Dry ice, the solid form of carbon dioxide, is a cooling agent prized for its extreme temperature and the fact that it leaves no messy liquid residue. Unlike traditional water ice, which melts into a puddle, dry ice dissipates completely, making it ideal for shipping frozen goods, preserving temperature-sensitive materials, and creating theatrical effects. The duration of its cold-keeping ability is entirely dependent on how quickly this solid carbon dioxide converts back into its gaseous state.
Understanding Sublimation
Dry ice maintains its intensely cold temperature because it undergoes sublimation, a direct phase transition from a solid into a gas. This change occurs at a consistently frigid temperature of approximately -109.3°F (-78.5°C) at normal atmospheric pressure. This process contrasts sharply with the melting of regular ice, which turns into liquid water before evaporating.
When solid carbon dioxide absorbs heat from its surroundings, the energy allows the molecules to bypass the liquid state entirely, transitioning straight into carbon dioxide vapor. This intense absorption of thermal energy makes dry ice a powerful refrigerant. Since the process creates a gas and not a liquid, there is no pooling or dampness, earning the substance the nickname “dry ice.”
Key Factors Determining Longevity
The rate at which dry ice sublimates is governed by a few primary physical factors, the most important being the quality of the insulation surrounding it. Storing dry ice in a poorly insulated container, such as a thin plastic bag, causes it to disappear in a matter of hours. Conversely, a high-quality, heavily insulated cooler with thick walls minimizes ambient heat transfer, allowing a five-pound block to last between 18 and 24 hours under optimal conditions.
The physical geometry of the dry ice itself is another significant variable, often summarized by the surface area to volume ratio. Large, dense blocks last substantially longer than the same mass of small pellets or chips because the block has less surface area exposed to warmer air. This lower ratio means less area is available to absorb the heat energy that drives sublimation. For example, a large block might lose 5 to 10 pounds every 24 hours in a standard cooler, but an equal weight of pellets will sublimate much faster due to their greater total exposed surface area.
The ambient temperature and airflow around the container also influence the sublimation rate. Storing the cooler in a warm environment, like a hot garage or a sunny vehicle, accelerates the process dramatically. Warm air carries more heat energy, causing the dry ice to absorb it faster. Increased airflow can also sweep away the insulating layer of cold carbon dioxide gas that naturally forms around the solid.
Practical Steps for Maximizing Cold Time
To extend the time dry ice stays cold, focus on minimizing heat entry and maximizing the density of the storage medium. The best containers are thick-walled, insulated coolers, but they must not be completely airtight. This is because the buildup of carbon dioxide gas creates an explosion risk. Once the dry ice is placed inside, fill any remaining air space with crumpled newspaper, towels, or foam peanuts to reduce the volume of warm air that can circulate.
Proper placement within the container is also important because carbon dioxide gas is heavier than air. Placing the dry ice on top of the items ensures that the descending cold gas continually blankets the contents, maximizing cooling efficiency. To further slow sublimation, loosely wrap the dry ice block in a layer of newspaper or a towel. This acts as a slight insulating barrier against the warmer air inside the cooler without trapping the gas.
Keep the container closed as much as possible; every time the lid is opened, cold gas escapes and is immediately replaced by warmer air. Storing the cooler in the coolest possible location, such as a shady spot or a refrigerated area, also helps by reducing the temperature difference between the dry ice and its environment.
Essential Safety Protocols
Due to its extreme temperature and gaseous output, dry ice requires specific safety precautions to prevent physical injury and atmospheric hazards. The most immediate risk is severe cold burns or frostbite, which can occur almost instantly upon direct contact with bare skin. Handle dry ice only with thick, insulated gloves, tongs, or a heavy towel to create a barrier against the -109.3°F temperature.
The second major hazard involves the high volume of carbon dioxide gas released during sublimation, which can quickly displace oxygen in confined spaces. One pound of dry ice produces a large amount of gas, creating a suffocation risk if stored in small, enclosed rooms, closets, or the passenger cabin of a vehicle without adequate ventilation. The storage container must never be sealed shut, as the rapidly expanding gas pressure can cause the container to rupture or explode.
For disposal, any remaining dry ice should be placed in a well-ventilated area, away from people and pets, and allowed to sublime naturally. Never dispose of dry ice in a sink, toilet, or trash can, as the extreme cold can damage plumbing fixtures and the gas can build up in enclosed systems.