Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)), which has an extremely low temperature of approximately \(-109.3^\circ \text{F}\) (\(-78.5^\circ \text{C}\)). Unlike water ice, dry ice transitions directly from a solid state to a gaseous state, a process called sublimation. This is why it is called “dry” ice, as it leaves no liquid residue behind. When dry ice is submerged in water, sublimation accelerates dramatically because the water provides a constant and efficient source of heat energy to the solid.
Factors Influencing Dry Ice Sublimation Speed
The duration dry ice lasts when submerged in water is highly variable and depends on its physical properties. The most significant factor is the overall size and mass of the piece, since a larger block requires more material to convert to gas. A small pellet will disappear in minutes, while a large, dense block weighing five to ten pounds may last up to 45 minutes before it is entirely sublimated.
The ratio of surface area to volume also plays a considerable role in determining the speed of sublimation. Smaller, irregularly shaped pieces expose a greater amount of surface area to the surrounding water, allowing heat transfer to occur more rapidly. A dense, solid cube will last longer than the same mass of loose pellets because the block has less exposed surface area for the water to transfer energy. The material and shape of the container holding the water can also influence the rate by affecting how quickly the water itself loses heat to the environment.
The Role of Water Temperature
Water temperature is the most important variable affecting the sublimation rate, as it is directly tied to heat transfer principles. Sublimation is an endothermic process, meaning the solid \(\text{CO}_2\) must continuously absorb heat energy from its surroundings to change phase into a gas. The speed of this phase change is directly proportional to the amount of heat energy the water can transfer to the dry ice.
Water is an extremely efficient medium for heat transfer, far surpassing air or insulating materials. This efficiency is due to water’s high specific heat capacity, which means it can hold a large amount of thermal energy. Warm or hot water contains significantly more heat energy available to transfer to the dry ice, causing rapid sublimation and an intense fog effect. As the water transfers its heat to the dry ice, the water temperature drops quickly, which then slows the rate of sublimation. If the water becomes cold enough, sometimes forming a layer of ice on the dry ice surface, the rate slows dramatically because the temperature difference and heat transfer efficiency decrease.
Safe Handling and Storage Practices
Handling dry ice safely requires specific precautions due to its extremely cold temperature and the gaseous product of sublimation. Direct contact with bare skin must be avoided, as the \(-109.3^\circ \text{F}\) temperature can cause severe tissue damage, similar to a burn, within seconds. Always use insulated gloves, tongs, or a towel when moving dry ice to prevent frostbite.
The gaseous carbon dioxide produced during sublimation is heavier than air and can accumulate in low-lying, confined areas. This accumulation displaces oxygen, creating a risk of asphyxiation. Dry ice must always be handled and stored in well-ventilated spaces, and it should never be transported in an unventilated vehicle cabin. Furthermore, dry ice should never be stored in an airtight container, such as a sealed glass bottle or jar. As the solid sublimates into a gas, the resulting pressure buildup can cause the container to rupture or explode.