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
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
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?