Ice Cream in a Bag

Big question: Why does salt make ice cold enough to freeze ice cream?

Shake sweet milk in a bag of salty ice and find out why salt makes ice colder than ice alone.

What you need

• 1 cup whole milk
• 2 tablespoons sugar
• 1/2 teaspoon vanilla extract
• 2 small zip-top bags and 1 large one
• 4 cups ice
• 1/2 cup salt
• A kitchen towel
• A bowl and spoons

What to do

1. Mix the baseWash your hands first and check every ingredient with your grown-up. Pour the milk, sugar and vanilla into a small bag, press out the air and seal it. Seal that bag inside the second small bag.
2. Build the ice bathFill the large bag halfway with ice and add the salt. Tuck in the small bags and seal it.
3. Shake itWrap the big bag in the towel and shake and squish it for 10 to 15 minutes, taking turns when your arms get tired. The milk turns from runny to slushy to soft ice cream.
4. Rinse and eatUse the towel to lift the small bags out of the salty ice, because they are very cold. Rinse them under the tap so no salty water gets in. Then open them and spoon your ice cream into the bowl.

What is happening?

Water freezes into ice at 32 F (0 C). That is called its freezing point. Sweet milk has to get even colder than that to freeze. Salt lowers the freezing point of water, so salty ice keeps melting even when it is colder than 32 F. Melting ice pulls heat out of everything nearby, including your milk, so the milk gets colder than plain ice could make it. Shaking keeps the ice crystals small, so your ice cream is smooth.

Salty ice can get cold enough to hurt bare skin, so keep the towel around the bag. Do not eat the salty ice.

Try it

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Check yourself