Instructor Information
JCE Classroom Activity: #35
Just Breathe: The Oxygen Content of Air Students place small quantities of fine (00) steel wool, available at hardware stores, into a test tube that is then inverted in a beaker of water. The steel wool rusts. Oxygen in the sample of air trapped in the test tube reacts with the iron to form rust. Within 30–45 minutes, the majority of oxygen is used up, but the experiment can be left overnight. A 50:50 mixture of vinegar and water is used to clean the surface of the steel wool before it is inserted into the test tube. A small amount of acid helps to speed the rusting process, but too much can result in the formation of H2, changing gas composition/pressure within the test tube (1). This Activity is adapted from a procedure described by Martins (2), which was based on the work of Birk, McGrath, and Gunter (1); it uses vinegar as suggested in a recent letter to the editor (3). Martins includes additional suggestions and questions for students (2). Instructors may wish to have students graph volume change versus time and perform error analysis. Step 8 asks students to determine percentage of the total height of the test tube represented by the final height of water in the test tube, for example, 28 mm/150 mm × 100% = 19%. Assuming that the height of the test tube is proportional to its volume for its entire length, that the change in the length of the column of air in the test tube is due only to the removal of oxygen, that all oxygen was removed from the air sample, that air is a mixture of ideal gases, and temperature and pressure are constant, this percentage should be equal to the percent oxygen in air.
Integrating the Activity into Your Curriculum Most students are familiar with the formation of rust. This Activity assumes students will come up with a simple form of the reaction 2Fe(s) + 3O2(g) → Fe2O3(s). A more complete representation of the reaction could be used as an example of a redox reaction and could be included in a unit on electrochemistry. Using this reaction, students should see that the decrease in the volume of gas in the test tube is due to the consumption of oxygen. They must then use their knowledge of the physical properties of gases (gas laws, partial pressure) to determine the initial concentration (percent volume) of oxygen in the air sample. This ties in well with atmospheric chemistry. The Public Broadcasting System (PBS) couples a similar experiment and topics such as air pollution with a NOVA program on climbing Mt. Everest (4). The need for supplemental oxygen while mountain climbing is discussed. You might pose a discussion question: if the Activity were done on the top of Mt. Everest, would the results be the same?
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Answers to Questions 1. The color changes from gray to reddish brown. The reaction is the oxidation of iron or rusting. 2. 2Fe(s) + 3O2(g) → Fe2O3(s). 3. The water level rises. Oxygen in the air trapped in the inverted test tube is incorporated into the rust (Fe2O3). The pressure and temperature remain approximately constant, so the removal of oxygen means a decrease in the volume of gas in the test tube, allowing water to rise in the test tube. 4. This is done to ensure that volume or height of gas in the tube is measured at atmospheric pressure. 5. See above. Students may find more accurate ways to measure the volume than the height measurement. 6. See above. Also consider that the end of the test tube is rounded, part of the volume of the test tube is occupied by the steel wool, and the gas in the test tube contains water vapor in addition to air. 7. Students probably should get within about 10% of the accepted value.
This Activity Sheet may be reproduced for use in the subscriber’s classroom.
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About the Activity
Optional In this section students are asked to consider a misconception described by Birk and Lawson (5). A QuickTime movie of the demonstration is provided on JCE Online as a supplement to the article (see http://jchemed.chem.wisc.edu/ Journal/Issues/1999/Jul/PlusSub/JCESupp/supp914.html). The problems titled Hexane #1 and Hexane #2 in General Chemistry Multimedia Problems (6 ) also explore this idea.
References and Additional Activities 1. 2. 3. 4. 5. 6.
Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804. Martins, G. F. J. Chem. Educ. 1987, 64, 809. Braathen, P. C. J. Chem. Educ. 2000, 77, 1410. NOVA Online. Teacher’s Guide, Everest: The Death Zone: http://www.pbs.org/wgbh/nova/teachersguide/everest2/ Birk, J. P.; Lawson, A. E. J. Chem. Educ. 1999, 76, 914. Whisnant, D. M. General Chemistry Multimedia Problems; J. Chem. Educ. Software 2000, SP27. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 78 No. 4 April 2001 • Journal of Chemical Education
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JCE Classroom Activity: #35
Student Activity
Just Breathe: The Oxygen Content of Air Breathe in. Breathe out. You perform this simple action thousands of times a day. What’s in the air you breathe? Air is actually a mixture of gases, one of which, oxygen, is necessary for survival. How much oxygen is in the air? How can you determine the composition of a mixture of colorless, odorless gases? In this Activity, you will determine the concentration (percent volume) of oxygen in air using the chemical reactivity of oxygen and the physical properties of gases.
Try This You will need fine (00) steel wool, vinegar, water, balance, small container, measuring cup, 250-mL beaker, 15-cm test tube, metric ruler, clock or stopwatch, stirring rod, and test tube brush. __1. Weigh a small piece of steel wool, about 0.5 grams, and record its mass. Fill a 250-mL beaker 3/4 full of water. __2. Make a 50:50 mixture of vinegar and water in a small container. 1/8 cup (~30 mL) of each works well. __3. Increase the surface area of the steel wool in step 1 by pulling it apart. Soak the steel wool in the vinegar/water mixture for 1 minute, making sure all of the steel wool is under the surface of the solution. Remove the steel wool and remove drops of solution that may be clinging to it by shaking it over a sink. Be careful not to spatter the vinegar. __4. Spread out the steel wool. Insert it into the bottom half of a 15-cm test tube. Use Test Tube a stirring rod to help position the steel wool in the tube, then quickly invert the test tube into the beaker of water and leave it there. Rest the mouth of the test Beaker tube on the bottom of the beaker and lean the tube against the side of the beaker. Steel __5. After five minutes, move the test tube so that the water level inside the test tube is Wool equal to the water level inside the beaker. Measure and record the height of the Water water in the test tube, then rest it on the bottom again. __6. Measure and record the height of the water in the test tube every five minutes using the procedure in step 5 until the water level stops changing. What happens to the steel wool as time proceeds? To the water level in the tube? __7. When the water level has stopped changing, remove the test tube from the beaker. Remove the steel wool from the test tube. Has its appearance changed? Clean the test tube with a test tube brush. __8. Measure and record the total height of the test tube. Calculate the percent of the total height of the test tube represented by the final height of water in the test tube. How can this be used to determine the percent (volume) oxygen in air?
Questions __1. What change to the steel wool do you observe that suggests a chemical reaction has taken place? What is this reaction called? __2. Write a balanced chemical equation for the reaction the steel wool undergoes. (Hint: to choose the proper product consider the color the steel wool changes to.) __3. What happened to the water level in the test tube? What caused this? __4. Why did step 5 have you move the test tube so that the water levels inside and outside the test tube were equal? __5. By what amount (%) did the volume of air inside the test tube change? (You may need to make additional measurements to answer this question.) __6. What are some things to take into account in question 5 that could affect the final answer? __7. The accepted value for percent oxygen in air is 21%. How closely did your response to question 5 match this?
Optional Consider an experiment in which a candle is placed in a dish of water. The candle is lit, then covered with a glass jar. (You may have seen this demonstrated, your instructor may demonstrate it, or your instructor may show you a video of the demonstration.) After a short time, the candle flame goes out and water rises up into the jar. __1. Explain why the water level rises. __2. How is this demonstration different from the reaction you performed in the first part of this Activity? __3. Could this experiment be used to determine the percent oxygen in air? Why or why not?
Information from the World Wide Web (accessed February 2001) 1. How does iron rust? http://antoine.fsu.umd.edu/chem/senese/101/inorganic/faq/how-iron-rusts.shtml 2. Oxygen. http://www.comptons.com/encyclopedia/ARTICLES/0125/01381120_A.html This Activity Sheet may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
