Ice and String

Imagine that you are given a glass of water with a couple of ice cubes floating in it. You are given a string and told to pick up the ice cube with the string, without touching the glass, the ice, or the water with your fingers. Could you do it? Of course you could, if you knew the science.

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For this experiment, you will need:

  • a glass of water
  • a couple of ice cubes
  • 2 feet of thread
  • salt

Start by placing the ice cubes into the glass of water.

Without reading any more, try to use the string to lift an ice cube out of the water, but you are not allowed to touch the glass or the ice cube.

Now lets add a bit of science. Lay the end of the thread across the ice cube. You may want to loop the thread a couple of times, to give it plenty of surface area to stick to. Then sprinkle about 1/2 teaspoon of salt on top of the thread and wait about a minute. Then lift up on the string. You will find that the string has frozen to the ice.

What happened? Salt lowers the melting point of water, so when you add the salt, the ice changes from a solid to a liquid. To make that change takes energy, so the ice absorbs heat from its surroundings, chilling the water around it. The water is cooled enough that it freezes the string to the ice cube.

The same process is used in making homemade ice cream. The ice cream mix is placed in a container which is surrounded by ice and salt. This lowers the temperature of the water enough to freeze the ice cream. You can quickly make some homemade ice cream by placing 1/2 cup of milk, 2 tablespoons of sugar, and a 1/2 teaspoon of vanilla extract into a zipper sealed sandwich bag. Seal the bag tightly and then place it into a larger plastic zipper sealed bag, along with 3 cups of ice and about 1/2 cup of salt. Seal the larger bag and start shaking and squishing it to make the ice cream freeze evenly. After about 3-4 minutes, the ice cream should become about as firm as homemade ice cream gets. Open the large bag and remove and rinse the smaller bag, so that no salt gets into your ice cream. Now grab a spoon and enjoy!

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