Two test-tubes, P and Q, were set up each containing a solution of red hydrogen carbonate indicator. Hydrogen carbonate indicator turns yellow when the carbon dioxide concentration increases and turns purple if the carbon dioxide concentration decreases. An aquatic plant was placed into tubes P and Q. Tube P was uncovered, tube Q was covered with a black lightproof cover. The tubes were left in a warm room in sunlight for four hours. What would be the colour of the hydrogen carbonate indicator in the two tubes after four hours? [Figure 6.1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.