Summer may be winding down, but there are still some hot days ahead. What better way to cool off than by making your own ice cream – no fancy machine needed!
Materials:
• Ziplock bags (quart and sandwich size)
• 1/2 cup of whole milk, half and half, or heavy whipping cream
• 1 Tbsp sugar
• 1 tsp vanilla extract
• 1/3 cup sea salt
• 2 cups ice
• Optional: sprinkles, chocolate syrup, caramel, fruit, etc.
Experiment:
Using the sandwich bag, add the vanilla, sugar, and milk, ½ and ½, or cream, and seal the bag. You may want to double-bag this one in case of leakage. To the quart bag, add the ice, salt, and sealed sandwich bag and seal the quart bag. Shake and squeeze the bag without breaking it for at least 5 minutes or until the consistency of the milk is that of ice cream. Open the quart bag and rinse the sandwich bag in very cold water. Open the ice cream and enjoy!
What’s happening?
Milk has a freezing point of 31 °F or -0.5 °C. This is colder than the temperature of ice, which is 32 °F or 0.0 °C. So, how can the milk freeze if the temperature of the ice is warmer than the temperature at which milk freezes? As the salt is added, it rearranges the water molecules such that they cannot crystallize into their normal pattern as easily. They pull the energy (heat) from the milk in order to do the reorganization, and that causes the milk to cool.
Salt with water causes a freezing point depression. Ocean water (3.5% salt) has a freezing point of 28 °F or –2 °C. The solution in the bag is approximately 20% salt water, which should have a freezing point of approximately 2 °F or –16 °C. That should be cold enough to freeze the milk. This is the same process that occurs when we throw salt onto the ice in the winter. The ice can’t stay as ice unless the temperature falls well below 32 °F or 0 °C.
Extension:
What happens if you don’t add salt to the ice? Will other types of salt work better or worse (rock salt, table salt, salt for melting snow, Epsom salt, etc.)? What about other substances to use to disrupt the water molecules, such as sugar?
References:
https://waylandlibrary.org/blog/2020/07/14/a-little-something-ice-cream-in-a-bag/
https://www.sciencebuddies.org/stem-activities/ice-cream-bag
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
