How does salt affect icewater?

• why salt is spread onto icy roads
• why salt is used to make ice cream
• why the water that passengers of the Titanic entered was colder than the freezing point

With most lab-activities I give the students a handout that includes pre-lab questions, procedures, data tables, follow-up questions, etc. Occasionally I like to get away from this and have the students listen to instructions and make their own tables and graphs. This activity works well with this approach.

• Two 8 oz. styrofoam cups filled with ice. We use small cubes from our athletic department's training room (1 cm x 2 cm x 2 cm).
• Two plastic teaspoons. . . one (keep dry) to be used for adding salt and the other to be used for stirring.
• Two thermometers. We use the short, cheap ones (see photo).
• Another cup containing salt (about 1/3 full).
• Clock with a second hand or stop watches.
• cold tap water.

1. Explain the procedures (see below) to the students.

• A. Fill two styrofoam cups with ice. Add cold water to fill in the space around the ice. The cups should be almost full. Place a thermometer in each cup as shown in the photo. Get a starting temperature and record it on the data table.
• B. One cup of ice will serve as the control. Add about half a teaspoon of salt to the other cup and stir it in. This should take 10-15 seconds. Let the two cups set for one minute and then take the temperature of each. Record these on the data table.
• C. Add another half teaspoon of salt to the same cup, stir, and then let set for a minute. Take the temperatures.
• D. Repeat step "C" until ten minutes have passed.
• E. At the end of the ten minutes place a finger from each hand into each cup for about 15 seconds to see which feels colder.

2. The students will need to use their own paper. On the paper they should include a data table, a line graph (two lines), observations, conclusion(s), and a theory. You may want to walk them through the design of the data table and the graph.

• The data table should include a starting temperature for both the experiment and the control (cup without salt). A temperature will be taken every minute for ten minutes.
• The graph will have time on the horizontal axis and temperature along the vertical axis. The temperatures should range from -12 C to +12 C, with 0 C in the middle.
• Observations: What do you notice during the activity when you compare the ice and water in the two cups?
• Conclusion: Do the results prove anything?
• Theory: How do you explain what happened?

1. Results will vary. Typically, the control may end up between 0 C and 3 C, whereas the cup that salt was added to may get as cold a -12 C. The students will notice the difference when they place their fingers in the cups.

2. A good experiment includes only one "variable". Ask the students if this activity had only one. They should realize that there were two . . . the addition of salt to only one of the cups, and the fact that only one of the cups was stirred. Ask them how they might design the experiment to eliminate this variable. (answer: stir both cups)

3. Explain why the temperature of the one cup dropped so low. The addtion of salt lowers the freezing point, forcing the ice to melt. To make this phase change, (from solid to liquid) the water molecules absorb heat from their surroundings. This causes the extreme cold observed in the cup. Ask them why adding salt to a glass of water (that doesn't contain salt) will not make the water colder? (answer: There is no phase change taking place.)