Did you know that you and your kids can make ice cream at your Father’s day picnic, or while you’re on camping, or just out in your backyard without electricity or a freezer? All you need is some rock salt and some ice.
You may know it by as coffee-can ice cream, or Ziploc bag ice cream, or no-machine ice cream, but what a lot of people don’t know is how and why you can make stuff freeze standing in a room temperature environment using ice and salt. I’ve revived this recipe from the past with some extra science. Because Yeah, Science.
Recipe is below. Science is first.
Ice Is A Structure
Did you know naturally-formed ice is technically a mineral? But only when it’s ice… not when it’s water.
H2O is not just another word for water, it’s an explanation about what atoms make it up. Two hydrogen, one oxygen. When liquid, the molecules bounce off one another–they flow. But when the temperature drops to 0 degrees celcius, and an impurity is present (perhaps natually occurring minerals in the water), crystallization begins to take place.
Incidentally, if you’ve ever wondered why ice is less dense than water, it’s all because of the structure. When water forms ice, the hydrogen atoms of one molecule can form bonds with the oxygen atom of another, kind of like this:
You can see that there’s a lot of empty space between the molecules, space that wouldn’t necessarily be there when the water is liquid. Fewer molecules of water take up more space when they’re frozen in lattice structure. More volume, less mass. This is why cans of pop explode when left too long in the freezer, and why your ice floats in your drink.
Why Ice and Salt Are Not BFFs.
Like most Canadians, I’ve known for decades that salt melts ice. When I asked why as a kid, I got the answer that salt acts like an antifreeze agent–which it does; salt lowers the freezing temperature of water. So when it’s maybe only -5 celcius outside, and your steps are icy, a judicious sprinkle of salt makes it so that it’s not cold enough for your ice to stay frozen any longer. It melts and goes away.
This answer is not wrong, but it doesn’t explain at all how you can drop the freezing point of water to the point where ice melts fast AND make ice cream at the same time while you’re wearing shorts at the park in the summer.
Would you believe that when the ice is melting, it’s actually getting colder?
What Happens at the Molecular Level?
As any chemistry teacher will be happy to tell you, energy is required to make changes at the atomic level. In the case of melting ice, the energy that’s used… is heat.
When ice melts, the bond that has formed between molecules to create the lattice structure must be broken apart. If you’ve ever felt the cold air that hovers around an ice cube or a cold drink, ice has had to absorb heat from the surrounding area to effect the chemical changes required for it to melt.
Energy naturally tries to achieve a balanced state. By adding salt, which decreases the freezing temperature of the water on top of the ice, more heat energy must be pulled from the air to try to reach an equilibrium between the ice water and air. Did you know that by using just rock salt and ice water, you can create temperatures as cold as -21 celcius?
That’s almost -6 degrees Fahrenheit. Plenty cold enough to freeze your ice cream!
Delicious Applications of Science – Time to Make Ice Cream With the Kids!
There are two main methods for Low-Tech ice cream: the “coffee can” and the “Ziploc.” Both are essentially the same method of preparation. You have a big (3lb) coffee can and a little (1lb) coffee can, or a big Ziploc (gallon) and a little Ziploc (sandwich) bag. You can even be like me, the lamer without the coffee can, and do this with a jar and a Tupperware tub.
Basically, just find something little and water-tight to hold the ice cream, and something big enough to hold the little thing, plus ice and salt. The big difference between the two is quantity. The baggie method is good for individual portions and highly individualized ice creams.
Then you make those kids work for their dessert. If using the coffee cans, tape the lids securely and have the kids roll, toss, or kick the can around for about 10-15 minutes. If you opt for the Ziploc version, ensure that both bags are sealed and have them take turns tossing and squishing their bags.
Constant movement helps keep the ice crystals that form in the cream small, providing a smoother, creamier version. But it’ll never happen; they’ll forget or check their bags or have to run to the bathroom. Kids don’t care. It’s ice cream that they made themselves! How awesome is that?
Baggie Version (serves 1):
- 1/2 cup Half & Half, alternative milk, or regular milk
- 1 tbsp sugar
- Flavour: 1/4 – 1/2 tsp vanilla extract (depending on how strong you like it) or a little chocolate syrup.
Mix and seal the ingredients in a sandwich-size Ziploc bag. In a gallon-size bag, fill about half full with ice. Add 1/2 cup (~ 2 handfuls) coarse kosher salt or rock salt. Insert the sandwich bag into the gallon bag and seal the gallon bag shut. Shake the bags until the mixture begins to harden (about 5 minutes).
Coffee Can Version (serves 4-6)
- 1 pint (2 cups) Half & Half, alternative milk, or regular milk
- 1/4 cup sugar
- Flavour: 1 – 2 tsp vanilla, or chocolate syrup to taste.
Same instructions as the baggie version, except with a 1 lb coffee can and a 3 lb coffee can. Ice cream may take up to 15 minutes to crystalize properly.
Top the ice cream as you please, or have the kids make their ice cream with a couple tablespoons of chopped fruit, homemade or mini chocolate chips, or if you want to bestow a special treat, candy. Not that I recommend this, but… Everything in moderation, right mommies?
Plus, how awesome do you look when you can prove that Skittles Ice Cream really does exist?
Just be careful about adding ingredients that may contain salt, as the same process that is helping you freeze the ice cream by melting your ice at supercold temperatures will also, unfortunately, keep ice cream with salt from freezing.
Happy ice cream making!