Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)), existing at an incredibly low temperature of approximately \(-78.5\text{°C}\) or \(-109.3\text{°F}\). Many people assume that, like regular ice, dry ice simply warms up and turns into a puddle of liquid. However, the unique behavior of dry ice when exposed to warmer temperatures involves a process that completely bypasses the liquid state.
The Direct Answer: Sublimation
Dry ice does not melt into water because it is not made of water, nor does it melt into a liquid at all under normal conditions. Instead of transitioning from solid to liquid, dry ice undergoes a phase change called sublimation.
This transformation occurs when the solid carbon dioxide absorbs heat from the surrounding air. As the temperature rises above \(-78.5\text{°C}\) at standard atmospheric pressure, the solid \(\text{CO}_2\) instantly converts into carbon dioxide gas. The visible fog that appears is not liquid vapor, but rather the cold carbon dioxide gas mixing with and condensing the water vapor present in the warmer air around it.
The Science Behind the Transition
The reason dry ice behaves this way comes down to a specific combination of pressure and temperature. The physical state of any substance—solid, liquid, or gas—is determined by its pressure and temperature conditions. For carbon dioxide to exist as a liquid, it requires a pressure much higher than the one we experience at sea level.
Scientists plot these relationships on a phase diagram, which includes a specific point called the triple point. The triple point is the only pressure and temperature combination where a substance can exist simultaneously as a solid, liquid, and gas in stable equilibrium.
The triple point for carbon dioxide occurs at a temperature of approximately \(-56.6\text{°C}\) and a pressure of about \(5.1\) atmospheres. Since the atmospheric pressure at sea level is only \(1\) atmosphere, we are operating far below the pressure required for liquid CO2 to form. When dry ice is warmed at \(1\) atmosphere of pressure, it crosses the boundary directly from solid to gas, bypassing the region where liquid CO2 is stable.
This pressure constraint ensures that dry ice will always sublime at room temperature and standard pressure. This unique property is why the material is called “dry ice,” as it leaves no wet residue behind.
Safe Handling and Practical Applications
Because of its extreme temperature, dry ice requires careful handling to prevent injury. Direct contact with the skin can cause severe frostbite or cold burns in a matter of seconds. It is necessary to always handle dry ice using thick, insulated gloves or tongs to maintain a protective barrier.
The sublimation process also creates two safety concerns related to the gas it releases.
Container Safety
Dry ice must never be stored in a sealed or airtight container, such as a glass jar or plastic bottle. As the solid converts to CO2 gas, pressure builds rapidly, which can cause the container to rupture or explode.
Ventilation Risk
The gas it produces can pose a risk of asphyxiation in poorly ventilated areas. Carbon dioxide is heavier than air and can displace oxygen in enclosed spaces like small rooms, car trunks, or walk-in freezers. Adequate airflow is therefore required whenever dry ice is in use to prevent the concentration of CO2 from reaching hazardous levels.
This lack of liquid residue and its intense cooling power make dry ice highly useful in several applications. It is commonly used for preserving perishable goods, like frozen food and medical samples, during shipping. It also provides the dramatic, low-lying fog effects seen in theaters and haunted houses, created by dropping the solid CO2 into warm water.