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Project Summary

Difficulty	6
Time required	Average (about one week)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	No issues

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Objective

The goal of this experiment is to measure the percentage of oxygen in air samples.

Introduction

We live near the bottom of an ocean of air that surrounds the earth. The atmosphere protects us from harmful radiation from the sun, yet captures enough of the sun's light and warmth to make the planet habitable. Speaking of habitable, the atmosphere also contains the oxygen we need to breathe to support cellular respiration, the metabolic process that provides the chemical energy necessary for life.

How much oxygen is in the air? This project will show you an interesting way to measure the percentage of oxygen in a sample of air in a test tube. The method depends on atmospheric pressure and a chemical reaction that removes oxygen from the air.

So, what kind of chemical reaction can remove oxygen from the air? Oxidation of iron, also known as rusting, will do the trick. Exposed iron will rust in the presence of oxygen and water. As you do your background research, study this chemical reaction, and you will see that oxygen becomes combined with the iron atoms and water to create iron oxides.

You'll use plain, fine steel wool (available at the hardware store) as your source of iron, placing it in the bottom of a test tube. Then, you'll invert the test tube, and mount it so that the mouth is submerged under water. This will trap the air in the test tube, and also provide water vapor for the oxidation reaction. You will have all of the chemicals necessary for the reaction: iron in the steel wool, plus oxygen and water vapor in the air in the test tube. As the iron rusts, oxygen is removed from the air sample in the test tube. With less gas, there will be lower pressure inside the test tube (fewer gas molecules bouncing around, pushing on the walls of the test tube and the surface of the water inside the tube). Meanwhile, your experiment will continue to be under the (more or less) constant pressure of the ocean of air, atmospheric pressure. So what will happen to the water level in the test tube as the oxygen in the air sample becomes sequestered in iron oxide?

That's how you'll measure the percentage of oxygen in your air sample. By measuring the water level at the start of the experiment, and at the end (when the water level has stopped changing), you can take the difference to find out how much oxygen was used to oxidize the steel wool.

How does oxygen content change with altitude? Can you use this method to find out?

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• atmospheric pressure,
• oxidation of iron,
• layers of the atmosphere.

• What are the gases that comprise Earth's atmosphere?
• What chemical reaction occurs when iron rusts?
• Why does the water level in the test tube rise as the steel wool oxidizes?
• Why does the water level eventually stop rising?
• What would happen if a larger piece of steel wool was used? A much smaller piece?

Bibliography

• This NASA website has information on atmospheric pressure:
  Sample, S., 2003. "It's a Breeze: How Air Pressure Affects You," NASA [accessed January 30, 2006] http://kids.earth.nasa.gov/archive/air_pressure/.
• Information on the atmosphere from the website "CloudsRUs.com":
  http://www.rcn27.dial.pipex.com/cloudsrus/atmosphere.html.
• Windows to the Universe Team, 2004. "Layers of the Earth's Atmosphere," University Corporation for Atmospheric Research (UCAR), The Regents of the University of Michigan, [accessed January 30, 2006] http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/layers.html&edu=mid.
• Wikipedia contributors, 2006. "Rust," Wikipedia, The Free Encyclopedia [accessed January 30, 2006] http://en.wikipedia.org/w/index.php?title=Rust&oldid=37112222.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• minimum of 6 test tubes, same size,
• masking tape,
• permanent marker,
• 6 clean, clear jars or bottles, with the same height,
• ring stands and clamps (or other means of holding test tubes over the jars),
• steel wool.

Experimental Procedure

Diagram of basic experimental setup. You'll want to use at least three test tubes for each condition you test, to assure that your results are consistent across multiple trials.

Alternate setup. If you don't have ringstands, you can make your own test tube holder with a piece of wood or pegboard. Mount the tubes between sets of holes, and hold them in place with rubber bands stretched tight and looped over nails or pegs on the back side. Add supports at the ends with height matched to your jars.

1. Attach a strip of masking tape to the side of each of your test tubes (for marking the water level).
2. Using permanent marker, make a mark on the tape about 1 cm down from the mouth of the test tube. This will be the starting water level.
3. Tear off enough steel wool to make a ball about 2.5 cm in diameter. Use a pencil to push the steel wool down to the bottom of a test tube. Repeat for a total of three test tubes with steel wool.
4. Invert the test tubes (3 with steel wool and 3 without) and mount them over the jars so that the water level is at the starting mark on each test tube.
5. You may want to cover your entire setup with a big plastic bag to minimize evaporation. Be careful not to knock the test tubes when covering and uncovering.
6. Check at least daily, and write your observations down in your lab notebook. Carefully mark the water level on the tape on each test tube.
7. When the water level is no longer changing in the test tubes, you're ready to analyze your results.
8. Measure is the difference in water level between the starting and ending position for each test tube.
9. For how many tubes did the water level change? For those that did:
   1. Calculate the volume that corresponds to this difference. (Remember that the volume, V, of a cylinder can be calculated from the formula V = πr²h, where r is the radius and h is the height of the cylinder.)
   2. Calculate the total starting volume of air in each test tube.
   3. Calculate the proportion of oxygen in each test tube.

• What is the average of your results?
• How does this compare with the value(s) for percentage of oxygen in air that you found in your background research?
• What are the potential sources of error in your measurement?

Variations

• If you ever take a vacation in the mountains, you can use this method to compare oxygen levels in the air at high and low altitude. Try doing this experiment at high altitude and comparing the results with same experiment done at a lower altitude.
• Can you use this procedure to detect decreased oxygen content in exhaled air? Do background research and find out how much oxygen we consume when we breathe. Do you think this method is sensitive enough to detect the difference? Design an experiment to find out.
• What do you think would happen if you collected air samples at high altitude, and then tested them at low altitude (or vice versa)? How easily does oxygen dissolve in water? Could dissolved oxygen affect your results? Design an experiment to find out.

• For more science project ideas in this area of science, see Weather & Atmosphere Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• WGBH Educational Foundation, 2005. "Everest: The Death Zone," NOVA, http://www.pbs.org/wgbh/nova/teachers/activities/2506_everest_01.html.

Last edit date: 2006-01-30 20:31:08

Career Focus

If you like this project, you might want to think about career opportunities in Weather & Atmosphere.

The atmosphere is a blanket of gases, surrounding Earth, that creates our weather. Meteorologists study the measurements and motion of the atmosphere, and changing events within it, so that they can predict the weather. This weather forecasting helps the general public and people who work in industries such as shipping, air transportation, agriculture, fishing, forestry, and water and power better plan for the weather, and reduce human and economic losses. Learn more about this career: Meterologist.