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A student is running an experiment to determine how light influences the rate of photosynthesis in aquatic plants. The student sets up a shallow dish with carbonated water and some bromothymol blue(BTB) indicator. The carbon dioxide in the water will make the solution acidic.

As the aquatic plants undergo photosynthesis, they will absorb carbon dioxide from the solution and the solution will become more alkaline, or basic. The BTB indicator is yellow in acidic solutions and will change color to blue as the solution becomes more basic.

The student places a sample of healthy water stargrass in each of several containers and then covers the different containers with various light filters. The containers are placed under a solar spectrum light source which is turned on for 12 hours, then turned off for 12 hours, with this light cycle repeated for 3 days.

Trial 1 shows the change in water color for samples with different light intensities, based on the use of different neutral density filters. These filters did not change the color of the light, only how bright the light was.

Light Intensity
(% of Maximum)
$\hspace{2cm}$
Color of Solution Initially Color of Solution after 24 hours Color of solution after 48 hours Color of Solution after 72 hours
25 Bright Yellow Yellow Yellow-Green Green
50 Bright Yellow Yellow Green Blue
75 Bright Yellow Green-Blue Blue Blue
100 Bright Yellow Blue Blue Blue

Trial 2 shows the change in water color for samples with differently colored filters. In these samples, the light for each sample had the same brightness but had differing color filters.

Color of Light Color of Solution Initially Color of Solution after 24 hours Color of solution after 48 hours Color of Solution after 72 hours
Black Bright Yellow Bright Yellow Bright Yellow Bright Yellow
Blue Bright Yellow Yellow Yellow-Green Green
Green Bright Yellow Yellow-Green Yellow-Green Green-Blue
Red Bright Yellow Yellow-Green Green-Blue Blue

In Trial 2, as the color of the light changes from red to black, which of the following BEST describes what is happening in the solution?

A

The more red the light is, the better it is absorbed by the plants, which then absorb carbon dioxide from the solution, making it more basic and changing the color to blue.

B

The more red the light is, the better it breaks down the bromothyl blue, changing the color of the solution from yellow to blue.

C

The black light has more energy than the red light and keeps the plants from absorbing carbon dioxide.

D

The plants use the light to produce carbon dioxide, and the more carbon dioxide is produced, the more blue the solution becomes.

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