Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)), and regular ice is frozen water (\(\text{H}_2\text{O}\)). Both are used for cooling, but their properties are vastly different. Water ice melts into liquid water, while dry ice skips this liquid phase entirely, transitioning directly into a gas. This difference in behavior is tied to the extreme temperature disparity between the two substances.
The Temperature Difference
Dry ice is significantly colder than frozen water, providing a much greater cooling capacity. Water ice maintains a temperature of \(32^{\circ}\text{F}\) (\(0^{\circ}\text{C}\)) as it melts under standard atmospheric pressure. This temperature is the freezing point of water.
In contrast, dry ice exists at a frigid \(-109.3^{\circ}\text{F}\) (\(-78.5^{\circ}\text{C}\)). This is the point at which solid carbon dioxide turns into a gas. The temperature difference of over 140 degrees Fahrenheit means dry ice can keep items much colder for longer periods than water ice can.
How Dry Ice Works
The extreme cold of dry ice is directly related to its chemical makeup and the physics of its phase change. Dry ice is composed of carbon dioxide molecules held together by relatively weak intermolecular forces. When dry ice absorbs heat, these forces are easily overcome, allowing the solid to transition directly into a gas.
This process, known as sublimation, occurs because carbon dioxide does not have a stable liquid state at normal atmospheric pressure. The triple point for \(\text{CO}_2\), the specific pressure and temperature where solid, liquid, and gas can coexist, is \(-69.5^{\circ}\text{F}\) (\(-56.4^{\circ}\text{C}\)) at \(5.1\) atmospheres of pressure. Since atmospheric pressure is only about one atmosphere, solid carbon dioxide immediately bypasses the liquid phase as it warms. This direct change makes dry ice a residue-free cooling agent.
Safe Handling of Dry Ice
Handling dry ice requires specific safety measures to prevent injury due to its extreme temperature. Direct contact with bare skin can cause immediate, severe frostbite because the cold temperature freezes skin cells rapidly. You should always use thick, insulated gloves or tongs when manipulating the solid carbon dioxide blocks or pellets. Thin gloves, like standard nitrile gloves, will not provide adequate protection against this extreme cold.
Proper ventilation is also a safety requirement, as dry ice continuously sublimates into carbon dioxide gas. Since \(\text{CO}_2\) is heavier than air, it can accumulate in low-lying or poorly ventilated areas, displacing the oxygen. This can lead to an asphyxiation hazard, causing symptoms like headaches or difficulty breathing. Never store or transport dry ice in a tightly sealed, airtight container, as the buildup of gas pressure can cause the container to rupture or explode. Storage containers should be insulated but allow the gas to vent safely into a well-ventilated space.