In my school test, the question is as follows:

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  1. Will the experiment work satisfactorily?
  2. What alterations can you make to obtain expected result?

What I wrote:

  1. Yes, it will work satisfactorily. Lime water absorbs carbon dioxide and hence there is no carbon dioxide for the leaf and hence photosynthesis does not take place and no starch is prepared. Hence on iodine test, the presence of starch is negative and thus proves that carbon dioxide is required for photosynthesis.

  2. We can put half of the leaf inside the flask and half of it outside. So, the inside part will not show presence of starch and the outside part will show presence of starch.

[Note]: It was marked incorrect by my biology teacher. It got it reviewed by another biology teacher and HE SAID WHAT I WROTE IS CORRECT and the teacher who marked it wrong gave the opinion as follows:

  1. It will not work satisfactorily because lime water does not absorb carbon dioxide. [which to me, seems incorrect as lime water ABSORBS carbon dioxide]

  2. The alteration is that you have to use Potassium Hydroxide instead of lime water. [which to me, seems incorrect too since both lime water and potassium hydroxide absorbs carbon dioxide and then there is no need to use potassium hydroxide when lime water works!]

Now I am confused what is correct and what should I write in my final examination?? Please help. Any help will be appreciated, Thank you.

  • 1
    $\begingroup$ Not an expert answer but from experience, I’ve seen limewater absorb $\ce{CO_2}$ mostly when $\ce{CO_2}$ is bubbled through the solution. However, in respirometers, KOH is used to absorb any neighboring $\ce{CO_2}$ in the chamber. Analogically thinking, it seems to me that limewater won’t be very efficient in removing $\ce{CO_2}$ from the flask in your experiment. Thus, you are correct in stating limewater absorbs $\ce{CO_2}$, but it might not work “satisfactorily,” making the teacher’s answer correct. For q2, I feel you should combine both your teacher’s and your answers. A control is needed. $\endgroup$ – lightweaver Apr 2 '16 at 5:23

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