This week I found myself driving to a welding supply store an hour from my home to pick up a block of dry ice for a science experiment. Although we only needed a small slab, the minimum amount they would sell me was 10 pounds (about the size of a bowling ball, as they described it). Figuring we should get the most out of our investment, I had the store slice our block into two halves so we could have some to play with.
Dry ice, of course, is frozen, solid carbon dioxide. CO2 freezes at about -78 degrees C (-109 degrees F), so it’s handy for making things super cold, super fast. It also creates interesting effects when combined with everyday substances such as water. We had never used dry ice at home before (although we’ve seen it in action at science events), but finding a few quick and easy activities only took a little searching on the Internet.
The simplest was the classic Halloween cauldron set-up, only on a miniature scale: we dropped a chunk of dry ice in a plastic cup of water and watched it boil away merrily while “steam” poured from the top. In another cup we added a few drops of dish soap and got instant overflowing bubbles. Slowly drawing a paper towel soaked in soapy water across the rim of a bowl filled with water and dry ice produced one giant bubble, as you can see in the video below.
But the highlight of the evening was Dry Ice Ice Cream. The methods I found online either used too many ingredients or too dangerous. (One called for mixing the dry ice with denatured alcohol, which creates a supercooled liquid that can instantly freeze your skin. Dry ice itself is relatively safe to use with kids, as long as you avoid touching it with your bare skin. We used winter gloves and tongs; safety goggles are also a good idea and always add a scientific flair to any activity.) Eventually I found a simple recipe and we improvised a technique which was somewhat successful. Here it is:
Ceceri Family Dry Ice Single-Serving Ice Cream Recipe
½ cup heavy cream
¼ cup milk
1 tablespoon sugar
¼ teaspoon vanilla
5 pounds dry ice, broken into small chunks (to break up a large piece put it in a paper bag and hit it with a hammer)
1.Mix ingredients in a plastic cup.
2.Place cups in a bowl filled with chunks of dry ice.
3.Stir every few minutes.
4.Ice cream will be ready in about 30-45 minutes.
Although quite tasty, even with frequent stirring our cups of ice cream came out frozen solid on the bottom and slushy on the top. I later found a blog post I had written about a Chemistry Week demonstration a few years ago where we made ice cream by combining the ingredients in a press-and-seal bag and placing it on a slab of dry ice for only a few seconds. Next time I make the trip to the welding store, we’ll have to try that again.
By the way, if you’re interested in how the kids and I used the rest of the dry ice to track alpha particle emissions from a uranium marble, go check out the post on our Home Physics blog.