What Do Concentrations Mean? Comparing Concentrations of Gases in Our Atmosphere

As discussed by your teacher, scientists use measurements such as parts per million (ppm) or parts per billion (ppb) to measure substances that occur in tiny amounts. Certain gases, such as the greenhouse gases (for example, carbon dioxide, water vapor, methane, and ozone), occur in the atmosphere in very, very small amounts. However, just because they are a very small percentage of the atmosphere does not make them unimportant. In a random air sample right outside your classroom, for example, you would likely find only about 350 molecules of carbon dioxide for every one million molecules of air mixture. Scientists would express this amount as 350 parts per million (ppm).

In this activity, you will explore how many dilutions it takes to achieve a part-per-million dilution of a common substance (food coloring). As you learn more about the concentrations of gases in our atmosphere, think about how these dilutions relate.


  1. Label ice cube tray "cells" 1 to 10 with a permanent marker.

  2. Fill the three plastic cups about half full of water for eye dropper (or pipette) cleaning.

  3. In cell #1, place 10 drops of food coloring. This represents a pure substance, or a concentration of 1 million parts per million.

  4. Take one drop of the food coloring from cell #1 and place it in cell #2.

  5. Rinse the dropper in one of the plastic cups to remove all traces of food coloring.

  6. Add 9 drops of clean water to cell #2 and stir the mixture. The mixture is now diluted to 1/10th of the original concentration, or 100,000 parts food coloring per million parts of solution.

  7. Take one drop from cell #2 and place it in cell #3.

  8. Rinse the dropper again.

  9. Add 9 drops of clean water to cell #3 and stir the mixture. How concentrated is the food coloring now, in ppms?

  10. Repeat the above procedure for cells #4 to 10 (remember to clean the dropper between uses). After each dilution, record the new concentration in the cell in ppm.

Observations and Questions

  1. In which cell is the color most intense? Why?

  2. In which cell is the color least intense? Why?

  3. Are there any cells where the liquid is colorless? Is there any food coloring in these cells? How do you know?

  4. Cell #1 contains food coloring with no water added. What is the percent concentration of food coloring in cell #1?

  5. 100 percent can be written as the fraction 100/100. Complete the following fraction so that both sides are equal:

    100/100 = ______/1,000,000

  6. The earth's atmosphere contains 78% nitrogen and 21% oxygen. Write these percentages as concentrations in ppm.

    78% = _______/100 = ________/1,000,000 = _______ ppm

    21% = _______/100 = ________/1,000,000 = _______ ppm

  7. Which of your cells of food coloring is closest in concentration in ppm to nitrogen? Which cell is closest to the concentration of oxygen?

  8. Carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons are gases that affect the temperature of the earth's atmosphere. Their concentrations are listed below. Which of your cells of food coloring is closest in concentration to the concentration of each gas? Convert the ppm concentrations to ppb.





    355 ppm = ___ ppb



    1.7 ppm = ___ ppb



    .3 ppm = ___ ppb



    .0005 ppm = ___ ppb



    .0003 ppm = ___ ppb


  9. How does the concentration of the greenhouse gases compare to the concentration of oxygen and nitrogen?

  10. How can gases such as carbon dioxide and methane, with their small concentrations, have such a large effect on our atmosphere? Give this question your best effort!

When you're finished with the activity, click on Back to Teacher Guide at the top of the page.