On the whole, eating and science projects don’t mix—you should never taste anything in a science laboratory—but in this project kids can use the power of salt to freeze their own ice cream and then they can eat it! And, even better, if you are doing this in the winter and there is snow on the ground, you can use snow from your backyard instead of the ice cubes.
For the ice cream mix:
1⁄2 cup (120 ml) milk
1⁄2 cup (120 ml) heavy (double) cream
1⁄4 cup (50 g) sugar
1⁄4 teaspoon vanilla extract
Measuring cup and spoon
Spoon for stirring
To freeze the ice cream:
1-quart (2-pint) zip-lock freezer bag, about 7 x 8 inches (18 x 20 cm)
A pair of warm gloves
2–3 cups of ice cubes
A clean tea towel
A rolling pin
1-gallon (8-pint) zip-lock freezer bag, about 10 x 11 inches (25 x 27.5 cm)
3⁄4 cup (120–180 g) salt
1. Put the milk, cream, sugar, and vanilla extract into the jug and give everything a good stir.
2. Pour this ice cream mixture into the smaller zip-lock bag, squeeze out the air, and zip it closed.
3. Put on your gloves and ask an adult to help you take ice cubes from the freezer. (Don’t touch them with bare hands or they could stick to your skin and hurt it.) Pile the ice cubes onto a clean tea towel. Wrap them up and gently bash them with a rolling pin to crush them.
4. Put about 2 cups of crushed ice (or snow) into the large zip-lock bag. If you have a thermometer, measure the temperature of the ice.
5. Add the salt to the ice and, with your gloves still on, mush the outside of the bag a bit to mix the salt and ice.
6. Still with your gloves on, put the bag with ice cream mix in it inside the bag with the ice. Make sure the smaller bag is surrounded by ice and salt. Squeeze out the air and zip seal the big bag. Still wearing gloves, gently squeeze and massage the ice around the inner bag. Keep doing this for about 10–15 minutes.
7. Check it! The ice cream mix in the inner bag will have frozen into ice cream, even though you haven’t put it in the freezer! Still wearing gloves, take the ice cream bag out of the bigger bag. Measure the temperature of the ice again and then eat your ice cream!
Ice needs energy (heat) to change from a solid to a liquid. If you put plain ice around the ice cream mix, the ice would take warmth from the mix and cool it—but not enough to freeze it. Salt lowers the freezing point of ice, so if you add salt to ice it will need even more energy (heat) to melt it. It takes all this energy from the ice cream, making it cold enough to freeze into ice cream. When you measure the temperature of the ice the second time, it will be much lower than the first time!