Making ice cream in a bag is a fun, tasty science project that both kids and adults enjoy. It’s not just an innovative way to whip up a delicious dessert—it’s also an educational experience that demonstrates fascinating concepts of science. The beauty of this project is that it doesn’t require a freezer or an ice cream machine. All you need are some everyday ingredients and materials, a love for ice cream, and a touch of curiosity about the world of science!
Ice Cream in a Bag Materials
Here’s a list of ingredients and materials you’ll need to make homemade ice cream in a bag:
- 1 cup half-and-half (or milk for a lighter version)
- 2 tablespoons granulated sugar
- 1/2 teaspoon pure vanilla extract
- Mix-ins like chocolate chips, fruit, or cookie crumbs (optional)
For a vegan variation, you can use a non-dairy milk alternative like almond, soy, or oat milk. Whatever milk you choose, a product with a higher fat content produces a creamier ice cream. You might get ice crystals if you use low-fat or skim milk.
For the Ice-Salt Mixture:
- 3 cups ice
- 1/3 cup rock salt or kosher salt (we’ll talk more about why we use these specific types of salt later)
The quantities of ice and salt are not critical and you don’t need to measure them. Basically, you fill the bag with ice and sprinkle on salt. Rock salt or kosher salt are ideal for this project because they have a larger grain size, so they mix well with ice rather than just sinking to the bottom of the bag. But, if regular table salt is what you have, it works just fine.
- 1 small zip-top bag (about a pint/quart size)
- 1 large zip-top bag (about a gallon size)
- Gloves or a towel to protect hands from the cold
Let’s Make Ice Cream!
Here’s how you can create your very own ice cream in a bag:
- In the smaller bag, combine the half-and-half (or milk), sugar, and vanilla extract. Squeeze out excess air and seal the bag tightly.
- Fill the larger bag halfway with ice, then sprinkle the rock or kosher salt over it.
- Place the sealed smaller bag into the larger bag with the ice-salt mixture.
- Seal the larger bag. If you’re using gloves, put them on now. If you’re using a towel, wrap it around the bag.
- Shake the bag vigorously for about 5-10 minutes, or until the mixture in the smaller bag hardens to a consistency similar to that of soft-serve ice cream.
- Carefully remove the smaller bag, being sure to wipe off any salt before opening it.
- At this point, add in any mix-ins you like, or even flavor your ice cream with chocolate or fruit puree.
The final product has a creamy, soft-serve consistency and a deliciously simple vanilla flavor that serves as a great canvas for various mix-ins and flavor variations.
The Science Behind It
This fun and tasty project showcases an important scientific concept called freezing point depression. When you add salt to the ice, it lowers the freezing point of water, which is normally 0°C or 32°F, down as low as -21 °C or -5 °F.
How does this work? Salt is sodium chloride (NaCl), which separates into sodium (Na+) and chlorine Cl–) ions. These ions interfere with water molecules getting close enough together for hydrogen bonding, which play a significant role in freezing.
So, as the ice melts, it dissolves some of the salt. Dissolving salt in water is an endothermic reaction that absorbs heat from the environment, which includes the ice cream mixture. The ice cream mixture in the bag actually freezes faster than it would in a regular freezer.
In this process, the type of salt matters. Salts that break into more than two ions lower the freezing point of water more than sodium chloride does. For example, calcium chloride (CaCl2) breaks into one calcium ion and two chlorine ions and lower the freezing point down to -29 °C or -20 °F. Sugar and other covalent compounds also dissolve in water and lower the freezing point. But, since they only dissolve into molecules rather than ions, their effect is not as significant.
There’s always more to discover with this project! You can experiment with different types of salts—like table salt, sea salt, or even Epsom salt—to observe their effects on the freezing process. You might find differences in how quickly the ice cream forms, or how smooth the final product is. Also, explore variations in the recipe, such as adding chocolate or other flavorings to the mixture. Enjoy the exploration and have fun experimenting!
- Atkins, Peter (2006). Atkins’ Physical Chemistry. Oxford University Press. ISBN 0198700725.
- Pedersen, U.R.; et al. (August 2016). “Thermodynamics of freezing and melting”. Nature Communications. 7 (1): 12386. doi:10.1038/ncomms12386
- Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry (8th ed.). Prentice-Hall. ISBN 0-13-014329-4.