Experimentally show that carbon dioxide is given out during respiration


To show experimentally that carbon dioxide is given out during respiration.

Materials Required

Conical flask, U-shaped delivery tube (tube bent twice at right angles), cotton wool or moist blotting paper, water, thread, beaker, test tube, rubber cork with one hole, 20% freshly prepared KOH sokution, vaseline, soaked gram seeds.


Respiration is a biochemical process during which food (glucose) is oxidised to liberate energy. It is a catabolic process.

In the experiment, moist gram seeds are taken as they are actively respiring and releasing CO2. The CO2 released is absorbed by KOH and forms K2CO3.

2KOH + CO2 ⟶ K2CO3 + H2O


  • Take about 25-30 seeds of gram and germinate these seeds by placing them on moist cotton wool or moist blotting paper for 3-4 days.
  • Place the germinated seeds into a conical flask and sprinkle a little water in flask to moist the seeds.
  • Take freshly prepared 20% KOH solution in a test tube and hang it in conical flask with help of thread.
  • Close the mouth of conical flask by placing a rubber cork containing one hole.
  • Through the hole of rubber cork, insert one end of the U-shaped glass delivery tube in the conical flask and place the other end into a beaker filled with water.
  • Seal all the connections of the experimental set-up with vaseline so as to make it air-tight.
  • Mark the initial level of water in the U-shaped delivery tube.
  • Keep the apparatus undisturbed for 1-2 hours and note the change in level of water in the tube.


After sometime, the level of water in U-shaped delivery tube dipped in water of the beaker rises.


Germinated grams seeds in a conical flask release CO2 during respiration. The CO2 released is absored by KOH present in the hanging test tube in conical flask. This creates a vacuum in conical flask which causes upward movement of water in the delivery tube leading to change in level of water in the delivery tube.


  • Germinating seeds should be kept moist.
  • All connections of the set-up should be air-tight.
  • Freshly prepared KOH solution should be used.
  • Keep one end of U-shaped delivery tube in conical flask and the other end immersed in water of the beaker.
  • The test tube containing KOH should be hung carefully.

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