At Camp Quest NorthWest we don’t just want to talk about how or why things work. Our goal is to teach critical thinking and science using hands-on exploration and testing. To that end, I’ve written up what I hope will be the first in a series of activities that you can do at home. These should take no more than an hour or so and be manageable using materials you might commonly have lying around the house, with the occasional exception usually easily available from a grocery or hardware store.
Let’s start off with something I tried for the first time myself last night to great success, dry ice ice cream. With very common kitchen ingredients and equipment and a small amount of dry ice, usually available at grocery stores, you can make up a cold, creamy treat that is sure to please the palates of young and old alike. To make this, you’ll need to gather the following ingredients for about two scoops (double or triple these amounts if you want more):
1 cup half-and-half
2 or 3 tablespoons sugar (to taste)
1/2 teaspoon vanilla extract
1/2 pound of dry ice
You’ll also need the following:
A reasonably thick bag (I used doubled-up paper lunch sacks)
A mallet or rolling pin
A metal or wood mixing spoon
A well-ventilated work space
Gloves or tongs to handle the dry ice, never touch it with bare skin!
A large bowl, at least 4 times the volume of however much ice cream you are making
Any mix-ins you want, M&M’s, chocolate syrup, candy corn, circus peanuts, whatever…
First, a warning: dry ice is extremely cold and should be handled with gloves at all times. Serious injury such as frostbite can occur if you don’t give it the respect it deserves.
Dry ice is the solid form of carbon dioxide (CO2), a common gas that makes up about .03% of Earth’s atmosphere by volume along with 78.09% nitrogen (N2), 20.95% oxygen (O2), and other trace gases. At standard pressures carbon dioxide does not have a liquid state. Instead, when cold enough it changes from gas directly to solid form, a process called deposition. At -78.5°C (-109.3°F) dry ice is very cold, and very good at freezing ice cream. Normally we would make ice cream at home by immersing a container with the appropriate ingredients into a bath of water, ice, and salt, the combination of which reduces the freezing point of liquid water allowing the ice cream to freeze. In the case of dry ice ice cream we’ll be mixing it directly into the ingredients. It is ideally suited for this because it quickly freezes the ingredients and converts directly from solid back to gaseous form, a process called sublimation. What’s left is a smooth, creamy, tasty treat.
Here are the step-by-step instructions:
1) Mix the half-and-half, sugar, and vanilla extract into your bowl and stir until combined.
2) Put the dry ice into a reasonably thick bag and smash it with the mallet/rolling pin. You’ll want the crushed dry ice to be as fine as possible. We don’t want large chunks of dry ice in the ice cream as we’ll have to get rid of them before we can eat it.
3) Scoop the crushed dry ice into the bowl while stirring the soon-to-be ice cream with your mixing spoon. It will start to bubble up almost immediately and a thick fog will obscure your view. Keep stirring, scraping what you can from the bottom of the bowl.
4) Continue step 3, adding more dry ice until you get the consistency you want. This can take anywhere from 3 to 8 minutes in total.
5) Add mix-ins and enjoy! Generally speaking it is safe enough to eat if you don’t see any chunks of dry ice and are still able to stir it without too much effort.
– While carbon dioxide does go from gas to solid at cold temperatures of -78.5°C and below and at pressures lower than 5.13 atm (atmospheres), that’s not actually how commercial dry ice is made. Instead, it comes from liquid CO2 formed at 59.2 atm through a process that converts it to snow, which is then compressed into the chunks we are familiar with.
– Have leftover dry ice? Try this: fill a bowl with hot water and drop a few chunks in for a great “bubbling cauldron” effect.