One in a Million
1. Explain to the class that tap water can contain various substances besides the water itself. For example, certain minerals, including iron, chloride and sodium, may be in tap water in different concentrations. Water is often tested to see what minerals it contains. In these tests, scientists look closely at the concentrations of substances in the water. They determine whether the concentrations of certain substances are at safe levels or are too high. The amount of any mineral or other substance in water is often expressed in parts per million or ppm. Tell the class that they will be doing a series of dilutions to see just how small a concentration of a few parts per million is.
2. Divide the class into pairs and give each pair a set of materials. Have each pair label their cups (1 through 7) and line them up in a row. Have each pair place ten tablespoons of mouthwash in cup 1. They then remove one tablespoon of the mouthwash from cup 1 and add it to cup 2. Have them rinse the measuring spoon with tap water, then add nine tablespoons of water to cup 2. Mix thoroughly.
3. Have the students use their measuring spoons to remove one tablespoon of dilute liquid from cup 2 and add it to cup 3. Rinse the measuring spoon thoroughly with tap water and add nine tablespoons of clean water to cup 3. Mix thoroughly. Have the students observe what is happening to the color of the solution as the mouthwash is diluted. Ask them when they think the solution will appear clear. Have them record their predictions.
4. Repeat the dilution procedure for cups 4-7. Each cup receives one tablespoon of liquid from the preceding cup and nine tablespoons of clean water.
5. Have the students look carefully at the seven cups. What do they notice about the colors of the solutions? Record observations on the Dilution Chart. How did the observations compare with the predictions?
6. Ask the students if the undiluted mouthwash has a strong odor and / or flavor. Can they detect any scent in cup 2? In cup 3? In which cups are they unable to detect any odor? What about the taste?
7. Have the class work together to determine the concentrations of the various cups in parts per million. Explain that cup 1 is 100% mouthwash (a million parts per million). In cup 2, one of the ten tablespoons are mouthwash; it is a 1/10 concentration (100,000 parts per million). Cup 3 is one tenth the concentration of the preceding cup. Can they figure out the concentration of cup 3? (1/100 or 10,000 parts per million.) Have the students continue until they have figured the concentrations of all seven cups.
8. Relate this dilution experiment to the actual concentration of substances in tap water. Find out some acceptable concentration levels for common minerals and other substances (see Maximum Allowable Levels chart).