All lessons › Kitchen science: squish, bubble and freeze › Lesson 3
Australian primary science · Years 3–4 · Lesson 3 of 4
Ice cream in a bag
Inquiry question: Why does salt help ice cream freeze?
Check practical materials
Fair testing
What students learn
Heat moves from warmer things to colder things. Adding salt makes ice colder, so it takes heat from the cream faster and freezes it.
Success criteria
- I can describe how heat moves from warmer to colder things.
- I can read a thermometer and record temperatures.
- I can compare ice with and without salt.
Preparation and materials
Before this lesson: Students know that liquids freeze into solids when cooled.
- Milk or thickened cream — 2 litres
- Sugar — 1 small bag
- Vanilla essence (optional) — 1 bottle
- Small zip-lock bags — 26
- Large zip-lock bags — 26
- Ice — 3 large bags
- Coarse salt — 1 kg
- Thermometers (digital if possible) — 13
- Tea towels or gloves — 13
- Spoons and cups for tasting — 26
Teacher setup
- Keep ice in an esky until the lesson.
- Pre-measure 1/2 cup milk, 1 spoon sugar and a drop of vanilla into each small bag and seal it.
- Food lesson: use a clean food-only area and new, food-only equipment. Check allergies and your school’s food policy first. Only taste if your teacher says so.
Safety and adaptations
- Food lesson: use a clean food-only area and new, food-only equipment. Check allergies and your school’s food policy first. Only taste if your teacher says so.
- Hold the bag with a tea towel or gloves: salty ice is cold enough to hurt skin.
- Glass thermometers break easily: use digital ones if you can.
- Wipe up spills so no one slips.
Review your school policies and curriculum requirements before teaching.
How to investigate
The lesson sequence should leave pupils with evidence to interpret, not only an activity to finish.
- Half fill both big bags with ice. Add 4 big spoons of salt to one big bag only.
- Measure the temperature of the ice in each big bag. Record it.
- Put one small bag of milk mix into each big bag. Seal the big bags.
- Wrap in a tea towel and shake gently for 5 minutes.
- Measure the ice temperature again. Feel the small bags through the towel. Which has frozen?
- Only taste if your teacher says so.
Teacher modelling prompt
Hold up a sealed small bag of water inside a big bag of plain ice. Show how to put the thermometer into the ice and wait until the number stops changing. Say: “I won’t tell you what salt does. Your two bags will show you.”
Record and explain evidence
Recording question: Does salt make ice colder and freeze ice cream faster?
| Bag | Ice temperature at start (°C) | Ice temperature after 5 minutes (°C) | Did the cream freeze? |
|---|---|---|---|
| ________________ | ________________ | ________________ | ________________ |
Example recording headings; record actual observations, including inconclusive results.
Discussion prompt
Which claim uses the idea of heat moving correctly?
Check understanding
What should pupils know? Which way does heat move between the cream and the ice?
Explain with evidence: Ava’s ice without salt was −1 °C and the salty ice was −12 °C. Which cream froze? Why?
Apply the idea: Why might an esky of drinks get colder faster if you add salt to the ice?
Teacher answer guidance and saved classroom work are available through the main lesson experience where supported.