Dry ice is the solid form of carbon dioxide (CO2), a substance used for cooling and shipping items that require extremely low temperatures. This solid CO2 maintains a temperature of approximately \(-78.5^\circ \text{C}\) or \(-109.3^\circ \text{F}\), making it far colder and more effective for refrigeration than regular water ice. Although many people refer to dry ice as “melting,” its phase change is unique because it bypasses the liquid state entirely.
The Science Behind Phase Change
The common term “melting” does not apply to dry ice because it undergoes sublimation, the direct transition from a solid to a gas. This transition occurs because of the specific pressure and temperature conditions on Earth’s surface.
For carbon dioxide to exist as a liquid, it must be held at a pressure of at least 5.11 atmospheres and a temperature above \(-56.6^\circ \text{C}\). This combination of conditions is known as the triple point, where solid, liquid, and gas phases can coexist. Since atmospheric pressure at sea level is only about one atmosphere, solid CO2 cannot turn into a liquid and instead immediately converts into a gas as it absorbs heat from the environment. The longevity of dry ice is determined by how quickly this heat absorption occurs.
Typical Sublimation Rates
Dry ice sublimates at a rate of roughly five to ten pounds every 24 hours when kept in a standard insulated container, such as a foam cooler. This rate translates to a daily mass loss of about 3% to 8% of the original amount, depending on the container’s thermal efficiency.
A five-pound block of dry ice can provide reliable cooling for up to 24 hours under optimal conditions. A larger 10-pound block often lasts closer to 30 to 36 hours in a similar container, potentially lasting up to 48 hours in a high-quality, thick-walled chest. These figures represent averages, and the actual sublimation time is a continuous process driven by external factors.
Factors Controlling Longevity
The speed at which dry ice sublimes is controlled by factors that determine the rate of heat transfer.
Ambient Temperature
The ambient temperature of the surrounding environment is a direct influence on this process. A warmer environment provides more thermal energy for the solid CO2 to absorb, which accelerates the phase change from solid to gas.
Size and Shape
The physical characteristics of the dry ice, specifically its size and shape, are important. Dry ice in the form of small pellets or nuggets sublimates much faster than large, dense blocks. This difference is due to the surface area to volume ratio; smaller pieces have a proportionally greater amount of surface area exposed to heat, allowing for quicker thermal energy absorption. A single large block minimizes the exposed area, which slows the sublimation process.
Airflow and Ventilation
Airflow around the dry ice influences its longevity. Moving air constantly carries away the cold CO2 gas that forms a temporary insulating layer around the dry ice. This moving air then replaces the cold gas with warmer air, which increases the rate of heat transfer and speeds up sublimation. Stagnant air is more effective at preserving the dry ice than a breezy or well-circulated environment.
Insulation Quality
The quality of the insulation surrounding the dry ice is a major determinant of its lifespan. Containers with thicker walls and a higher R-value, a measure of thermal resistance, minimize the heat flow from the outside. Specialized shipping containers with two inches of urethane insulation, for example, can keep dry ice much longer than a thin-walled foam cooler.
Maximizing Storage Time
To slow the sublimation process and maximize the lifespan of dry ice, users should employ several practical techniques:
- Use a thick-walled insulated cooler or specialized dry ice chest.
- Ensure the container is not completely airtight and has a loosely fitting or vented lid, as CO2 gas must escape to prevent pressure buildup.
- Wrap the dry ice in newspaper or a heavy cloth before placing it in the cooler for an added layer of insulation.
- Fill the empty space within the container with insulating material, such as crumpled newspaper, towels, or foam peanuts, to minimize the air volume that can hold warm air.
- Limit the frequency with which the container is opened to prevent warm, outside air from rushing in.
- Store the container in the coolest available location, such as a shaded spot or an air-conditioned space, and keep it out of direct sunlight.