TO THE STUDENT

Carefully read through these directions and the directions on the next page before you begin to work.

You may be part of a group for the first part of this exercise. If you are, the group should carry out the experiment and collect the data together, but each student must record the data in his or her own booklet. Be sure to record the data exactly as you observe them. After the data have been collected, each student should answer the questions independently.

After you have finished your experiment and have recorded all of the data, you will be asked to answer some questions about the experiment and the data you recorded. Your answers must be written in this test booklet in the space provided. Make sure that you understand each question before you begin to write. At any time while you are writing your answers, you may look back at the directions for the experiment and the data you collected. Be sure that your answers are written as clearly and neatly as possible.

Before you begin the exercise, read the list of materials given below and check to make sure that your group has everything listed.

Materials

• 5 test tubes
• 1 glass stirring rod
• dry yeast
• sugar
• 3 balloons
• 1 25 ml graduated cylinder
• 1 dropper bottle of bromothymol blue indicator (BTB)
• 2 measuring spoons
• 1 large plastic cup
• 1 thermometer, 0-100^oC
• 1 bottle distilled water
• 1 test tube rack
• hot water
• safety glasses
• aprons
• 1 straw
• paper towels
• stopwatch or clock
• markers

You and your best friend have just started a bakery business for fun and profit. You hope to sell homemade baked goods to your family, friends, and neighbors. You both realize that yeast is important to the rising of bread, rolls, and other tasty morsels

Yeast is a living one-celled organism that produces carbon dioxide gas and alcohol as a result of its "eating habits." The holes in bread are really spaces where carbon dioxide bubbles were trapped.

Working in a group, you will use yeast to produce carbon dioxide. You will verify the presence of carbon dioxide using an indicator called bromothymol blue (BTB). BTB will turn light green or yellow in the presence of an acid formed by carbon dioxide and water.

Procedure

1. Label one of the test tubes as "1A", one as "2A", one as "1B", and one as "2B". Do not label the fifth test tube.

2. A water bath is a container of water at a given temperature. An object is placed in the water bath in order to keep the object at the temperature of the water. To prepare a water bath, pour warm tap water into the plastic cup. Adjust the water's temperature to 40^oC by adding hot or cold tap water as needed. Total volume of the 40^oC water should be at least 200 ml

3. To Test Tubes 1A and 2A, add 20 ml of distilled water. Place Tubes 1A and 2A in the water bath, as shown in Figure A.
   
   Keep the tubes in the water bath when they are not in use. From time to time, check the temperature of the water bath, and add hot water to the water bath as needed to keep the temperature near 40^oC.

4. Add yeast and sugar in the amounts specified in Table 1 below. Using the glass rod, gently stir the contents of each tube. (NOTE: To avoid contaminating the contents of one test tube with the contents from another tube, after you stir the contents of any test tube, thoroughly rinse the stirring rod with clean tap water and then dry the rod.
   Table 1

   Test tube	Volume of water (ml)	Water temperature (oC)	Amount of sugar (teaspoons)	Amount of yeast (teaspoons)
   1A	20	40	none	1/4
   2A	20	40	1/4	1/4

5. Measure the temperature in each tube. Record the temperatures in Table 2 below.

   NOTE: To avoid contaminating the contents of one test tube with the contents from another tube, thoroughly rinse the thermometer with clean tap water and then dry the thermometer after each measurement.

   Table 2

   Test tube	1A	2A
   Temperature (oC)

6. Squeeze as much air as you can out of one of the two balloons. Place the balloon over Tube 1A, so that any gas that escapes from Tube 1A will be trapped by the balloon and cause the balloon to inflate. Repeat the procedure using the other balloon and Tube 2A.

   Set the water bath containing Test Tubes 1A and 2A aside for 10 minutes, to allow the yeast time to make carbon dioxide.

7. While you are waiting, drop 5 ml of BTB indicator into the unlabeled test tube. Using the straw, gently blow air into the BTB for one minute and observe any color change. Record your observation in the following space.

    

    

    

   The change occurs when carbon dioxide in your breath dissolves in the BTB and lowers its pH. When you are finished with this step, carefully rinse the test tube and set it aside.

Please answer the following questions by yourself.

1. Describe the difference between the contents of Test Tubes 1A and 2A, and explain the purpose in having this particular difference.

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2. For yeast cells that are growing in a sugar solution and producing alcohol, Figure B below shows the relationship between the number of live yeast cells per ml of solution and time.
   
   Describe two conditions that could prevent the number of living yeast cells per ml from increasing with time beyond Point A.

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