Source: USGS LearningWeb
To develop an understanding of parts per million as a concept, teams of students will create successive dilutions of a solution to reach a parts-per-million concentration.
The atmosphere is a mixture of gases. Similarly, the world's oceans and fresh waters contain dissolved chemicals. Many substances dispersed in air or water are measured in parts per million. Some of these substances are colorless, odorless, and tasteless, yet even in small quantities they can be toxic.
For each group of three students:
The procedure can be copied and handed out to students.
Before beginning the activity, put a piece of masking tape on each cup and label them "Sample 1," "Sample 2," and "Sample 3."
Once the students are familiar with the procedure required to create a parts-per-million solution of a pollutant, have a selection of substances available for them to dilute and observe. Encourage the students to create experimental tests for determining if other substances are observable in the part-per-million concentration. Some suggested substances to experiment with are detergent and acid (vinegar). You can ask:
Answers will vary.
Discussion note: Is a diluted substance "gone" just because it is no longer visible? How can these ideas be transferred from a liquid to a gas like CO2?
Answers to the Questions in the Lesson
Sample 1: Because you have added one drop of food coloring to 99 drops of water, the concentration is one part per hundred, which can also be expressed as 1/100 or 1 percent. A calculator can be used to visualize the answer. Divide 1 by 100. The answer is 0.01. The color should be visible.
Students might answer that filtering the water through a substance like sand or through paper might "clean" it, but filtering will not remove a chemical solution. The teacher might use this question as an opportunity to discuss the removal of CO2 from the atmosphere. Just as no such simple process as filtering the water will remove food coloring, no simple process will remove excess CO2 from the atmosphere. Reducing the amount of CO2 emitted by human activity reduces the need to remove it later.
Sample 2: To 99 drops of new water, you add a drop of the solution from sample l, which consists of .99 parts water and .01 part food coloring. Because you have now diluted the .01 drop of food coloring in a total of 100 drops of solution, divide .01 by 100 on the calculator. Your answer is .0001. This means you now have 1 part food coloring in ten thousand, or 1/10,000. Depending on the color used, the food coloring in sample 2 should be faintly visible.
Sample 3: Again you have 99 drops of new water and one drop from the solution in sample 2. The one drop is .9999 parts water and .0001 parts food coloring. To calculate the concentration of food coloring in sample 3 divide .0001 by 100 (the total number of drops in the solution). The answer is 0.000001 or one part food coloring in one million (1/1,000,000). The food coloring will not be visible at this concentration.
Making a parts-per-billion sample: Continue the procedures described above. Begin with 99 new drops of water. Use one drop of the parts-per-million solution. You will get 0.00000001 parts food coloring or one part food coloring in one-hundred million (1/100,000,000). For the final step, take nine new drops of water and add to it one drop of the previous solution. This yields 0.000000001 or one part per billion.
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