For kinetic study of the reaction of iodide ion with $$H_2O_2$$ 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

This question is about the iodine clock reaction, which involves the reaction of iodide ions with hydrogen peroxide ($$H_2O_2$$). Let us analyze each statement:

(A) Always use freshly prepared starch solution.

This is correct. Starch solution degrades over time due to bacterial decomposition and may not give a sharp blue-black colour with iodine. Freshly prepared starch is essential for reliable endpoint detection.

(B) Always keep the concentration of sodium thiosulphate solution less than that of KI solution.

This is correct. In the iodine clock reaction, sodium thiosulphate ($$Na_2S_2O_3$$) reacts with the iodine produced and keeps the solution colourless until all the thiosulphate is consumed. The thiosulphate concentration must be less than KI concentration so that iodine eventually accumulates and turns the starch indicator blue.

(C) Record the time immediately after the appearance of blue colour.

This is correct. The appearance of blue colour marks the endpoint — all thiosulphate has been consumed and free iodine reacts with starch. The time should be recorded at this point.

(D) Record the time immediately before the appearance of blue colour.

This is incorrect. You cannot predict the exact moment before the colour appears. The time is recorded when the blue colour actually appears.

(E) Always keep the concentration of sodium thiosulphate solution more than that of KI solution.

This is incorrect. If thiosulphate concentration exceeds KI concentration, the blue colour may never appear during the observation period, making the experiment impractical.

The correct statements are A, B, and C.

Therefore, the correct answer is Option A: A, B, C only.