Name: ________________________________________________________ Date: ____________ Period: _____
Finding the Percentage of Oxygen in the Atmosphere
Background: The atmosphere of the Earth is composed of a mixture of gases. The two most
abundant gases are nitrogen and oxygen. Oxygen also appears in the Earth’s crust combined with
other elements to form minerals.
In this investigation, you will study how much of the air is consumed during combustion. To prove
that all the air is not just oxygen and to find out one way to calculate what the percentage of
oxygen gas might be, we will burn a candle in air. The candle will burn only the oxygen
component. The loss of the oxygen gas will create a partial vacuum and draw the water up the tube
so that we can measure the volume of oxygen consumed. By measuring volume, you will be able to
determine the percentage of oxygen in the air.
Problem: What percentage of the Earth’s atmosphere is oxygen?
Guiding Question:
Hypothesis: (if-then statement)
Group Claim (conclusion):
Evidence:
Justification of the evidence:
Procedure:
1. Place your candle on a small piece of clay in the center of the petri dish.
2. Fill the test tube with water; measure the volume of this water with a graduated cylinder. This
volume represents the volume of air when the test tube is empty. Record this volume in the
data table.
3. Pour the water into the petri dish, filling it close to the lip of the dish.
4. Light the candle and rapidly invert the test tube over the lit candle. Make sure that the open
end of the test tube is under the surface of the water but is NOT touching the bottom of the
dish.
5. As the candle uses the oxygen, it will go out and water will be pulled into the tube to replace the
oxygen. Draw a picture of this set up in your data section.
6. When the candle goes out, mark the level of water in the test tube with a wax pencil.
7. Fill the test tube with water to the line you marked. Measure the volume of water with the
graduated cylinder. This is the volume of air in the test tube minus the oxygen that the candle
used. Record this volume in the data table.
8. Subtract the volume of air after the candle goes out from the original volume. This is the
original oxygen volume in the test tube. Record the volume in the data table.
9. Determine the % of oxygen in air by using the formula below. Record this data.
% oxygen in air =
Volume of oxygen in test tube
x l00
Total volume of air in test tube at start
10. Add your data to the class data sheet. Record the class average.
Data Table:
Trial 1 (mL)
Trial 2 (mL)
Average (mL)
Initial volume of air in the test tube
Volume of air after the candle goes out
Volume of oxygen
Percentage of oxygen in air
Percent of error (%O2)
Your avg-21 *100
21
Analysis Questions:
1. Would the same result for the percentage of oxygen in air be obtained if a larger test tube was
used? A larger candle? Explain.
2. Why does the water rise in the test tube as the candle goes out?
3. Nitrogen is the other major component of air (78.1%). What property of nitrogen have you
discovered as a result of this experiment? Is it combustible?
4. Why is oxygen an important part of the Earth’s atmosphere?
5. The background states that the percentage of oxygen has fluctuated over time. What do you
think could have caused oxygen levels to rise and fall over Earth’s long history?
6. “As the altitude of an area increases, the density of the atmosphere decreases.” How can this
statement be used to explain why it is more difficult to breathe in Denver, which has an
altitude of more than 1500 m, that at the shore, which is sea level?
Conclusion:
Why is oxygen important in our atmosphere? How did the modern atmosphere on Earth today
form? When did this begin to happen? What else began to happen as the atmosphere evolved? How
does science know this?