Which of the following sets includes all the species that will change the orange colour of K$$_2$$Cr$$_2$$O$$_7$$ in acidic medium?

K$$_2$$Cr$$_2$$O$$_7$$ in acidic medium furnishes the dichromate ion $$Cr_2O_7^{2-}$$ which is a strong oxidising agent.

The relevant reduction half-reaction is
$$Cr_2O_7^{2-}+14H^++6e^- \rightarrow 2Cr^{3+}+7H_2O$$
with standard potential $$E^\circ = +1.33\;V$$. Any species that can supply electrons (i.e. can be oxidised) will reduce $$Cr_2O_7^{2-}$$ from +6 (orange) to +3 (green), hence “change the orange colour”.

Therefore we must pick only those ions/molecules that behave as reducing agents in acidic solution.

Testing the species listed in each option

1. Fe$$^{2+}$$: readily oxidises to $$Fe^{3+}$$; standard potential $$Fe^{3+}+e^- \rightarrow Fe^{2+},\;E^\circ = +0.77\;V$$. Since $$0.77\;V \lt 1.33\;V$$, $$Fe^{2+}$$ can indeed reduce dichromate.

2. Sn$$^{2+}$$: oxidises to $$Sn^{4+}$$; $$Sn^{4+}+2e^- \rightarrow Sn^{2+},\;E^\circ = +0.15\;V$$. Again $$0.15\;V \lt 1.33\;V$$, so $$Sn^{2+}$$ is a good reducing agent for dichromate.

3. I$$^-$$: oxidises to $$I_2$$; $$I_2+2e^- \rightarrow 2I^-,\;E^\circ = +0.54\;V$$. Because $$0.54\;V \lt 1.33\;V$$, iodide reduces dichromate.

4. S$$^{2-}$$: oxidises to elemental sulphur or sulphate; $$S+2e^- \rightarrow S^{2-},\;E^\circ \approx +0.14\;V$$ (to $$S$$) and even smaller versus sulphate. Hence sulphide is a powerful reducing agent for $$Cr_2O_7^{2-}$$.

All four species listed above will definitely decolourise (actually turn green) the orange dichromate solution.

Checking the other options

• Option B contains $$Fe^{3+}$$, which is the oxidised form; it cannot supply electrons. • Option C contains $$Sn^{4+}$$, another fully oxidised ion. • Option D contains $$Fe^{3+}$$, $$SO_4^{2-}$$ and $$Sn^{4+}$$ — none of these is a reducing agent under the given conditions. Hence those sets are incomplete or incorrect.

Therefore the only set that contains all species capable of reducing $$Cr_2O_7^{2-}$$ is:

Option A which is: Fe$$^{2+}$$, Sn$$^{2+}$$, I$$^-$$, S$$^{2-}$$