Salt and Ice
So You Want to Do a Project about Ice!
To demonstrate the effect of salt on ice.
- 2 saucers
- 2 ice cubes
- 1/2teaspoon (2.5 mL) salt timer
- Use the tape and marker to label one of the saucers "Salt" and the other "No Salt."
- Place one ice cube in each saucer.
- Place the salt on the ice in the Salt saucer.
- Place both saucers in a freezer.
- Observe the contents of the saucer every 10 minutes for 30 or more minutes.
The ice covered with salt begins to melt.
When salt and water are mixed together, the salt dissolves, which means the salt breaks into small particles that thoroughly mix with the water. A solution made with a liquid solvent is called a liquid solution. The temperature at which a liquid freezes (changes to a solid) is called the freezing point. A liquid solution of salt water has a lower freezing point than water alone. The greater the concentration of the salt solution, the lower its freezing point.
At or just below freezing temperatures, ice is said to have a "wet" slippery surface. But at temperatures well below freezing, ice has a "dry" surface and is barely slippery. This is because as the ice gets colder, the surface molecules are bound together more tightly. So when salt is sprinkled on the surface of "wet" ice, the salt dissolves in the more loosely bound molecules on the surface layer of the ice and lowers the freezing point of the water on the surface. Even though the solution is at or slightly below the freezing point of water, the salt water does not refreeze. At very low temperatures, it is difficult to melt ice with salt because the ice has a dry surface and salt cannot dissolve in the tightly bound surface water molecules. So the ice doesn't melt.
The ice cube in the No Salt saucer is called a controL A control is a test in which all of the variables (things that can have an effect on the results of an experiment, such as containers, light, and heat) are the same except one. In this experiment, salt is the variable that is different. By omitting salt from the control, it can be determined if salt is the variable that causes the ice to melt.
For Further Investigation
In wintertime, both salt and sand are used to keep cars from slipping on icy roads and people from slipping on icy sidewalks. Salt applied to an icy sidewalk will melt the ice, but the salt may damage plants along the sidewalk. This is why sand is often used. Sand increases friction, but does sand melt ice? A project question might be, What effect does sand have on the melting of ice?
Clues for Your Investigation
- Repeat the investigation, using salt on some ice cubes and sand on others. Use equal amounts of sand and salt with equal amounts of ice.
- Record the appearance of the ice in each sample after specific time intervals. Photographs of the ice cubes could be taken to represent the results.
References and Project Books
Churchill, E. Richard. 365 Simple Science Experiments with Everyday Materials. New York: Black Dog & Leventhal,1997.
Reader's Digest. Why in the World? Pleasantville, N.Y.: Reader's Digest, 1994.
Smith, Alastair, ed. The Usborne Big Book of Experiments. London: Usborne, 1996.
VanCleave, Janice. Janice VanCleave's Chemistry for Every Kid. New York: Wiley, 1989.
Science around the Year. New York: Wiley, 2000.
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.