Dry ice, the solid form of carbon dioxide (CO2), is known for its extremely cold temperature and unique behavior. Unlike water ice, dry ice does not melt into a liquid under normal atmospheric conditions. Instead, it transitions directly from a solid to a gas, a process called sublimation. This property makes it an effective cooling agent for applications where liquid residue is undesirable, such as preserving perishable goods or creating special effects.
The Exact Temperature of Dry Ice
The temperature of dry ice is a fixed point determined by its physical properties at standard atmospheric pressure. Dry ice maintains a frigid temperature of approximately -109.3°F. This is significantly colder than the freezing point of water and most household freezers. This precise temperature is the point at which solid carbon dioxide transitions to a gas.
Understanding Sublimation
The transformation of dry ice is called sublimation, where the solid skips the liquid phase and converts directly into carbon dioxide gas. This occurs because the triple point of carbon dioxide—the pressure and temperature combination where its solid, liquid, and gaseous phases coexist—is above standard atmospheric pressure. At Earth’s atmospheric pressure, solid CO2 cannot exist as a liquid, so it moves straight to a gas.
Sublimation is an endothermic process, meaning it actively absorbs heat energy from its surroundings. This absorbed heat is required to break the intermolecular forces holding the carbon dioxide molecules in their solid structure. As dry ice absorbs this heat, it effectively cools the remaining solid block and anything touching it. The cold gas released is dense and causes water vapor in the surrounding air to condense, creating the thick, low-lying fog often associated with dry ice.
Essential Safety Precautions
The extreme cold of dry ice requires strict safety measures to prevent immediate injury. Direct contact with the skin, even briefly, can cause severe thermal injuries known as frostbite. It is necessary to always handle dry ice using thick, insulated gloves, such as cryogenic gloves, or tongs to avoid direct skin exposure. Standard kitchen gloves or thin nitrile gloves do not provide sufficient thermal protection.
Another significant hazard is the rapid expansion of CO2 gas as the dry ice sublimates. One pound of dry ice can produce about 250 liters of carbon dioxide gas. If stored in a completely sealed container, this rapid gas production can cause a dangerous buildup of pressure, potentially leading to the container rupturing or exploding. Therefore, dry ice must always be stored in an insulated, non-airtight container that allows the gas to vent safely.
The sublimated CO2 gas presents an asphyxiation risk in poorly ventilated spaces. Since carbon dioxide gas is heavier than air, it tends to settle in low-lying areas, displacing oxygen. Transporting or storing dry ice in a closed vehicle or small room without adequate airflow can quickly lead to dangerous CO2 concentrations. Proper ventilation is mandatory to ensure the released gas is safely dispersed.
Common Applications and Uses
The combination of extreme cold and the absence of liquid residue makes dry ice highly valuable across many industries. It is commonly used as a refrigerant for transporting temperature-sensitive goods, such as frozen foods, pharmaceuticals, and biological samples. Its ability to keep items frozen without leaving a watery mess is a significant advantage over regular ice.
In the entertainment sector, dry ice is widely used to create dense, theatrical fog effects that hover near the ground. This is achieved by placing the solid carbon dioxide in warm water, which accelerates sublimation and creates a cloud of condensed water vapor. Industrially, dry ice blasting is a cleaning method where pellets are propelled at surfaces to remove residues like paint, oil, or mold. The impact and subsequent sublimation of the pellets remove the contaminant without causing abrasion or leaving secondary waste.