THE RECIPE
Making Ice Cream in a Ziploc Bag
This recipe makes one serving. You can increase the ingredients proportionally to make more ice cream.
You Will Need:
- 1 Large Ziplock Bag (around gallon sized)
- 1 Small Ziplock Bag (around pint sized)
- 1 Tablespoon Sugar
- ½ Cup milk, cream, or half-and-half (Because ice cream is supposed to be creamy, it is generally better to use milk with a higher fat content. Whole milk will produce richer ice cream than nonfat milk.)
- 6 Tablespoons of Salt (Table salt will still work. However, the larger the salt crystals, the better, so if you can, try to use rock, kosher, or ice cream salt instead.)
- Enough ice to fill the large bag halfway
- Bowl(s) and spoon(s) - Unless you’re just going to scarf it straight out of the ziploc bag like an animal. Then you don’t have to wash anything! (except your dignity, and possibly your clothes)
Optional:
If you want to add flavoring, add ¼ teaspoon of it. Popular flavorings for this recipe include vanilla extract (makes vanilla-flavored ice cream), chocolate syrup (makes chocolate-flavored ice cream), and strawberries (makes strawberry flavored ice cream).
Oven Mitts/Mittens/Gloves - The bag is going to get very cold, so these may be helpful to avoid your hands becoming uncomfortably chilly.
How to Make:
1. Put the milk/cream, sugar, and flavoring (if applicable) in the small ziploc bag. Make sure it is completely sealed.
2. Put half of the ice and salt in the large bag and mix it around so the ice and salt are evenly distributed.
3. Put the smaller bag in the larger bag.
4. Cover the smaller bag with the rest of the ice and salt. Again, mix the ice and salt so it is evenly distributed. The goal is to have an even ice-and salt mixture covering every side of the smaller bag. Once you have that, completely seal the larger bag as well.
5. Shake like your life depends on it. 5-10 minutes should be good, or whenever the mixture in the smaller bag is solid. If you don’t shake long enough, the mixture will still taste good, but it’ll be more of a milkshake than ice cream. If the ice-salt mixture is melting too much, pour out the liquid and add more ice and salt.
What’s happening? What’s the salt for?
This activity is often done in chemistry classes because it explores an important chemical concept: freezing point depression.
Heat is basically kinetic energy on the molecular level. The hotter something is, the more its molecules move. Ice is a solid because its water molecules have very little energy. They must gain energy (heat) in order to start moving more rapidly and break out of their solid crystalline structure. Adding salt to the lowers (“depresses”) the freezing point of the ice. This means that the ice has to absorb more energy from the environment in order to melt, making the ice colder. This rapid cooling allows your mixture inside the small Ziploc bag to freeze.
Large crystals of salt work better because they dissolve more slowly into the ice water. This allows for more even cooling. Seasoning salt (NaCl) is used because it’s a common, easily obtainable ionic compound, but any soluble ionic compound should work. Sugar will not work because it’s a covalent molecule. The added ionic particles move between the water molecules and disrupt their ability to form the crystalline structure necessary to form ice.
Now you can savor your frozen treats even more this summer knowing all the interesting science and hard work that goes into making their deliciousness a reality!