Here is a simple experiment which you can do with your little ones.
All you need is:
- A large bag filled with icecubes
- salt
- milk – plain or flavoured
- 1 tablespoon sugar
- 1 teaspoon vanilla essence (optional)
- Ziploc bags
- Tea towel
Method:
- Pour 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 the milk bag into the ice bag and give it a good shake.
- Keep the milk in contact with the ice as much as possible.
- Keep rolling the ice over the milk, the ice in the bag will get VERY cold, so put a towel around the bag to protect your hands.
- Check the milk after 5 minutes, it should be a similar consistency to ice cream, 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 that is added to the 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.
The ice will feel VERY cold, which is why you’ll need a towel to cover the bag after a few minutes.
You can also follow this video, with regards to the experiment:



























