You will need:
- Half a cup of milk.
- Half a cup of cream.
- Chocolate or strawberry topping.
- A medium strong plastic freezer bags
- 2 Zip-loc bags.
- Half a cup of salt
- Crushed ice and a hand towel
- A mixing bowl
- A spoon
Mix the milk, cream and topping in the mixing bowl.
Pour the mixture into a Zip-loc bag.
Squeeze the air out of the bag and seal it.
Place the bag with the mixture into another bag, and seal it again.
In the mixing bowl, mix the crushed ice and table salt.
Place the Zip-loc bag containing the mixture into a freezer bag.
Fill the freezer bag with the crushed ice and salt mixture.
Place your sealed bag of cream mixture into the freezer bag with crushed ice and salt. Push out the air and seal the bag.
Holding the bag in a hand towel, gently massage or shake the freezer bag to cool the ice cream mixture. Make sure the crushed ice surrounds the inner Zip-loc bag.
After about 15 – 20 minutes of gently moving the ice around the bag of cream, your ice cream is ready to eat!
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Why Does This Happen?
Water normally freezes at 0oCelsius, however, adding the salt causes the freezing point of the ice to be lowered. A 10% solution of salt in water lowers the freezing point of water to -6oCelsius, whereas a 20% salt solution lowers the freezing point to -16oCelsius. Saltwater, therefore, needs a lower temperature to become a solid. This means that you end up having ‘supercooled water’ touching the inner bag, which helps to freeze the ice cream inside.
Variables to test
- With gloves on, try dry ice instead. Does this work better or worse?
- Try the experiment without salt.
- Try pure thickened cream vs adding some water to it. Does this change the texture and freezing time?
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