What you’ll need
- 125 ml milk
- 125 ml cream
- 1.5 tbsp sugar
- food coloring, e.g., red (optional)
- flavoring, e.g., vanilla (optional)
- approx. 0.5 kg ice cubes
- approx. 120 g salt
- 1-liter freezer bag with pressure/zip seal
- 3.5- or 4-liter freezer bag with pressure/zip seal
- winter gloves
- measuring jug
How to do it
When the bag is shaken, the ice melts and the salt gradually dissolves. This makes the ice water very cold and after a while the milk turns solid.
What’s the secret?
Salt dissolves in water. Because there is also a small thin film of water on the ice cubes, the salt dissolves when it touches the ice.
If this thin film of water is absorbed by the salt and there is still enough salt, the ice on the surface begins to melt again. The dissolving process repeats until there is no salt left.
The dissolving of salt consumes energy in the form of heat. The heat is removed from the environment, i.e., the ice. The saltwater-ice mixture is therefore significantly colder than ice without salt.
When it comes into contact with the cold saltwater-ice mixture, the milk mixture also loses heat, becoming cold and firm.