Science JunctionLesson 2

 How does organic material break down and effect the oxygen levels of an aquatic ecosystem?

Students will be able to: Materials:
3 different water samples (dishwater, well water, rain water, pond water, etc.)
3 test tubes
graduated cylinder
test tube rack
methylene blue in a dropper bottle

Note: Bacteria act on methylene blue indicator solution and cause it to change from blue to a colorless liquid. The more bacteria a water sample contains, the faster the color of the methylene blue breaks down.

Note: Safety goggles and aprons should be worn at all times during this lab activity.

After discussing the procedure with your partner, write a hypothesis about the relationship
between the amount of organic matter, the amount of bacteria and the oxygen levels in
a water sample.


Working with a partner:
1. Label three test tubes 1, 2 and 3. Put 5 ml of a different water sample in each tube.
2. Add 20 drops of methylene blue to each test tube. Put the test tubes in a test tube rack
and observe them at 10 minute intervals for 1 hour. Record your observations in the data

Data Table:

After the observations are complete, answer the following questions
1. Does the data collected support your hypothesis? Why or why not?

2. In which test tube did the blue color disappear first?

3. Which water sample contains the most organic matter? Which contains the least organic matter?

4. Which water sample contains the most oxygen? Which water sample contains the least oxygen?

5. Least 3  possible sources of organic matter in each positive sample.

6. Write a short paragraph explaining the relationship between oxygen levels, bacteria and the breakdown of organic matter.  What can you conclude about the dissolved oxygen levels and the amount of bacteria found in a water sample?


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©1998 April J. Cleveland for Science Junction, NC State University. All rights reserved.
page design by Lisa Leonor Grable and April J. Cleveland 
Last Modified: 6/8/98
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