AUSTIN, Texas - WHAT YOU'LL NEED:
- Half-and-half creamer
- Vanilla extract
- Two Ziploc bags (one quart size and one gallon size)
- 4 cups of ice
- 1/2 cup of rock salt
- A digital thermometer if you want to check the temperature of the bag
- Mix 1/2 cup of half-and-half into the smaller bag with 1 tbsp of sugar and a 1/4 tsp of vanilla extract
- Seal it well and put it to the side
- Pour ice and rock salt (or regular salt) into the larger bag
- Place the sealed quart size bag into the larger bag and seal it
- Shake the bag for 5 minutes making sure to hold the bag at the top where the zipper is
- After 5 minutes, open the bags
The mixture in the smaller one should have frozen into ice cream! Also, the temperature of the ice should be between 10 and 20 degrees.
Here's what's going on. When you add the salt to the ice, the sodium and chloride ions get in the way of the bonds between the water molecules that form the ice. As those bonds break, the ice begins to melt. Now you have liquid water at temps lower than 32 degrees, so you're actually lowering the freezing point by adding salt.
Now when something melts, it absorbs energy from the environment. There always has to be a balance so as something absorbs energy, something has to lose that energy. In this case, that's the mixture in our smaller bag. As it loses energy, the temperature falls, causing the mixture to change from a liquid to a solid: ice cream!