.

But at higher pressures, it is possible to skip the liquid phase altogether. The process of going directly from a solid to a gas is called sublimation. Going the other direction from a gas to a solid is called deposition.

The illustration below shows the states of CO2 at various pressure and temperature combinations. At 1 atmosphere of pressure--which is the everyday air pressure we experience--carbon dioxide gas will freeze and turn to dry ice at about -78° Celsius. If you put the gas in a compression chamber and increased the air pressure about 5 times the normal pressure or 5 atmospheres, then CO2 will go from gas to liquid to solid. In other words, at say 7 atmospheres, dry ice will melt and become liquid CO2.

Change of state of CO2 at different pressures and temperatures

## Uses

Dry ice is often used to keep food and other items cold in a container, such as an ice box.

Another common use is seen in movies and novelty displays. When pieces of dry ice are dropped in a container of water, they rapidly melt, causing a bubbling effect in the water, as if it was boiling. The CO2 gas bubbles out and looks like smoke. Movies about mad scientists and Halloween displays often use this effect.

When CO2 is at higher pressures, it exists in the liquid state. Liquid carbon dioxide is used in high-pressure fire extinguishers.

## Summary

Dry ice is frozen carbon dioxide. It is extremely cold and will go directly from the solid form to gaseous state under normal conditions. Dry ice will melt into a liquid under high pressure. It is often used for refrigeration and special-effects demonstrations.

Learn how special effects work

## Resources and references

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