Halloween is coming up which means we will all be investing in some blocks of dry ice. But what is dry ice? Have you ever stopped to consider the inherent contradiction in its name?

When we think of ice what we normally refer to is water ice, which is to say, frozen H2O. There are a few other substances also referred to as ices but they are not water at all. Dry ice is, in fact, just frozen carbon dioxide. The gas that we expel every time we breathe is what we use to create chilling effects and super cold temperatures.

What makes dry ice so useful in effects is something called sublimation. Sublimation is when an element or substance skips a material phase. The three phases we commonly see are gas, liquid, and solid matter. Dry ice sublimates from solid matter to gas, without an intervening liquid phase, which is why it is so terrific for making fog and mist. 

But what about the temperature? Dry ice is commonly used in industrial shipping because it can keep foods and medicines much colder, for much longer, than water ice. And even better? It doesn’t leave any residue. Because it skips the liquid phase you’re never stuck with wet marks or pools as it “melts.” In order to freeze carbon dioxide we have to take it to an extremely low temperature. Water freezes at 32 degrees, but carbon dioxide freezes at -109.3 degrees. It begins to sublimate (change from solid to gas) at -69.5 degrees. 

It will begin to mist at room temperature, but if you want an even better fog, add dry ice to a bucket of water. This speeds up the sublimation process (water is a much better conducted of heat than air) and produces a thick fog that clings to the ground instead of something that floats up and fills the room.

Beware! Because of its incredibly low temperature you must wear gloves to handle dry ice. It will leave you with frostbite if you touch it with bare skin. It’s also not safe to store in your home freezer. Because it is so extremely cold it tricks your freezer’s thermostat into shutting off, which can be bad for your food once the dry ice evaporates. Speaking of that evaporation leads me to the other reason we shouldn’t store dry ice in our home freezers. Frozen CO2 sublimates from solid to gas, which means that it can build up excess carbon dioxide pressure in your freezer and blow open the door! The best way to store dry ice at home is in a cooler. Place the lid on top, but don’t lock it in place. As long as there is even a small seem for the gas to vent everything is fine.