Water and salt can make for some great basic chemistry activities. Your group will see what happens to water and ice when a little salt gets thrown into the mix. Note: part of this activity shows results one day later. As an alternative, you can have the kids prepare the cups of water ahead of time, perhaps at the end of the day before you do this activity (see steps 3 and 4 below).
Ever think about why salt is so effective on snowy roads and sidewalks? It’s not really that the salt melts the ice. It lowers the temperature at which water freezes into ice—zero degrees (32° F)—thereby preventing the snow from turning into an icy glaze. The main difference between water and ice is the speed of the molecules. In water, the molecules are moving around more quickly. (Think of boiling water. Then, the molecules are really jostling around.)
Less heat and slowing down the molecules a certain amount results in water becoming ice. When you lay a piece of string on an ice cube and sprinkle salt on it, it lowers the freezing temperature and melts the ice in that area. As the salt sinks its way through the ice cube, the water above it refreezes.
Have kids sit in a circle. Have them pass an ice cube around the circle as you discuss a few things about water and ice. They may notice that the ice cube is slowly melting, or that they have water on their hands. Ask them: “Do you know what is happening to the ice cube and why?” (They should answer that the ice cube is melting with the help of warm hands.) Ask: “How do you make ice cubes in the first place? (Take this opportunity to introduce the three different states of water: solid [ice], liquid [water], and gas [fog or steam].)
Then ask: “Do you think there are other things that can make ice melt? Something other than temperature?” (You may have to bring up salt and how it is used on icy sidewalks and roads.)
Now, do the first part of the activity as a group (this can be done ahead of time, perhaps at the end of the previous day). Tell students: “We will do an experiment that involves freezing water.”
Give each pair of kids a Cool Idea activity sheet and pencil and point them toward the student web page.
Fill two cups each halfway with water and put a tablespoon of salt into one of them. Ask your group: “What do you think will happen with each cup of water?” Ask the kids to write their hypotheses in their science journals and talk about their predictions if time allows.
Now students are ready to do the second activity on their own. Give each pair of kids an ice cube, a piece of string, a paper plate, and a little pile of salt.
Since your kids may not know about hydrogen bonds yet, after listening to their observations, you may want to offer up that after the salt melts the ice around the string, it refreezes. That’s why the string gets embedded into the ice cube.
If you live in a place where there is snow or ice in the winter, you probably see trucks sprinkling salt on the road. Have you ever wondered, "Why salt? Why not sugar or flour or baking soda?"