Bromothymol blue (BTB) is a chemical indicator for carbon dioxide (CO2). In the presence of carbon dioxide (CO2), BTB turns from blue to yellow. An aquatic plant was placed into two test tubes containing BTB, and the test tubes were closed with a stopper. Then, one test tube was placed in the dark, and the other test tube was placed in direct sunlight. The test tube in sunlight stayed blue indicating no CO2 was present, while the test tube in the dark changed to yellow. Which statement best explains why the solution in the test tube placed in the dark changed to yellow?

The Chemical reactants( internal factors) for the process of photosynthesis are C02 and H20, and enzymes. Other external factors are sunlight and chlorophyll.Therefore for a plant to undergo photosynthesis these reactants must be present in the presence of sunlight.

In the present scenario the plant in the test tube exposed to sunlight must used up the C02 with water and exposed sunlight to produce O2 -the by products and CHO,Protein and Lipids in plants.

Since the C02 has metabolised,no CO2 would be present in the test tube of the plant and the water, hence BTB is negative,blue- no C02.

However,the plants in the dark lacks the supply of sunlight therefore no photosynthesis could occur.Consequentially the C02 is not metabolized,and its concentration is high in the leaf in the test tube placed in the dark.This is indicated as yellow positive by the BTB which change from Blue to yellow.