You will need:
1. Mix the milk, cream and topping in the mixing bowl, or you can mix it inside one of the bags.
2. Seal the freezer bag with ice cream mixture by tying a knot or using the Zip-loc bag. Try to get most of the air out of the bag before you seal it.
3. Place the ice cream bag inside another bag, i.e. double bagging to avoid breakage.
4. Mix the salt through the crushed ice in the mixing bowl.
5. Place the tied bag inside the large freezer bag (check for holes- you don't want salty ice cream!).
6. Fill the large bag with the crushed ice and salt mixture. Push out the air and seal the bag.
7. Holding the bag using a hand towel, gently massage or shake the bag to cool the ice cream mixture. Make sure the crushed ice surrounds the inner ice cream bag, it will help with cooling.
8. After 15 minutes your ice cream should be ready!
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Water normally freezes at 0o Celsius, 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 - 6oC, where as a 20% salt solution lowers the freezing point to - 16oC. Salt water therefore needs a lower temperature to become a solid. This means that you end up having 'super cooled water' touching the inner bag, which helps to freeze the ice cream inside.
People use salt to melt ice on roads, an important safety concern for motorists.