Why do snow plows put salt on the roads in the wintertime? We’ll do a little experiment to see what’s going on here.
Materials
- Glass
- Ice cubes
- Salt
- String, yarn or thread
- Scissors
- Optional: Food coloring
Be Safe
Food coloring will stain skin and clothes. For best results, work with an adult to supervise and guide you.
Experiment
Cut a piece of string or yarn about 12" long, so that you can place one end in the glass and leave a tail that overhangs the side of the glass. Fill the glass with water and add an ice cube. Notice that the ice cube floats. Place one end of the string on top of the ice cube and leave the other end hanging over the glass. Sprinkle a layer of salt on top of the ice cube and string. Wait for two or three minutes and then carefully lift the string hanging over the edge of the glass. You should be able to lift the ice cube out of the water with the string! If it doesn’t work, try adding more salt or wait a bit longer.
What’s happening?
When water freezes, it goes from the liquid state to the solid state. What we would see if we were the size of atoms or molecules is water molecules moving around really fast in the liquid state. In the solid state, however, the molecules would be organized and almost locked in place. The only motions you would see in the solid state are atoms and molecules rotating and vibrating.
Water freezes at 32 °F or 0 °C. When salt is added to the ice, it lowers the temperature at which the water freezes. This effect is called the “freezing point depression.” So instead of water freezing as it does normally at 32 °F, it might be 26 °F when salt is added. You should have noticed the ice cube start to melt right away. As the ice melts, it must pull energy from its surroundings, making the surroundings colder. This is why your fingers feel cold if you pick up an ice cube. As the “salted” ice cube melts, any water on the string gets colder and refreezes, which allows you to pick up the ice cube with the string frozen on its surface.
Optional
To better see the ice cube beginning to melt as salt is added, drip food coloring on top of the ice. With added color, you should notice water channels forming in the ice as it melts.
Explore more at
https://www.rookieparenting.com/why-does-salt-melt-ice-freezing-point-experiment/
https://www.scientificamerican.com/article/lift-ice-cubes-with-chemistry/
To view more “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts
