Health Advice



Dry ice, the unsung hero of the COVID-19 vaccine rollout, explained

Tom Avril, The Philadelphia Inquirer on

Published in Health & Fitness

To understand why, let's start with water. Molecules of water are "polar," meaning they are slightly positive on one end (the hydrogen atoms in H2O) and slightly negative on the other (the oxygen). Opposites attract, so the molecules stick together fairly well. This is why water molecules tend to cluster in little beads on a drinking glass or car windshield, a process called cohesion.

But molecules of carbon dioxide (CO2) have an oxygen atom (the negative part) on both ends, with carbon in the middle. No attraction between the end of one molecule and the next. The result: it doesn't take much for molecules of carbon dioxide to skedaddle apart from each other. Once you get higher than negative 109 degrees, whoosh! The molecules spring apart into the gaseous phase, without pausing in the liquid form.

At room temperature, it happens in a hurry.

To demonstrate, Levis placed chunks of dry ice in a plastic bottle and attached a balloon over the top. He then did the same for another bottle with cubes of what he called "water ice" — that is, frozen water, not the summertime Philadelphia treat. (Note to those who might try this on their own: Wear protective gloves to handle the dry ice, and do so in a ventilated area.)

Almost immediately, the balloon on the dry-ice bottle began to inflate, as the solid chunks sublimated into carbon dioxide gas. In just a few minutes, bang! No more balloon.

"Ha ha!" he chortled. "Worth the wait."


Yet in the other bottle, the regular ice simply melted into water. Small amounts were evaporating, but not enough to see any impact. The balloon remained limp.


Whether a substance is solid, liquid, or gas depends on more than temperature. Another factor is pressure. If the pressure is high enough, even carbon dioxide can be compressed from a gas into its liquid phase.

In fact, that's how dry ice is made, said Rich Gottwald, president of the Compressed Gas Association, an industry trade group. (Among its members is Radnor-based Airgas, one of the leading U.S. producers of dry ice.)


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