It is common to place dry ice on a scale and watch the displayed weight decrease over time. The solid block appears to shrink without leaving any liquid residue, which is counterintuitive for many people accustomed to the melting of water ice. This visible mass loss is a direct result of the substance’s unique physical properties and the atmospheric conditions it encounters. Understanding this phenomenon requires examining the composition of dry ice and the specific phase transition it undergoes when exposed to normal room temperature.
What Exactly Is Dry Ice?
Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)), the same colorless and odorless gas that humans exhale. It is exceptionally cold, with a surface temperature of approximately \(-109.3\) degrees Fahrenheit (\(-78.5\) degrees Celsius) at standard atmospheric pressure. This extreme cold makes it an effective cooling agent for shipping perishable goods, such as frozen foods or pharmaceuticals.
Unlike water ice (\(\text{H}_2\text{O}\)), dry ice is non-polar, which contributes to its unique behavior as it warms. Its density ranges between \(1.55\) and \(1.7\) grams per cubic centimeter, making it heavier than water ice. This composition as solid \(\text{CO}_2\) dictates the phase change process that causes the scale reading to drop.
The Science of Sublimation
The mass loss is driven by sublimation, a phase transition that bypasses the liquid state entirely. Sublimation is the direct change from a solid to a gas, meaning dry ice does not melt like a regular ice cube. This unique behavior is due to the relationship between temperature and pressure, which can be visualized on a phase diagram for \(\text{CO}_2\).
For carbon dioxide, the triple point—where solid, liquid, and gas phases can coexist—is at a pressure significantly higher than Earth’s surface pressure (approximately \(5.13\) atmospheres and \(-69.5\) degrees Fahrenheit). Since atmospheric pressure at sea level is only about one atmosphere, \(\text{CO}_2\) cannot exist as a liquid under normal conditions.
Any solid \(\text{CO}_2\) exposed to temperatures above \(-109.3\) degrees Fahrenheit at one atmosphere will instantly transition into a gas. This process is endothermic; the dry ice absorbs heat from the environment to fuel the phase change. This continuous conversion of solid mass into gaseous mass is the mechanism behind the shrinking of the block.
Why the Scale Reading Drops
The scale reading decreases because the solid mass is converted into an invisible gas that no longer rests on the weighing platform. As sublimation occurs, the carbon dioxide changes from a tightly packed solid structure into a rapidly moving, diffuse gas. This gas then escapes and disperses into the surrounding air. The observed weight loss is a direct measure of the amount of solid \(\text{CO}_2\) that has undergone this phase transition.
Safe Handling and Storage
Handling dry ice safely is important due to its extreme cold and the gas it releases. The temperature of \(-109.3\) degrees Fahrenheit is cold enough to cause immediate frostbite if it touches bare skin. Always use thick, insulated gloves or tongs when moving dry ice to prevent thermal injury.
Dry ice should never be stored in an airtight container, such as a sealed jar or cooler. As the solid sublimates, it generates a large volume of \(\text{CO}_2\) gas, and the pressure buildup can cause the container to rupture or explode. Always store dry ice in a well-ventilated area, as the \(\text{CO}_2\) gas is heavier than air and can accumulate in confined spaces, displacing oxygen and posing an asphyxiation risk.