Bromothymol blue (BTB) is a pH indicator that is also used to detect carbon dioxide (CO2). BTB is blue when pH is basic and CO2
is low. BTB is yellow when pH is acidic and CO2 is high. BTB is green when pH is neutral. A group of students are planning to perform an investigation in which they will place either a stalk of the aquatic plant elodea or a snail in a test tube that contains water with a neutral pH of 7 and BTB. The students will also include a test tube that contains elodea and a snail. Observing color change once the tubes have been placed under a growth light for several hours will allow the students to answer which TWO of the following questions? A. Do both elodea and snails require oxygen to survive?
B. Does elodea produce oxygen during photosynthesis?
C. Do snails respire faster when placed in a tube with elodea?
D. Do snails use the CO2 produced by elodea to produce oxygen?
E. Does photosynthesis performed by elodea remove CO2 from the water?
F. Does cellular respiration occur at a higher rate than photosynthesis in the tube with only elodea?
E. Does photosynthesis performed by elodea remove CO2 from the water?
F. Does cellular respiration occur at a higher rate than photosynthesis in the tube with only elodea?
Explanation:
The tubes were placed under a growth light and as the elodea is growing, it would need photosynthesis to provide food. The color changes will therefore allow the students to see the effect photosynthesis has on the CO2 in the water by checking to see if the BTB turns blue which would mean that CO2 was removed by photosynthesis.
The students will also be able to see which process occurs faster in the tube with only elodea because the dominant color would show if cellular respiration occurs faster than photosynthesis or if the case is the reverse. For instance, if respiration is occurring faster, there will be more CO2 which would turn the BTB towards yellow, if photosynthesis is faster the BTB will turn towards blue and if the rates are similar, the BTB will remain green.
