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Freezing Point: Why We Sprinkle Salt on Icy Roads

By — John Wiley & Sons, Inc.
Updated on Dec 13, 2010

If you live where there is lots of ice and snow in the winter, you may have noticed that the highway department spreads salt on the roads to prevent automobile accidents. Salt lowers the freezing point of water, the temperature at which water changes from a liquid to a solid, causing ice to melt. This activity will illustrate the effect of salt on ice.

Materials

Large ice cube; Small bowl; Table salt; Small cup of water; Piece of thin string about eight inches long

Freezing Point

Activity

  1. Place the ice cube in the bowl and put the bowl on a table in front of you.
  2. Dip one or two inches of the piece of string in the water to get that end of the string thoroughly wet.
  3. Gently place the wet part of the string on top of the ice cube and sprinkle salt over the wet string until the salt completely covers the part of the string that's lying on the ice cube.
  4. Wait about thirty seconds and lift the dry end of the string. What happens to the ice cube?

Follow-Up Questions

  1. What happened to the ice cube when you pulled upward on the string?
  2. What do you think salt had to do with the success of this activity?
  3. When making homemade ice cream, you add salt to the ice in the churn. Why is salt needed? What does the salt in an ice cream churn have to do with this activity?

Answers

  1. he string stuck to the ice cube.
  2. The salt lowered the freezing point of the ice, causing it to melt and refreeze around the string.
  3. The salt lowers the freezing point of the ice, so that you can make ice cream quickly. The salt causes the ice around the canister of an ice cream churn to melt. The lower freezing point of the melted ice draws heat from the ice cream mixture and results in the rapid cooling and freezing that causes the ice cream to solidify.

Extension

Repeat the activity using different types of salt. In this activity, you used table salt. Try using rock salt, ''No Salt'' (potassium chloride), and calcium chloride. Do they all work the same as the table salt?

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