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

In this activity, students will use a dilution experiment to understand the concept of part-per-million (ppm) and part-per-billion (ppb) measurements. Through discussion, they will be able to relate these dilutions to concentrations of gases in our atmosphere.

Background

Certain gases, such as the greenhouse gases (for example, carbon dioxide, water vapor, methane, and ozone), occur in the atmosphere in miniscule amounts. In a random air sample from the troposphere, 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).

Gas concentrations can also be expressed in mass units. With gases in the atmosphere, we usually think in terms of volume and may express this as parts per million by volume (ppmv). Some substances occur in such small amounts that scientists measure them in even smaller amounts such as parts per billion by volume (ppbv) or even parts per trillion by volume (pptv).

Because these measurements are very important to atmospheric scientists, it is useful for students to realize just how important even vanishingly small amounts of certain gases can be. This exercise is designed to give students an appreciation of how many dilutions it takes to achieve a part-per-million dilution of a common substance (food coloring).

You may want to share some of the following interesting comparisons with your students and encourage them to come up with their own:

Part per million:

• 1 12-oz can of soda pop/30-meter swimming pool
  
  
• 1 3-oz chocolate bar/football field
  
  
• 1 bogey/3,500 golf tournaments

Part per billion:

• 1 square foot/36 square miles
  
  
• 1 bad apple/1,000,000 barrels of apples
  
  
• 1 pinch of salt/10 tons of potato chips

Part per trillion:

• 1 square inch/250 square miles
  
  
• 1 postage stamp/an area the size of Dallas, Texas
  
  
• 1 flea/360,000,000 elephants
  
  

Learning Goals

1. Students will understand the concept of part-per-million (ppm) and part-per-billion (ppb) measurements.
   
   
2. Students will appreciate the small scale of ppm and ppb measurements, as compared to the concentration scales that they may be familiar with.
   
   
3. Students will relate the concepts of ppm and ppb to the concentration of gases in our atmosphere.
   
   

Alignment to National Standards

National Science Education Standards

• Unifying Concepts and Processes, Grades K to 12, pg. 118, Paragraphs 3 and 4: "Different systems of measurement are used for different purposes. Scientists usually use the metric system. An important part of measurement is knowing when to use which system. For example, a meteorologist might use degrees Fahrenheit when reporting the weather to the public, but in writing scientific reports, the meteorologist would use degrees Celsius. Scale includes understanding that different characteristics, properties, or relationships within a system might change as its dimensions are increased or decreased."

Benchmarks for Science Literacy, Project 2061, AAAS

• Common Themes, Scale, Grades 9 to 12, pg. 279, Item #1: "Representing large numbers in terms of powers of ten makes it easier to think about them and to compare things that are greatly different."
  
  

Grade Level/Time

• Grade level: 6 to 10
  
  
• Time:
  
  
  • Teacher introduction: 10 minutes
    
    
  • Student activity: 25 minutes
    
    
  • Discussion/assessment: 15 minutes
    
    
    

Materials Per Team

• Ice cube tray (preferably white)
  
  
• Water jug filled with water
  
  
• Three small plastic cups
  
  
• Pipette or eye dropper
  
  
• Food coloring
  
  
• Marker
  
  
• Data chart
  
  

  Cell	1	2	3	4	5	6	7	8	9	10
  Color
  ppm	1,000,000	100,000

  
  

Procedure

Instructions are provided in the student guide.

Have students work in pairs. They should fill out the data chart as they proceed with the activity.

Assessment Ideas

Ask students to creatively prepare a drawing that shows the abundances of five of the most important gases in the earth's atmosphere. They may use pictures, charts, graphs, or drawings.

* = Greenhouse gases

Modifications for Alternative Learners

• Serial dilution activities tend not to be a problem for English Language Limited (ELL) learners, although the questions may prove challenging for some. Pairing ELL students with others may help.