We realize it’s not summertime just yet, but that’s no reason why we can’t enjoy America’s favorite dessert. Each year, the average American eats an average of 23 pints of ice cream. I am definitely one of those people! Wouldn’t you just love some ice cream with chocolate syrup, sprinkles, and whipped cream right about now? Mmmm!
Check out our previous post about ice cream. It gives you the recipe to make your own at home. Get creative and invent new flavors!
Steve Spangler explains the science behind making ice cream. Check it out!
Did you know: Sugar makes the dessert sweet, but it also serves another important purpose. In the freezer, plain cream turns into a solid that’s hard as a rock. Sugar helps keep the ice cream softer by lowering the mixture’s temperature.
I am a member of the AmeriCorps team at the Providence Children’s Museum, and we just finished running an ice-cream making program a few weeks ago! We had a number of conversations about the science of ice cream, specifically about the ice/water/salt mix that’s used to cool down the freezing cream very quickly.
Like the video mentions, adding salt makes the temperature of the water/ice slurry drop below 0C. But why? We knew that salt lowered the *freezing point* of H20, but if that were the whole story, it wouldn’t change temperature: changing the freezing point would just make the ice water melt, not get colder.
We finally figured out that it’s because the ice and water mixture tends to reach an equilibrium, where individual water molecules in the mixture are becoming frozen or liquid at equal rates. This equilibrium happens because both freezing and melting involve a transfer of energy (i.e. heat): melting takes energy to occur, and freezing gives off energy. The freezing and melting rates in an ice water mixture will tend to become equal to balance out those energy transfers.
Basically, when you add salt (lowering the freezing point), you push the mixture out of equilibrium – more melting than freezing is suddenly going on. The extra melting that is happening requires extra energy/heat, which is pulled from the ice water itself, causing a temperature drop. As the temperature drops, the rate of freezing begins to rise again. Equilibrium is achieved once the mixture gets cold enough that the freezing and melting rates are the same again.
More info on equilibrium and the salt water temperature drop:
http://www.worsleyschool.net/science/files/saltandfreezing/ofwater.html
http://www.chillyphilly.com/salt.html
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml
Melissa – thanks so much for this awesome information! I hope that you had to do many taste-tests during your experimentation process!