Purpose: To allow students to
observe what happens when salt is added to
icewater. This will help them understand . .
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.
Materials per group (shown in photo below)
Here's what to do.
1. Explain the procedures (see below) to the students.
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.
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.)
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