Learning important principles of chemistry can be combined with making tasty treats by using the process of making ice cream to demonstrate the concept of freezing point depression.
When a solute (such as salt) is added to a solvent (such as water) there are changes in the properties of the resultant solution. These changes are referred to as colligative properties, and describe why salt water boils at a higher temperature and freezes at a lower temperature than fresh water.
Ice cream is essentially just frozen sweetened cream infused with air, and is traditionally made using rock salt and ice to chill the ice cream mix, while the ice cream churn stirs the mix to give it the texture associated with everyone’s favorite delicious frozen treat.
However, you don’t need anything more high-tech than a few food-storage bags to make ice cream, and any future scientist over the age of six can explore chemistry with this simple experiment.
Supplies:
1 gallon size zipper-style food storage bag
1 quart sized zipper-style food storage bag
1/3 cup rock salt
2 cups of ice cubes
1/2 cup whipping cream or half and half
1/2 cup milk
1/4 cup sugar
1/4 tsp vanilla
ice cream add-ins of your choice: chocolate syrup, chocolate chips, sprinkles, diced fruit, etc
How to do it:
1) Mix the cream, milk, sugar, and vanilla until all sugar is dissolved.
2) Pour mixture into a quart sized bag and add desired add-ins
3) Put rock salt and ice into the gallon sized bag, and then placed the sealed quart sized bag into the larger bag as well.
4) Hold onto the top of the gallon bag and shake gently for 5 15 minutes. The bag will get cold enough that you could get frostbite from holding it directly in your hands, so try to only hold the top of the bag, where no ice is.
5) When ice cream reaches the desired texture, remove the smaller bag, and enjoy!
What’s Happening?
Salt, or sodium chloride, mixes with water and the resulting solution has a lower freezing point than fresh water. As water molecules start to mix with the salt, they must absorb more energy from the surrounding environment (the ice cream mix) to melt faster, and will be colder than plain melted water, which provides the right conditions for the ice cream mix to freeze. Shaking the bag traps air between frozen molecules of ice cream mix, which makes the fluffy texture of ice cream
Making it Science
To add educational value to this project, consider having a “control” ice cream bag, and shake a bag of ice cream mix in a gallon bag with ice, no salt. Compare the two bags of ice cream mix after 5, 10, and 15 minutes of shaking.
Also, you can measure the volume of the mix before placing it in the bag and then measure the volume of the ice cream produced.
Experiment with the amount of ice and salt used to make the ice cream and compare that to the time required to freeze the ice cream with a particular combination of salt and ice.
Science can be a lot of fun, and getting kids excited about chemistry doesn’t require explosive fireworks or other showy displays. Simple, at-home activities, like making ice cream in a bag, can provide a positive experience learning science and also make a great memory.
