Showing oxygen goes in
Showing that oxygen is absorbed in respiration is the craftier of the two — something that has gone missing has to be measured.
Activity 03
Put germinating seeds in a flask. Close the flask with a stopper carrying a rubber tube and a U-shaped glass tube. Dip the far end of the tube into a vessel of coloured water. Watch the water level in the tube.
Setup A — there is KOH inside the flask. Setup B — no KOH. Everything else is identical.
Figure to be added
Observation
In setup A the water is drawn up the tube. In setup B no such change is seen.
What happens in setup A
Both the carbon dioxide already in the flask and the carbon dioxide the seeds give out in respiration dissolve in the KOH in the test tube. Meanwhile the seeds absorb the oxygen inside the flask. So gas leaves and nothing is added back. Air in the U-tube flows into the flask to fill that space, and the water in the vessel is drawn up the tube behind it.
Why the water does not move in setup B
The seeds in setup B absorb oxygen too. But with no KOH, the carbon dioxide they give out stays in the flask. The volume of CO2 released and the volume of O2 absorbed balance, so the total volume does not change.
B is not a control — it is part of the answer
The water not moving in setup B is not evidence that the seeds are not respiring. What it shows instead is that the volume of oxygen taken in during respiration and the volume of carbon dioxide given out are close to equal.