STEM
Detecting Atmospheric Carbon Dioxide
This activity is adapted from the LHS GEMS book, “Global Warming & the
Greenhouse Effect” published by the Lawrence Hall of Science at the University
of California at Berkeley in 1990.
The STEM Context
A study of the concentration of carbon dioxide gas in air samples requires an
understanding of effective sampling techniques, the use of pH indicators, an
understanding of chemical reactions that result on the formation of acidic
compounds, and an understanding of the chemical processes that contribute to
the composition and characteristics of Earth’s atmosphere.
Examples of Applicable National Learning Standards:
As a result of activities in Grades 5-8, all students should develop an
understanding of properties and changes in properties in matter. Content
Standard B; The National Science Education Standards; Page 149.
One example of a fundamental concept and principle that underlies that
standard is: “Substances react chemically in characteristic ways with other
substances to form new substances (compounds) with different characteristic
properties.” Page 154
As a result of activities in Grades 9-12, all students should develop an
understanding of chemical reactions. Content Standard B; The National Science
Education Standards; Page 176.
One example of a fundamental concept and principle that underlies that
standard is: “Reaction rates depend on how often the reacting atoms and
molecules one another, on the temperature, and on the properties – including
shape- of the reacting species.” Page 179
Procedural Issues
Students should be reminded to bubble the air sample through the BTB
solution to maximize the opportunity for CO2 molecules to react with water
molecules. The collection of an air sample from a car may require that an adult
collect the sample for students to analyze.
www.umassk12.net/ipy
A STEM ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by
the Climate System Research Center in conjunction with the International Polar Year
Bromothymol blue (BTB)
Source: http://en.wikipedia.org/wiki/Bromothymol_blue
The WikipediA web site is one example of many web sites that provide
information about the use of bromothymol blue as an indicator of a small change
in the pH of a solution
BTB indicator solutions have a blue color when the pH of the BTB solution is
slightly higher than 7.0. As the pH of a BTB decreases, the color of the solution
changes from blue to yellow as the pH changes from 7.6 to 7.0 and then to 6.0.
The use of a BTB solution to analyze air sample provides an opportunity to
discuss the characteristics of weak and strong acids as well as the reversibility of
many chemical reactions as is the case with the formation of carbonic acid.
CO2 +
H2O
↔
H2CO3
Sample Answers to Questions
The calculation of the mass percent concentration of the more dilute BTB
solution provides an opportunity to discuss solution concentrations that include
molarity, normality, molality, and mass percent concentration.
The expression of the mass percent concentration of the more dilute BTB
solution also provides an opportunity to remind students of the general rules
associated with significant figures.
Examples of Answers to Questions
Question 1: Equal volumes of air samples will be collected. The rate at which the
air sample is bubbled through the BTB solution will be the same of all samples.
The air sample will be bubbled through the solution slowly to maximize the
interaction between carbon dioxide and water.
Question 2: 131.98 g − 126.95 g = 5.03 grams of BTB solution
Question 2: 3.03 g x 0.0004 = 0.002 grams of BTB solute
Question 3: 182.04 g − 126.95 g = 55.09 grams of a more dilute BTB solution
Question 4: 0.002 grams of BTB ÷ 55.09 grams of solution x 100 = 0.0036%
Question 5: The accuracy of mass and volume measurements will influence the
accuracy of the calculation. .