WHAT IS THE EFFECT OF BIODEGRADABLE WASTE

ON THE AMOUNT OF DISSOLVED OXYGEN?

 

INTRODUCTION:

Biodegradable means that a substance can be broken down by living organisms (decomposers). Since decomposition is a form of respiration, oxygen is consumed. In this lab you will learn that a great deal of biodegradable waste in water consumes much dissolved oxygen leaving little for plant and animal life.  In this lab the yeast will represent the decomposers. The milk is the biodegradable material and the methylene blue is an indicator. It will change from "blue" to "white" when there is no more oxygen present in your mixture. (Actually, the color change is from blue‑to colorless but because the milk is white this causes the color change to be to white.

 

MATERIALS:

3 test tubes of equal size            test tube rack                pipette

yeast mixture                             milk                              Methylene blue

 

PROCEDURE:

 

1.   Take a small beaker of the yeast mixture and a small beaker of the milk to your lab station as well as a dropper bottle of methylene blue.

 

2.   Place three test tubes in the test tube noting which is one, two and three.

 

3.   Using your pipette, add the amount of materials to each test tube as shown in the chart below:

      Be sure that the height of the liquid in each of the three test tubes is exactly the same.

 

4.   Add three drops of methylene blue to each test tube.

 

5.   Mix the methylene blue with the milk/water mixture by placing your thumb over the test tube and inverting each test tube four times.

 

6.   The yeast mixture will already be prepared. It is a mixture of 2 mI of dry yeast to 20 ml of water.

 

7.   For the next part of the lab one of you will need to carefully time the experiment. As you mix each test tube with the yeast you will need to begin timing at once.

 

8.   Add 20 drops of the yeast mixture to test tube one. Mix th6roughly by inverting four times and record the exact time that you add the yeast to the tube.

 

9.   Now do the same for test tubes two and three. Timing is critical. Begin timing each test tube as soon as you add the yeast. Fill out the data table on the next page.

 

10. When the color has changed from blue to white in each test tube record the exact time at which the change was complete. The surface of each test tube will remain blue. Why?

 

11. BEFORE YOU CLEAN UP, SHAKE ONE OF THE TEST TUBES WHICH HAS TURNED WHITE. What happens? Why?

 

It can take as long as 15 minutes for the color change but the average time is four to five minutes.

 

QUESTIONS:

 

1.   What is the gas taken in by the microorganisms?

 

2.   What is the gas given off by the microorganisms?

 

3.   Where do microorganisms living in water get the oxygen that they use in decomposition?

 

4.   Where do the green plants living in water get the carbon dioxide that they need?

 

5.   Why is the oxygen in this experiment used up?

 

6.   Which part of your experiment represents the decomposers?

 

7.   Which part of your experiment represents the sewage dumped into the water?

 

8.   In which test tube did you have the most sewage? In which test tube did you have the least sewage?

 

9.   Using a piece of graph paper, graph your results. Construct a line graph.

 

10. What does this graph tell you about the relationship between the amount of biodegradable waste in water and the amount of dissolved oxygen in the water?

 

11. What would be the effects of dumping a great deal of raw sewage into a river as regard to the dissolved oxygen in the same river?

 

12. Write a good conclusion as to what you learned by performing this lab.