CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 26(b)

(i) The flask is kept airtight so that the carbon dioxide given out by the germinating seeds cannot escape and outside air (with its oxygen) cannot enter; any change in the gas inside is then due only to the seeds. (ii) Potassium hydroxide ($\text{KOH}$) solution. Role: it absorbs the carbon dioxide given out by the germinating seeds. (iii) $\text{KOH}$ absorbs the carbon dioxide, so the amount of gas in the flask decreases and a partial vacuum is created (the pressure inside falls). The air pressure on the water in the beaker then pushes water up the delivery tube.