Freezing Point of Water

What you need:

• Water
• Ice cubes
• Salt
• String
• Cup

What to do:

1. Fill a cup with water, ice and a string.
2. Place the wet string on top of the floating ice cube.
3. Carefully pour a teaspoon of salt on top of the string and watch closely and the wet string freezes to the ice cube.
4. Slowly lift the ice cube out of the water with the string now attached.

What’s the Science?

When the salt is sprinkled on the ice cube, it lowers the freezing point to below 32 degrees F. Since the ice cube started out at 32 degrees and cannot get any colder, the surface of the cube will melt a little bit. The string is now lying in a puddle of melted salt water. As the cube continues to melt, the salt water solution is diluted and the freezing point rises. The water around the string refreezes and attaches the string to the ice cube.

This “depression” by salt of the freezing point of water is the basis of the use of salt to melt ice on roads during the winter and also the use of ice and salt in an old-fashioned ice cream maker.

Standards Alignment:

Ohio, Physical Sciences, 3-5, A

Michigan, Changes in Matter, Elementary, IV.2