Columbia University Summer Research Program for Science Teachers

What are the effects of biodegradable waste on dissolved oxygen?

Ken Wasserman, MS 88, Brooklyn, NY

1995

Suitable for average-above average 8th grade regents earth science students

Experiment: What are the effects of biodegradable waste on dissolved oxygen?

Purpose: To understand the relationship between biodegradable waste and dissolved oxygen found in the polluted waters surrounding Brooklyn.

Background : When microorganisms (bacteria and fungi) eat biodegradable wastes, they use large amounts of oxygen. Thus, a lot of biodegradable waste means very little dissolved oxygen for fish and aquatic life. In this experiment, we will investigate that relationship. Yeast will represent the microorganisms, milk is the biodegradable waste, and methylene blue (a dye) will indicate when oxygen is used up. The methylene blue will change from blue to white when no more oxygen is present in the test tubes. (The color change is actually from blue to colorless. The white color you will observe in the test tubes is due to the milk’s color). [Content Standard Unifying Concepts- Change, constancy, measurement]

Materials:

2 small beakers or baby food jars stirring stick

2 ml (about 1/2 teaspoon) dry yeast one 5-ml pipette or an eye dropper

one 10 ml graduated cylinder methylene blue solution

3 test tubes in rack masking tape

Procedure:

1- Fill a baby food jar about half full of milk. Take it to your lab table.

2- Place the three test tubes in the rack and put masking tape numbers (1,2,3) on them.

3- Use the pipette or eye dropper to add the amount of materials to each test tube as shown below. (Approximately 15 drops equals 1 ml.)

Before you continue, check the liquid’s height. It should be the same in all three tubes. You should have exactly 2.5 ml of solution in each tube.

4- Add three drops of methylene blue to each test tube. The methylene blue is an "indicator" solution. It will change from blue to white when the oxygen is used up.

5- Mix each tube by putting your thumb over the top and inverting it (turning it upside down) quickly four times.

6- Prepare a sample of yeast by adding 2 ml (about 1/2 teaspoon) dry yeast to 20 ml of water in a beaker or baby food jar. Mix the yeast and water thoroughly with your mixing stick.

7- You are now ready to mix the yeast and milk solutions. Follow these directions very carefully:

a- Watch the clock for exact timing. Proceed to the next step (b) when the second hand passes the "12". Record the exact time of mixing- on the minute- in the table below, next to test tube 1.

b- Mix the yeast solution vigorously with the tip of your pipette or eye dropper. Then carefully put exactly 2.0 ml (30 drops) of yeast solution into test tube 1. Mix by inverting four times.

c- Now repeat the procedure with test tubes 2 and 3. Be sure you record the exact time you added the yeast to each tube.

8- Relax and wait until each tube’s color changes from blue to white. (It usually takes about 15 minutes). note: the surface of each test tube always will remain blue. Can you guess why?

9- When the color change is complete, figure the total time by subtracting the time of mixing from the time the tube changed color (column B-A). Record this time to the nearest minute. It may take several minutes for this change to occur- be patient!

Questions: Answer on a separate sheet

1- Name the gas "inhaled" (taken in) by microorganisms.

2- Name the gas "exhaled" by microorganisms.

3- Where do microorganisms living in water get the oxygen they need to live?

4- Where do green plants living in water get the carbon dioxide they need to live? [Content Standard C- Interdependence of organisms]

5- Shake one of the test tubes that turned white. What happens to the color? Why does the color change? [Teaching Standard B- Orchestrate discourse among students]

6- Air is added naturally to rivers when water goes through rapids and over falls. How does shaking the test tube prove that air is added to water when it tumbles over rocks?

7- Why is the oxygen in the experiment "used up?" [Content Standard B- Chemical reactions]

8- Name the part of the experiment that represents microorganisms.

9- Name the part of the experiment that represents waste.

10- In which test tube did you have the most waste? The least waste?

11- Graph your results on a separate sheet ( time for color change to occur, in minutes Vs ml of milk) and answer the following questions:

12- What does the line you plotted tell you about the relationship between the amount of waste and oxygen in a body of water? [Content Standard Unifying Concepts- Change, constancy, measurement]

13- If large amounts of waste were dumped in a river, such as our Hudson River, what would be the effects of the dissolved oxygen in the water? [Content Standard F- Natural and human induced hazards]

14- Some bacteria, given ample food and water, can multiply every 20 minutes. Calculate the number of bacteria present after twelve hours at this rate (starting with one bacteria cell).

Teacher notes about this experiment:

1- You may use fresh or powdered milk. Students will get faster color changes if you dilute the milk to 50% strength and warm slightly before the experiment.

2- Two liters of milk are enough for five classes.

3- Many students make mistakes in volume measurements- be careful!

4- Since color changes take up to 15 minutes, consider the length of your lab period.

5- Students must know graphing skills before this experiment.

6- All glassware must be clean and free of soap residue. Soap residue may kill the yeast.

7- All variable in this experiment are relative to one another. For example shaking two times, instead of four will change the results.

8- The yeast solution should be well-mixed before it is added to the test tubes. It is important to get approximately the same number of yeast cells in each test tube.

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