Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)), valued for its extremely cold temperature and the fact that it does not melt into a liquid. While temporary placement in an unsealed, vented plastic container, such as a specialty foam cooler, is common practice, placing dry ice in any sealed plastic container is extremely dangerous. Sealing dry ice can lead to an explosion due to the rapid buildup of gas pressure.
Understanding Dry Ice and Sublimation
Dry ice is solidified carbon dioxide, typically at about \(-78.5^\circ\text{C}\) (\(-109.3^\circ\text{F}\)). Unlike water ice, dry ice undergoes sublimation, transitioning directly from a solid state into a gaseous state without passing through a liquid phase.
The solid carbon dioxide is constantly converting into \(\text{CO}_2\) gas, creating a substantial volume expansion because gas molecules occupy significantly more volume than their solid counterparts. One pound of dry ice sublimates to approximately \(8.3\) cubic feet of carbon dioxide gas at atmospheric pressure.
The constant and significant production of gas dictates all safety protocols for handling and storing dry ice. Even in a good thermal container, dry ice will sublimate by about \(3\%\) to \(8\%\) of its volume per day, depending on the insulation quality and ambient temperature. This ongoing phase change means that any container holding dry ice must allow for the gas to escape safely and continuously.
The Hazard of Pressure Build-Up
The physical danger of placing dry ice in a sealed container stems directly from the massive volume expansion that occurs during sublimation. When solid dry ice turns into \(\text{CO}_2\) gas, the gas becomes trapped if the container is airtight, rapidly increasing the internal pressure.
Common household plastics, such as thin plastic bags, screw-top soda bottles, or rigid food storage containers, are not engineered to withstand this extreme internal force. As the pressure builds, the container will eventually fail, resulting in a sudden and forceful rupture or explosion. This failure can propel plastic shards with enough force to cause serious physical injury, including severe eye damage.
The risk is not limited to thin plastics; even robust containers made of glass or metal can rupture if they are airtight. The sheer volume of gas produced from even a small amount of dry ice is sufficient to create dangerous pressure levels. For this reason, dry ice should never be placed in any container with a screw-top lid or a tight seal that prevents gas from venting.
Safe Handling and Material Suitability
Safe storage requires a container that is both insulated and specifically designed to be non-airtight. Specialized, thick-walled foam coolers, made of polystyrene plastic, are commonly used because they provide excellent insulation while naturally allowing gas to escape through the lid or seams. This design ensures that the sublimating \(\text{CO}_2\) gas can vent to the atmosphere, preventing the buildup of dangerous pressure.
When transporting or storing dry ice, it is acceptable to place it in an unsealed plastic bag inside a vented container. The container must be able to withstand the extremely cold temperature without becoming brittle or fracturing, which eliminates materials like glass. The primary rule for any container, regardless of material, is that it must be vented to allow the constant escape of carbon dioxide gas.
In addition to using a vented container, personal safety precautions are necessary when handling the dry ice itself. Due to its frigid temperature, direct contact with dry ice can cause severe frostbite within seconds, so insulated gloves or tongs must always be used. When transporting dry ice in a vehicle, the container must be placed in a separate area from the passengers, and the vehicle must be well-ventilated to prevent the \(\text{CO}_2\) gas from displacing oxygen.