Dry ice is solid carbon dioxide (CO2), and it constantly releases CO2 gas into the surrounding environment. This substance is a powerful cooling agent because it is constantly changing state through a natural physical process that bypasses the liquid phase. The continuous release of gas gives dry ice its utility but also creates the need for specific safety considerations.
The Composition and Properties of Dry Ice
Dry ice is the solid form of carbon dioxide. It is manufactured by compressing and cooling CO2 gas until it liquefies, and then allowing the liquid to expand, which causes it to freeze into a snow-like solid. This solid is then compressed further into the blocks or pellets commonly used for commercial purposes. Dry ice maintains an extremely low temperature of approximately -109.3°F (-78.5°C) at standard atmospheric pressure.
This exceptionally low temperature makes it a powerful refrigerant. Unlike water ice, dry ice does not melt into a liquid as it warms up, which is a major advantage in transportation and temporary storage. The absence of liquid residue makes it ideal for preserving perishable goods, biological samples, or creating special effects without the mess of water. It is also colorless, odorless, non-flammable, and chemically stable.
Understanding Sublimation
The process by which dry ice transforms directly into carbon dioxide gas is known as sublimation. This direct transition happens because, at standard atmospheric pressure, CO2 cannot exist as a liquid.
As the solid CO2 absorbs heat from the surrounding environment, its molecules gain enough energy to break free from the solid structure and escape as a gas. This physical change involves a massive increase in volume, which is the scientific detail that underpins the safety concerns. For example, one pound of solid dry ice expands into approximately 850 times its volume as a gas. This significant volume expansion is why even a small amount of dry ice can produce a very large volume of carbon dioxide gas.
Safety and Ventilation Requirements
The large volume of CO2 gas released during sublimation poses the primary safety hazard, not because the gas is inherently toxic, but because it is an asphyxiant. Carbon dioxide gas is denser than the air we breathe, displacing the oxygen necessary for life. If the concentration of CO2 in the air exceeds about 0.5%, it can begin to cause symptoms, and higher concentrations can lead to unconsciousness and even death.
Because of this risk, dry ice must always be used and stored in a well-ventilated area to allow the released gas to disperse safely. Never store dry ice in a sealed or airtight container, because the continuous sublimation will cause a dangerous buildup of pressure. This pressure accumulation can cause the container to rupture or explode.
When transporting dry ice in a vehicle, limit the quantity and ensure there is a constant supply of fresh air by keeping windows open or using the vehicle’s fresh air intake setting. Beyond the asphyxiation risk, the extreme cold of -109.3°F also requires careful physical handling. Direct contact with the skin can cause severe cold burns or frostbite in a matter of seconds, so insulated gloves or tongs should always be used when manipulating the solid material.