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You would never add a heap of salt to your ice cream, but this tasty treat can be made by taking advantage of a chemical property of salt and a bit of elbow grease. Here’s how!
Safety: The ice in this activity can get very cold, well below freezing. Take care not to touch it for long periods of time. Use clean hands and clean equipment when making food.

You will need

• Large mixing bowl
• Smaller metal bowl
• Wooden spoon
• Whisk
• 2 cups ice
• ¼ cup water
• 6 tablespoons salt
• 1 tablespoon sugar
• ½ cup milk (whole milk is ideal)
• ½ cup cream
• 1 teaspoon of vanilla extract
• A buddy to help you whisk your ice cream (optional)

What to do

1. Fill the larger mixing bowl with the ice and water.
2. Mix in the salt well with the wooden spoon. When you’re done, put the spoon away so you don’t accidentally use it to stir the ice cream.
3. In the metal bowl, pour in the sugar, milk, cream, and vanilla extract, and mix them with the whisk.
4. Put the metal bowl into the ice-filled bowl. Be careful you don’t get salty ice in your ice cream!
5. Use the whisk to briskly mix the milk mixture for about 10 minutes (take turns!) or until the mixture thickens up into an ice cream texture that you’re happy with. Stay persistent, your mixture will thicken slowly at first.
6. And as your ice cream thickens up, be sure to scrape the bottom of the bowl to prevent lumps from forming.
7. If you’d like your ice cream to get firmer, put the thickened mixture in the freezer for 30 minutes to an hour.
8. Eat and enjoy!

What’s happening?

In this recipe, the icy cold water cools down your ice cream mixture. But there’s something special going on when you add salt to the ice.

Ice is made of water molecules, tightly stuck to each other. Adding salt to water disrupts the water-to-water bonds that make up ice, melting it into liquid water. The salt water that you get afterwards has a freezing point below 0 degrees Celsius. In fact, very salty water can reach temperatures of nearly -20 degrees Celsius before freezing into ice.

In addition to lowering the freezing point, adding salt actually makes the water colder! This happens because it takes heat energy to melt ice cubes. In our case, we didn’t add extra heat to change ice into water so instead that heat was lost from the ice cubes themselves. And that means the salted ice water ends up much colder than the unsalted ice cubes were.

Scientists call this clever technique ‘freezing point depression’. It has many uses, from de-icing roads to measuring milk quality. For us ice cream makers, super-cool salt water is super handy for freezing our ice cream mixture more efficiently.

A special combination

Did your whisking arm get tired? Making ice cream takes work because you are creating a special combination of ingredients called an emulsion. An emulsion is a combination of two liquids that don’t normally mix, like oil and vinegar in salad dressing. In our case, it’s the milk fat globules and the sugary water, which are also mixed with air from your whisking. This specific mixture of ingredients gives ice cream its deliciously creamy texture.