For kinetic study of the reaction of iodide ion with H₂ O₂ at room temperature :
(A) Always use freshly prepared starch solution.
(B) Always keep the concentration of sodium thiosulphate solution less than that of KI solution.
(C) Record the time immediately after the appearance of blue colour.
(D) Record the time immediately before the appearance of blue colour.
(E) Always keep the concentration of sodium thiosulphate solution more than that of KI solution.
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Thiosulphate is the clock's timer: keep it small, and stop the watch once you see blue.

1. Iodine is made. H₂O₂ slowly oxidises iodide in acid: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O. This is the slow reaction whose rate you measure.
2. Thiosulphate removes it. A small fixed amount of thiosulphate turns each bit of I₂ back into iodide almost instantly: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. While any thiosulphate is left, no free I₂ builds up and the solution stays colourless.

   Statement	Verdict	Reason
   (A) Fresh starch	Correct	Old starch solution breaks down and gives a weak, late blue colour, so the end point is missed.
   (B) Thiosulphate less than KI	Correct	Thiosulphate must run out first so that free I₂ can appear. Iodide must stay in excess.
   (C) Time just after blue appears	Correct	The blue colour is the signal that thiosulphate is used up. You stop the watch the moment you see it.
   (D) Time just before blue appears	Wrong	Before the blue appears nothing changes in the flask, so you cannot tell when that moment is.
   (E) Thiosulphate more than KI	Wrong	Excess thiosulphate removes all the I₂ that forms. The blue may never appear.

3. So. The correct statements are (A), (B) and (C), which is option (a).

The idea to keep. In any clock reaction, a small amount of a scavenger hides the product until it runs out. Keep the scavenger limiting, use a reliable indicator, and time the first visible colour.

Answer from the NTA final answer key. Solution by PrepWiser.