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.
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.
For each group of three students:
- One eyedropper
- Supply of water
- A cylinder with 10-milliliter graduations
- Three 12-ounce clear plastic cups
- Masking tape
- Marking pen
- One bottle of food coloring (darker colors will work best)
- A calculator (optional)
- One box of crayons, pastels, or colored chalk
- A notebook for recording results.
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."
- Put 99 drops of water in the graduated cylinder. Record the volume of this amount of water in the notebook. (You will need this measurement later to avoid having to measure another 99 drops.) Pour the water from the 99 drops into the cup marked "Sample 1."
- Add one drop of food coloring to sample 1. Stir the water. Record the color in your notebook using crayons, pastels, or chalk.
- Answer the questions in the question section. You can use a calculator. Write the answers on the sheet or copy the information into your notebook.
- Pour an amount of water equal to 99 drops into the graduated cylinder. Pour this into the cup marked "Sample 2."
- Add one drop of sample 1 to sample 2 .
- Stir and record the resulting color.
- Answer the questions in the question section.
- Pour an amount of water equal to 99 drops into the graduated cylinder. Pour this into the cup marked "Sample 3."
- Add one drop of sample 2 to sample 3. Stir and record the color of the solution.
- Answer the questions in the question section.
- What is the concentration of food coloring in sample 1?
- Can you see the food coloring in sample 1?
- Suppose the food coloring was a harmful substance, how would you "clean" the water?
- What happened to the color of the water in sample 2? Describe and explain.
- What is the concentration of food coloring in sample 2?
- What is the concentration of food coloring in sample 3?
- Can you see the food coloring in sample 3? Explain why or why not.
- How could a parts-per-billion solution be made?
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:
- Are the new substances observable in any way? (Do they form a film, or foam, or is there discoloration?)
- Has there been a change in a Ph test for the acid or base? (Use litmus paper to test the solutions.)
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?
For the Teacher
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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