After we mentioned dry ice in a recent question on transporting food, we received several letters – and more science lessons than we expected. Thanks to all who gave us a reason to look more closely into the fascinating world of dry ice.
First, we shouldn't have said that ice water is colder than ice. We were trying to take a shortcut – we should have said that ice water can chill more efficiently than ice alone.
Second, while we correctly noted that dry ice doesn't melt – it reverts to a gas – one writer took us to task for writing that it dissolves or converts, rather than using the words “dissipates” or “sublimes.” Sorry, we aim for common language that is easy to understand. Not many readers need to know the term “sublimates.”
Dry ice is made from a gas, so it goes back to being a gas. That's part of what makes dry ice so convenient for shipping – since it doesn't melt, you don't have to worry about water leaking out of a package.
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Several people also asked about our advice to not keep an open cooler of dry ice in a small, enclosed space for a long period of time. Dry ice is made from carbon dioxide, which can displace oxygen. The chance of a chunk of dry ice displacing enough oxygen to cause a problem is remote, but there are reports of people noticing their respiration increase in the presence of dry ice. Having your heart beat faster while you're driving a car can be unsettling.
The official handling instructions suggest keeping it in a container or using it in a ventilated space. You also shouldn't handle it with bare skin.
At least we all can agree on our final suggestion: Dropping dry ice in hot water is fun. Sublime, even.