Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)). This colorless, odorless material is unique among common refrigerants because it does not melt into a liquid state. The name “dry ice” refers directly to this property, distinguishing it from regular water ice. Its physical characteristics make it a highly effective coolant used across many fields, from food preservation to industrial cleaning, but these properties necessitate strict safety precautions.
The Specific Temperature and Sublimation Process
The temperature of dry ice at standard atmospheric pressure is precisely -78.5°C or -109.3°F. This frigid temperature makes it far colder than regular water ice, which freezes at 0°C (32°F). The defining characteristic of dry ice is sublimation, where the solid material transforms directly into a gas without ever passing through a liquid phase.
This unusual behavior is governed by the phase diagram of carbon dioxide. For \(\text{CO}_2\) to exist as a liquid, it must be at or above its triple point, which occurs at 5.11 atm and -56.6°C. Since normal atmospheric pressure is only about 1 atm, solid \(\text{CO}_2\) cannot melt into a liquid.
Instead, as the dry ice absorbs heat, it transitions into carbon dioxide gas, leaving behind no liquid residue. This energy absorption provides its strong cooling effect. One pound of dry ice yields approximately 250 liters of \(\text{CO}_2\) gas.
Common Uses Based on Extreme Cold
The extreme cold and clean sublimation of dry ice make it an ideal temporary refrigerant for shipping perishable goods. It is routinely used to maintain ultra-cold temperatures for medical supplies, such as certain vaccines, and to preserve frozen foods during transport. This method is favored over water ice because it prevents spoilage without the risk of water damage.
A widely recognized application is the creation of dense, low-hanging fog for special effects in theater and entertainment. When dry ice is placed in hot water, the accelerated sublimation produces a cloud of cold \(\text{CO}_2\) gas that condenses the water vapor in the air, forming the characteristic ground-hugging fog.
In industrial settings, the intense cold is leveraged for shrink fitting, a process where metal parts are momentarily shrunk by cooling to allow for tight assembly. Another application is dry ice blasting, where high-pressure \(\text{CO}_2\) pellets are used to clean surfaces; the pellets sublime upon impact, lifting contaminants without leaving abrasive residue.
Essential Safety Precautions
The -78.5°C temperature of dry ice poses an immediate and severe contact hazard, leading to instant frostbite or cryogenic burns upon bare skin exposure. Handling the material requires the use of insulated gloves, such as thick cloth or leather mittens, or tongs. Standard nitrile or latex gloves offer insufficient thermal protection against this extreme cold.
A second significant risk is the buildup of \(\text{CO}_2\) gas, which occurs as the dry ice sublimates. Because carbon dioxide is heavier than air, it tends to pool in low-lying or confined spaces, displacing oxygen. Exposure to high concentrations can cause dizziness, headaches, difficulty breathing, and rapidly lead to asphyxiation.
Proper storage is paramount for mitigating both cold and gas risks. Dry ice must never be placed in a completely airtight container, such as a glass jar or a sealed plastic bottle. As the material sublimates, the accumulating \(\text{CO}_2\) gas generates immense internal pressure, which can cause the container to rupture or explode.
Instead, dry ice should be stored in an insulated, but loosely covered, container like a Styrofoam cooler. The container must allow the continuously produced \(\text{CO}_2\) gas to vent safely into a well-ventilated area. This avoids pressure buildup and maintains a safe gas concentration in the environment.