Why do you need salt to make ice cream?

By Marshall Brain


http://youtube.com/watch?v=l9c5rmDQ9DE
http://video.google.com/videoplay?docid=-8492222814591948948

In the video there are two different things to try: 1) Making ice cream, and 2) Measuring the change in temperature when adding salt to ice.

If you would like to try making ice cream at home, here is what you need:

  • 1 cup milk
  • 2 tablespoons sugar
  • 1/4 teaspoon vanilla
  • 6 cups of ice cubes (does not have to be precise - use 8 cups if you want)
  • 1/2 cup normal table salt
  • 1 small (sandwich size) ziplock plastic bag
  • 1 large (gallon size) ziplock plastic bag
Mix the milk, sugar and vanilla together and pour into the small plastic bag. Seal the bag and make sure it is not leaking. Put the small bag into the large bag, add the ice and sprinkle all of the salt over the ice. Now mash the bag for about 10 minutes until you can see that the milk has turned to ice cream. [If you do not mash the bag, the milk will freeze solid and it will be a "milk ice cube" instead of "ice cream".] Pull the small bag out of the big bag, rinse off the small bag (to get the salt off), open the bag and enjoy!

To experiment with different chemicals that could be used to melt ice as shown in the video, you need:

  • A small plastic cup (8 ounce size)
  • 4 ounces of water
  • Thermometer that can measure down to 0 degrees F
  • Different chemicals to melt the ice (e.g. - salt).
Put the water in the cup. Add the thermometer or temperature probe so it is submerged in the water. Freeze the water overnight. Now pull the ice cube out of the freezer. Immediately start graphing the temperature as shown in the video, recording the temperature every minute. Once the temperature of the ice has stabilized at 31 to 32 degrees F, add 2 tablespoons of salt to the ice cube. Now graph the temperature as it falls, as shown in the video.

You can try all sorts of chemicals besides salt. Try sugar, laundry detergent, baking soda, rubbing alcohol (with adult supervision), etc. It is said that chlorine bleach works well, but you definitely need adult supervision for that. DO NOT LET KIDS PLAY WITH BLEACH.

Here's some great information on why salt melts ice:

Have fun performing your own science experiments!