LAB: "Burning Up the Atmosphere". . . . . . name:

Her e’s the deal. . . Let’s say that, until recently, scientists still hadn’t figured out how much oxygen is present in the air we breathe. However, earlier this week, in an article published in a major scientific journal, a scientist named I. M. Knotreel claims to have discovered that (dry) air is 21 % oxygen.

The discovery has caused quite a stir in the scientific community, so this week scientists all over the world are attempting to repeat the experiment used in the study. As they do so, they are carefully evaluating the experimental technique in order to decide whether or not the scientist’s claim is legitimate. Today you will join this endeavor by repeating Knotreel’s simple experiment.

1. What is "air" ?

2. Which gas in air is "used up" as a candle burns ?

3. Change “.18” to a percentage.

1. Obtain the materials listed above. Use the metric ruler to measure the height of your test tube to the nearest .1 cm.

height =. . . . . . . cm

1. Position a candle in the holder. Be sure that the top of the candle is at least 5 cm above the base.

2. Pour 1 cm of water into the pan and set the candle into the center of the pan. Add two drops of food coloring to the water.

3. Light the candle and then, carefully, in one motion, invert the test tube over the candle so that the bottom of the tube is completely under water but not completely touching the bottom of the pan.

4. Observe what happen. Record your observations here:

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Why did the candle stop burning?

Why did the water rise into the tube?

5. Once you have the technique down, you will perform the "burning of air" technique five more times. Each time, measure the height to which the water goes up into the tube to the nearest .1 cm, and record the data below. If your candle burns too short, replace it with a taller one. To get fresh air into the tube, rinse the tube with water after each trial.

. . . . . TRIAL . . . . . . . . #1 . . . . . . . . #2 . . . . . . . . #3 . . . . . . . . #4 . . . . . . . . #5

ht. of water . . . . . . . . . . . cm . . . . . . . . cm . . . . . . . cm . . . . . . . . cm . . . . . . . . cm

Knotreel assumed that amount of the water that entered the tube is equal to the amount of oxygen that was present in the air.

6. Calculate the % of oxygen for each of your trials, write them in the spaces on back of this sheet. To calculate the percentages simply divide the height of the water by the height of the test tube.

% oxygen = height of water divided by the height of tube

trial #1: _____ % . . .#2: _____ % . . .#3: _____ % . . . #4: _____ % . . .#5: _____ %

7. Average your results from the five trials. . . . . average = _____ % Oxygen

1. There are several flaws in the technique that you’ve just used to determine the percentage of oxygen in air. Identify at least three factors that Knotreel failed to take into consideration. All reasonable answers will be accepted. Think critically!

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2. Approximately 99 % of the atmosphere's weight is found within about 32 km of the Earth's surface. How many miles is this ? (hint: km x .62 = miles)

3. The chemical formula for the oxygen we breathe is 02. “Ozone” is another from of oxygen. What is its formula?

Burning Up the Atmosphere . . . for the teacher

1. The students like it! Its a “discrepent event” that stimulates thinking.

2. It provides an interesting way to show students how scientists share “new discoveries”

3. It gets them thinking about what air is.

Pre-Lab . . .

1. Have the students read the introduction, or read it together.

2. Demonstrate Knotreel’s technique that will be used in the experiment (described in the procedures). Discuss what happens.

3. Have them do the “pre-lab questions”, or do them together. Answers:

. . . . #1. Air is a mixture of gases.

. . . . #2. Oxygen is “burned up” as the candle burns.

. . . . #3. 18 %

1. The test tubes that we used were 15 cm long and 2.5 cm in diameter.

2. We used glass "culture dishes", however any similar clear plastic or glass container should work fine.

3. Buy the birthday candles and then go to a hardware store and find some hex nuts to serve as holders.

4. The purpose of the food coloring is to make it easier for the students to see the water.

1. Here are some of the factors that Knotreel failed to take into consideration.

A) Although the oxygen is burned up, other gases such as carbon dioxide would be produced. Knotreel did not account for this.

B) Humidity (water gas) was not taken into consideration.

C) Temperature was not taken into consideration. The air above the flame would be very warm. When the flame goes out, the air would be cooled quickly by the water, etc. This would reudce the volume of the air and decrease the pressure inside the tube. This temperature-induced pressure change may have been a major factor in causing the water to rise.

D) The air above the flame is probably not typical air. It is hot and has a different mixture of gases.

2. about 20 miles . . . 19.8 to be exact

3. O3