Since carbon dioxide dissolves in water to form an acid, an acid-base indicator such as bromthymol blue is used to track the acid concentration during plant photosynthesis and respiration. Bromthymol blue turns yellow in an acid solution and remains blue in a basic solution. The color change is reversible. A student used the following procedure to track carbon dioxide.
In an experiment, a student placed a solution of bromthymol blue into each of two flasks, inserted a drinking straw in each flask, and blew through the straw until the solution turned from blue to yellow. Carbon dioxide, a waste product in the student’s breath, caused the color change. The student then placed a small Elodea water plant in each flask and closed the flasks with rubber stoppers. Flask A was placed in bright sunlight, while flask B was
placed in the dark for 24 hours, as shown in the figure below.
After 24 hours, the bromthymol blue in flask A had turned back to blue, while in flask B it was still yellow.
If living things use oxygen and release carbon dioxide during respiration, what is the most probable reason that the atmosphere does not become depleted of oxygen and become filled with carbon dioxide?
A . Plants replace the oxygen in the atmosphere and use carbon dioxide during photosynthesis.
B . The atmosphere supplies large amounts of oxygen and absorbs large amounts of carbon dioxide.
C . Carbon dioxide in the atmosphere breaks down in sunlight to release oxygen and water.
D . Carbon dioxide is used by many invertebrate animals in place of oxygen during respiration.
E . Plants don’t use oxygen from the atmosphere in the dark, but they do use carbon dioxide.
Answer: A
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