Dry ice is the solid form of carbon dioxide (CO2). It serves as a powerful, residue-free cooling agent due to its extreme temperature of -78.5°C (-109.3°F). You can reuse dry ice, but only the portion that remains solid after its initial use. Unlike water ice, dry ice constantly transforms into a gas, meaning reuse depends entirely on how well the material is preserved and stored between applications.
Understanding Sublimation and Material Loss
The unique nature of dry ice is defined by sublimation, the direct phase transition from a solid state to a gaseous state without ever becoming a liquid. This process occurs at any temperature above -78.5°C at standard atmospheric pressure. Since the ambient temperature is always significantly higher than this value, the dry ice continuously converts to CO2 gas.
This constant loss means that dry ice cannot be “refrozen” like a gel pack. The rate of sublimation can vary widely, but a general estimate is a loss of about 5 to 10 pounds every 24 hours in a typical insulated storage container. Factors like the block’s size and the quality of insulation directly influence the speed of this material loss. Larger blocks with less surface area sublimate slower than pellets.
Practical Steps for Extended Storage
Slowing the natural rate of sublimation requires isolating the material from warmer air. The most effective storage method involves using a thick, insulated container, such as a Styrofoam or specialized plastic cooler. These containers provide a thermal barrier while also allowing the gas to escape safely. Using a standard household freezer is ineffective because they cannot reach the required temperature and are typically airtight.
To further reduce the sublimation rate, minimize the air space inside the storage container. Packing any remaining empty space with crumpled newspaper or other insulating material helps reduce the amount of warm air circulating around the dry ice. The container should be placed in the coolest, darkest area possible, away from direct sunlight or heat sources. Reducing the frequency with which the container is opened also helps maintain the temperature gradient and conserve the remaining solid.
Safety Considerations for Stored Dry Ice
Strict safety protocols are required due to the extreme temperature and the nature of the sublimating gas. Never place dry ice in any tightly sealed container, such as a glass jar, a screw-top cooler, or an airtight freezer. As the dry ice converts to gas, the pressure inside a sealed container builds up rapidly, creating a severe explosion hazard. One pound of solid dry ice can yield approximately 250 liters of CO2 gas, which explains the extreme risk.
The storage area must be well-ventilated to prevent the buildup of carbon dioxide gas. CO2 is heavier than air, so it tends to settle in low-lying or confined spaces, displacing oxygen and posing a risk of asphyxiation. Storing the material in a garage or an open laboratory is preferable to a closet or small, unventilated room. Handling the dry ice also demands caution, requiring insulated cryogenic gloves to prevent direct skin contact and severe frostbite.