Dry ice is the common term for solid carbon dioxide (CO2), a compound that exists naturally as a gas in Earth’s atmosphere. This substance is manufactured by compressing and cooling gaseous carbon dioxide until it solidifies into blocks or pellets. The material is strikingly cold, maintaining a temperature of approximately \(-78.5^\circ\text{C}\) (\(-109.3^\circ\text{F}\)) while it slowly disappears. The unique behavior of solid carbon dioxide makes it an example of a physical transformation rarely seen in everyday life.
The Phenomenon of Sublimation
Dry ice is the most widely known example of a scientific process called sublimation, which is the direct transition of a substance from a solid state to a gaseous state, bypassing the intermediate liquid phase entirely. This unusual phase change occurs because of the relationship between a substance’s temperature, pressure, and its three states of matter. For carbon dioxide, the triple point—the specific pressure and temperature where solid, liquid, and gas can coexist—is relatively high.
The triple point for carbon dioxide is found at a pressure of \(5.1\) atmospheres and a temperature of \(-56.4^\circ\text{C}\) (\(-69.5^\circ\text{F}\)). Since the standard atmospheric pressure at sea level is only about one atmosphere, this pressure is significantly lower than the triple point. As a result, when solid carbon dioxide is warmed at normal air pressure, it skips the liquid phase and turns directly into CO2 gas.
This direct transition from solid to gas is what gives the substance its “dry” name, as it never produces a wet residue like water ice. The phase diagram for CO2 shows that at any pressure below \(5.1\) atmospheres, the solid-gas boundary is crossed directly upon heating.
Unique Characteristics of Solid Carbon Dioxide
The physical properties of dry ice distinguish it from ordinary water ice, beginning with its extreme coldness. Dry ice maintains a surface temperature of \(-78.5^\circ\text{C}\) (\(-109.3^\circ\text{F}\)), which is far colder than the \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) of frozen water. Unlike water ice, which changes into liquid water upon melting, solid carbon dioxide turns into a colorless, odorless gas.
The resulting carbon dioxide gas is non-toxic, but it is much denser than the air around it, meaning it tends to sink and pool in low-lying areas. The density of the solid is also high, typically ranging between \(1.55\) and \(1.7\) grams per cubic centimeter. This combination of extreme cold and the direct sublimation into a gas makes dry ice a highly effective cooling agent.
Common Uses and Safe Handling
The unique characteristics of dry ice make it valuable for a variety of commercial and specialized applications. Its intense cold and lack of liquid residue make it a preferred material for shipping perishable goods, such as frozen food and biological specimens, where moisture would cause damage. When dry ice is dropped into water, the rapid sublimation creates a dense, billowing cloud of water vapor, which is widely used for theatrical fog effects.
Handling dry ice requires specific precautions due to its extremely low temperature and the gas it produces. Direct contact with bare skin can cause severe frostbite, so insulated gloves or tongs must be used for any manipulation. The carbon dioxide gas released during sublimation poses a risk of asphyxiation in enclosed spaces because it displaces oxygen.
It is necessary to store and use dry ice only in areas with ample ventilation to prevent the buildup of gas. Moreover, dry ice must never be placed in an airtight container, as the pressure from the accumulating gas can cause the container to rupture or explode. Letting the dry ice sublimate completely in a well-ventilated space is the safest method for disposal.