Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)). It transitions directly from a solid to a gas, a process known as sublimation, at \(-78.5^\circ\text{C}\) (\(-109.3^\circ\text{F}\)) under normal atmospheric pressure. This characteristic makes it highly effective for cooling and creating dramatic visual effects, as it leaves no liquid residue.
Essential Safety and Handling Procedures
The extreme cold temperature of dry ice poses an immediate hazard, as direct or prolonged contact with bare skin can cause severe frostbite, similar to a thermal burn. It is necessary to wear thick, insulated gloves, such as leather or specialized cryogenic gloves, whenever handling dry ice. Thin latex or nitrile gloves offer insufficient protection and can freeze to the skin upon contact.
The carbon dioxide gas released during sublimation is odorless and colorless. As solid dry ice converts to gas, it expands significantly—one pound of solid \(\text{CO}_2\) produces about 250 liters of gas—and this gas can quickly displace oxygen in confined spaces. Working with and storing dry ice requires excellent ventilation, such as an open room, garage, or outdoors, to prevent the buildup of carbon dioxide, which can lead to difficulty breathing or unconsciousness. Never use dry ice in small, enclosed areas like closets, walk-in refrigerators, or unventilated vehicles.
Understanding and Accelerating Sublimation
Sublimation is an endothermic process, meaning solid carbon dioxide must absorb heat from its surroundings to transform into a gas. The sublimation rate is directly controlled by how quickly heat energy is transferred. Exposure to ambient air results in slow sublimation.
To intentionally “activate” the dry ice, the goal is to maximize heat transfer. The most effective method is to introduce the dry ice to warm or hot water, as water transfers thermal energy more efficiently than air. Higher water temperatures result in a faster sublimation rate and a burst of gas.
Two other factors influence the speed of sublimation: surface area and air circulation. Smaller pieces or pellets sublimate faster than a large block because they expose more surface area to the heat source. Moving air, or the bubbling action in water, helps carry away the cold \(\text{CO}_2\) gas and constantly replaces it with warmer air or water, maintaining a high heat transfer rate.
Key Methods for Practical Application
The principles of rapid heat transfer are demonstrated in the creation of a fog effect, which is condensed water vapor, not \(\text{CO}_2\) gas. When the extremely cold carbon dioxide gas mixes with the warm, humid air above the hot water, the water vapor quickly condenses into a dense white cloud. To create a large volume of fog, place five to ten pounds of dry ice into a large container holding four to eight gallons of hot water.
The hotter the water, the more fog will be produced, though extremely hot water may cause the fog to rise too quickly. The fog naturally flows downward and across the floor because the cold, \(\text{CO}_2\)-rich air is heavier than the surrounding air. Since the dry ice rapidly cools the water, the fog effect will diminish after 5 to 10 minutes, requiring the addition of more hot water or dry ice to continue the effect.
For chilling or freezing, dry ice must be used to cool an insulated space, like a cooler or freezer. Dry ice maintains a lower temperature than regular water ice and is excellent for preserving frozen goods during transport or power outages. When cooling, the dry ice should be placed on top of the items, as the cold \(\text{CO}_2\) gas is dense and sinks. It should not be placed directly into food or drinks unless using specialized accessories, as accidental ingestion can cause internal injury.
Safe Storage and Final Disposal
Proper storage focuses on slowing the sublimation rate while preventing pressure buildup. Dry ice must be stored in an insulated container, such as a cooler, to minimize ambient heat transfer, but the container must never be sealed. The rapidly expanding \(\text{CO}_2\) gas can cause a sealed container to burst.
Use a non-airtight insulated cooler, which slows sublimation while allowing the gas to vent. Dry ice sublimates at a rate of five to ten pounds every 24 hours in a standard cooler, so purchase it close to the time it is needed.
Remaining dry ice should be allowed to fully sublimate in a well-ventilated area, such as outdoors or in a room with open windows, away from children and pets. Never dispose of dry ice by flushing it down a toilet or sink, as the extreme cold can damage plumbing fixtures. It should not be thrown in the trash or a sealed receptacle.