Ice Cube on a String {Learning about Freezing Point Depression}

IMG_0127It’s been a few days since I posted a new experiment. I hope you’ve been experimenting without us! We’ve been busy preparing for Ruby’s dance recital… and I’ve been resting my pregnant body. This sudden summer heat is taking a lot out of me!

That’s why we decided to do something quick, easy, and COLD. This is a fun experiment that can be turned into a party game. You’ll stump a lot of people when you ask them to get an ice cube out of a glass with only a salt shaker and piece of string – no touching allowed.

 

Ice Cube on a String

Materials:

  • A full cup of cold water
  • Ice cube(s)
  • 1′ long length of string (we used butcher twine from the kitchen drawer)
  • Table salt

Procedure:

  1. IMG_0962Place your ice cube in the cup of cold water. Make sure the water is cold, so that the ice cube doesn’t melt too quickly before you even begin. We learned that through our first failed attempt!
  2. Lay your string carefully across the top of the cup, making sure it has good contact with your ice cube.
  3. Pour salt over the ice cube and string.
  4. Wait a few seconds and grab the ends of your string. Your ice cube and string will have frozen together and voila! You have an ice cube on a string!

 

The Science:

IMG_0964Ice is frozen water. When liquid water reaches 32 degrees Fahrenheit or 0 degrees Celsius, it becomes ice, a solid. That means the freezing point for water is 32 degrees Fahrenheit.

However, when you add salt (a solute) to the water (a solvent), you create a solution, which is two substances mixed together. The salt and water solution must get even colder to freeze into ice, than just plain water alone. This is called freezing point depression. With a 10% salt (and 90% water) solution, it must be 20 degrees Fahrenheit or -6 degrees Celsius to freeze. With a 20% salt (and 80% water) solution, it must be even colder at 2 degrees Fahrenheit or -16 degrees Celsius. Brrrr! As nice as that sounds on a hot, humid summer day like today, that’s reeeeeally cold! Frostbite cold!

IMG_0966If you’ve ever been to or lived up north, you may have seen big salt trucks driving around pouring salt onto icy patches of road or sidewalks. That’s because when the salt mixes with the ice on the ground, it melts. Many people think that the salt somehow heats up the ice and melts it, but that’s not the case! As we talked about above, the salt just lowers the freezing point for the water, converting it back into a liquid instead of allowing it to stay frozen as solid ice. It should be noted that salt can’t melt solid ice directly – it must be mixed with water and then applied to the ice. However, most ice and snow is actually covered in a thin layer of water, so when you apply salt on top, nature has already helped you create your salt water solution with no effort needed from you!

For this specific experiment, the salt lowers the melting point of the water that’s been frozen into the ice cubes. Because there’s so little salt and so much water, the salt doesn’t do a very good job of lowering the freezing point for long, so the water quickly freezes back  into solid ice, trapping your string in the process.

Let me know if you try this experiment – or better yet, if you challenge your friends to figure out how to make it work! We plan on trying to stump a couple of our friends with it this week!

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