Ice cream - it's in the bag!
Materials list, per student or group
- ½ cup of milk (plain or any flavour)
- Ice cubes
- 6 tablespoon of salt
- 1 tablespoon of caster sugar
- Small zip-seal bag
- Large zip-seal bag
- ½ teaspoon of vanilla essence (optional)
- Warm pair of gloves (optional)
At the end of this activity your students will want to test the ice cream, so check for any dietary restrictions before starting. Ensure that the area of the lab or classroom that you use is clean, and wipe down tables before starting. All the equipment needed is disposable for hygiene purposes.
Vanilla essence can be added if you use unflavoured milk.
Make sure the ice cream mixture is securely sealed in the small bag when you insert it into the large bag of salt and ice, otherwise the mixture will taste very salty.
To speed up the freezing process, students may want to squeeze and handle the bag of ice. This will make their hands very cold and start to ache so encourage them to bring in a pair of warm gloves for comfort.
Although it is tempting to keep checking the inner bag, remind the students that the longer they shake the bag, the better the ice cream will be. It should only take about five minutes for the ice cream to set. Feel through the outer bag to check if it is solid.
- How does the ice cream change state?
- Is this change reversible or irreversible? Why is that?
- How has the structure of the ice cream mixture changed?
- Why is the salt important? What is it doing to the ice? Salt lowers the freezing point of the water (a salt solution will freeze at -10 °C).
- Will adding new ingredients change the consistency of the ice cream?
- Why do we need to shake the plastic bags? What happens if you don’t?
Transition to KS4:
- Is ice cream an emulsion, suspension or mixture?
- What properties do salt and ice have?
- How has shaking the mixture changed the structure of bonds in the ice cream?
- How can you make ice cream more quickly? As a demonstration, try putting the small zip-seal bag of flavoured milk into a bowl of dry ice (solid carbon dioxide) or add liquid nitrogen to the milk while stirring. (Please be mindful of health and safety considerations and take all necessary precautions.)
Links to everyday life
Ice lollies have been recorded in history as far back as Ancient Egypt. Roman Emperor Nero would demand snow be brought down from the mountains and mixed with fruits for his enjoyment. Since then ice lollies and ice cream have never gone out of fashion.