Hold up a jar of water with a sample of water from a local source. Ask: How can we tell whether this water is polluted? Then, pass around samples of water (one should be tap water, one with salt added, one should smell different -consider adding vinegar, and one should be an odd color) or ask for a volunteer to come up and examine them. Ask: Would you drink any of these water samples? Record results on board. Ask: If water is clear and doesn’t smell, is it safe to drink? Is it safe to swim in? How is drinking water different from water you swim in? Students might point out that you can swim in salt water but not drink it. Encourage students to think of ways in which we could test the “health” of water that “looks clean” without tasting it or touching it. Ask: What kinds of pollutants might we find in our local streams, ponds, and rivers? Generate a list on the board, and then discuss with students which of these pollutants might cause problems for people (drinking, swimming, fishing) and other organisms.
Hand out the student worksheet, “Salt Water in Streams”.
To help students start thinking about salt pollution in the context of a larger ecosystem, watch the 2 minute video clip from Rochester Channel 6 news on the side effects of road salt: http://www.youtube.com/watch?v=hppc5tH_Tdk.
Hold up an eggplant for all of the students to see. Slice it in half. Ask students to make a prediction about what will happen to the eggplant when you sprinkle it with salt (question #2 on the worksheet). Set up the demonstration and return to the eggplant after the next steps.
Ask: Do all pollutants cause problems to all organisms? Students may have heard of the microbe that eats arsenic, and they may have thought about salt being a pollutant. Point out that concentrations of pollutants are important as well; not just concentrations at the moment but concentrations that build up over time (remind students of the concept of bioaccumulation). Ask: Are there animals that can live in both fresh and salt water? Can all animals live in both fresh and salt water; why or why not?
Hand out the “Case studies” pages and ask students to fill in the chart based on what they have read. Review student answers.
Check for understanding by reviewing the answers to #4, which asks students to evaluate a claim. If this is new terminology for the students, here are a few examples:
Claim: Ice cream is good for you.
Evidence: Ice cream contains calcium.
Reasoning: Since calcium is needed for fat metabolism, and it is found in ice cream, it is good for you to eat ice cream.
A more scientific set:
Claim: Day and night are caused by a spinning earth.
Evidence: A photo taken of the Pole star with a long exposure shows all the stars going around the pole star.
Reasoning: Either all the stars are rotating around the Pole Star (and Earth is not spinning), or the ground on which the camera is sitting is turning.
For this initial lesson, students only need to evaluate a claim, not make their own. An answer key is provided with the lesson resources.
Review student answers to question #5, which explores the ways in which organisms are affected by different salt levels. Return to the eggplant demonstration, and either pass around the eggplant or walk around the room and show it to students. There should be a lot of water on the surface of the vegetable. Ask students to explain why this happened and write down their ideas in question #6. Student answers should focus on the process of osmosis in organisms, which will help them understand why chloride is toxic to aquatic organisms.