Bromothymol blue (BTB) is a chemical that can be used as an indicator of the pH of a solution. Because CO2 produces an acid in water, BTB can indirectly measure how much CO2 is present in a solution. When placed in solutions with neutral pH (little CO2), the solution appears green. In solutions with low pH (high CO2), the solution appears yellow.
A teacher performed a demonstration to investigate how photosynthesis and respiration cycle carbon and oxygen.


What color would the solution in the test tube be after being in the light? What would it be if it were then placed in the dark for 24 hours, and why?