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You can make ice cream in a plastic bag — for science! This is a fun and tasty food science project that explores freezing point depression.
Please ensure you are dressed properly for this experiment including suitable clothing and shoes, apron and gloves.
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You Will Need:
- 1/2 cup milk
- 1/2 cup heavy whipping cream
- 1/4 cup sugar
- 1/4 teaspoon vanilla or vanilla flavouring
- 1/2 to 3/4 cup salt as table salt or rock salt
- 2 cups ice (Use Distilled Water and freeze)
- Medium sized Ziploc bag
- Large sized Ziploc bag
- Measuring jars and spoons (Choose a big Jar)
- Jars and spoons for eating your treat (Choose a big Jar)
Add 1/4 cup sugar, 1/2 cup milk, 1/2 cup whipping cream, and 1/4 teaspoon vanilla to the medium ziploc bag. Seal the bag securely. Put two jars of ice into the large ziploc bag.
Use a thermometer to measure and record the temperature of the ice in the large bag. Add 1/2 to 3/4 cup salt to the bag of ice. Place the sealed medium sized bag inside the large sized bag of ice and salt. Seal the large sized bag securely.
Gently rock the large bag from side to side. It’s best to hold it by the top seal or to have gloves or a cloth between the bag and your hands because the bag will be cold enough to damage your skin. Continue to rock the bag for 10-15 minutes or until the contents of the quart bag have solidified into ice cream.
Open the large sized bag and use the thermometer to measure and record the temperature of the ice/salt mixture. Remove the medium sized bag, open it, serve the contents into jars with spoons and ENJOY!
How does this work?
Ice has to absorb energy in order to melt, changing the phase of water from a solid to a liquid. When you use ice to cool the ingredients for ice cream, the energy is absorbed from the ingredients and from the outside environment.
When you add salt to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt. This makes the ice colder than it was before, which is how your ice cream freezes.
The sodium chloride causes the ice to absorb more energy from the environment (becoming colder), so although it lowers the point at which water will re-freeze into ice, you can’t add sodium chloride to very cold ice and expect it to freeze your ice cream or de-ice a snowy pavement (water has to be present!). This is why NaCl isn’t used to de-ice sidewalks in areas that are very cold.