The question: How can salt help ice freeze ice cream?
Make a prediction
Will the ice cream freeze with plain ice, or does it need salt? Write your guess on the worksheet before you start. Scientists call this a hypothesis.
What you need
1/2 cup half-and-half or milk
1 tbsp sugar
1/4 tsp vanilla
1 small zip-top bag and 1 large zip-top bag
About 3 cups ice
1/3 cup coarse salt (kosher or rock salt)
A towel or gloves
A thermometer (optional)
Steps
Mix half-and-half, sugar and vanilla in the small bag and seal it well (squeeze out the air).
Fill the large bag halfway with ice and add the salt.
Put the small bag inside the large bag and seal it.
Wrap in a towel and shake for 5 to 8 minutes until the mix firms up.
Wipe the salt off the small bag, then eat your ice cream with a spoon!
🔬 Do it twice: once with salt and once without. Measure the ice temperature each time.
⚠️ Safety: The bag gets very cold, so use a towel or gloves. Don't eat the salty ice.
What's happening? The science
Pure ice melts at 32°F. Adding salt lowers the freezing point of water, so the salty ice melts and the mix gets colder than 32°F, often well below it. To melt, the ice pulls heat from everything around it, including your ice cream mix, which freezes. This is the same reason salt is spread on icy roads.
Think further
Why do cities put salt on icy roads in winter?
What would happen if you used sugar instead of salt on the ice?
