Did you know you can make homemade ice cream in a bag in less than 10 minutes with just milk, salt and ice?
This easy kitchen science experiment involves lots of interesting chemistry and you end up with a great tasting dessert at the end!
What is ice cream?
Ice cream is made up of droplets of fat from milk jumbled up with millions of tiny crystals of ice and pockets of air.
We’re going to use the freezing power of salt and ice to create ice crystals in milk without a freezer!
Homemade Ice Cream In A Bag
A large bag of ice
Milk – we used chocolate milk
A tablespoon sugar and a tea spoon vanilla essence – optional
How to make ice cream in a bag
Pour about a cup of milk into a ziploc bag. Add the sugar and vanilla essence ( or just use flavoured milk ). Make sure the bag is properly sealed.
Half fill a bigger ziploc bag with ice and add a good amount of salt.
Put your milk bag into the ice bag and give it a good shake.
Try and keep the milk in contact with the ice as much as possible.
Keep rolling the ice over the milk, you should feel the ice in the bag get VERY cold, so might need to put a towel around the bag.
Check the milk after 5 minutes, it should be a similar consistency to ice cream, but if not keep going for a bit longer.
Why does ice cream in a bag work?
Pure water freezes at 0 degrees Celsius. The addition of salt lowers the freezing point by a few degrees ( freezing point depression ). This means when salt is added to the ice in the outer bag, the ice (which is at 0°C) is above its freezing point, so starts to melt. Melting needs energy which in this case comes from the milk mixture in the inner bag. Heat energy is absorbed from the milk making ice crystals form between the tiny fat molecules.
The more salt you add to ice, the lower the freezing point. For the ice to melt heat must be absorbed from the surroundings ( in this case the milk mixture ) causing it to freeze.
You should find the ice feels VERY cold, which is why you’ll need a towel to cover the bag after a few minutes.
Practical applications of salt – why is salt added to roads in cold weather?
During cold weather salt and grit are applied to roads, the salt makes the ice melt even if the air temperature is below freezing point.
Do you think a solution of ice and salt would freeze in a normal freezer?
Watch our video here: