Dry ice is the solid form of carbon dioxide (CO2). This substance is exceptionally cold, maintaining a temperature of approximately -78.5°C (-109.3°F) at standard atmospheric pressure. Unlike regular water ice, dry ice transitions directly into a gas rather than melting into a liquid, which is why it is often used as a refrigerant. Its ability to cool items without leaving messy liquid residue makes it valuable for shipping perishable goods, creating theatrical fog effects, and various industrial applications.
The Process of Sublimation
The unique behavior of dry ice is a result of sublimation, the direct phase change from a solid to a gas that skips the liquid phase entirely. This occurs because the triple point of carbon dioxide is above normal atmospheric pressure. The triple point is the specific pressure and temperature where a substance can exist simultaneously as a solid, liquid, and gas. For carbon dioxide, the triple point is at a pressure of about 5.1 atmospheres and a temperature of -56.6°C.
Since Earth’s atmosphere is only about one atmosphere of pressure, solid carbon dioxide warms above its sublimation point of -78.5°C and turns straight into gas. The sublimation process is endothermic, meaning it constantly absorbs heat from the surrounding environment to fuel the phase change and provide effective cooling. One pound of solid dry ice is capable of producing about 250 liters of carbon dioxide gas as it sublimates.
Key Characteristics of Carbon Dioxide
The gas released by dry ice is carbon dioxide. At normal temperatures, this gas is colorless and odorless, making it impossible to detect without specialized equipment. The visible fog often associated with dry ice is not the carbon dioxide gas itself, but condensed water vapor chilled by the extremely cold gas.
A primary characteristic of carbon dioxide gas is its density compared to air. Carbon dioxide gas is about 1.5 times denser than the air we breathe. This increased density causes the gas to sink and accumulate in low-lying areas, such as the bottom of containers, cellars, or along the floor. This tendency to pool is a major factor in the safety precautions required when using dry ice.
Safety and Ventilation Requirements
The primary hazard associated with the large volume of carbon dioxide gas released by dry ice is the risk of asphyxiation. When CO2 accumulates in an enclosed or poorly ventilated space, it physically displaces the oxygen in the air. As oxygen levels drop, this can lead to unconsciousness and potentially death.
To mitigate this risk, dry ice must always be used and stored in areas with adequate ventilation to ensure the gas disperses safely. Never store dry ice in completely airtight containers, such as sealed plastic bottles, as the constant sublimation will build up pressure that can cause the container to rupture or explode. Direct contact with the solid dry ice should also be avoided, as its extremely low temperature can cause severe cold burns or frostbite in a matter of seconds.