An indicator 'X' is used for studying the effect of variation in concentration of iodide on the rate of reaction of iodide ion with H₂O₂ at room temp. The indicator 'X' forms blue coloured complex with compound 'A' present in the solution. The indicator 'X' and compound 'A' respectively are :

Solution

Chemistry ma’am · 4 steps

Ask what the reaction makes: iodide turns into iodine, and starch is the indicator that turns iodine blue.

1. What forms. In acid, H₂O₂ oxidises iodide to iodine. Iodine is the product that builds up as the reaction runs, so it is the compound to watch.

   H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O

2. Pick the indicator. Starch forms a deep blue complex with iodine, more precisely with I₃⁻ formed from I₂ and I⁻. Methyl orange is an acid-base indicator: it only changes between red and yellow with pH, and it never gives blue with iodine.
3. Why it suits rates. The blue colour appears once iodine is present, so the time taken for the blue to show tells you how fast the reaction runs. Change the iodide concentration and this time changes.
4. So. Indicator X is starch and compound A is iodine, which is option (d).

The idea to keep. Starch is the indicator for iodine and gives a blue colour. Whenever a reaction makes I₂ from iodide, starch tracks it.

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