Potassium Dichromate

Potassium dichromate is a well-known oxidising agent whose colour change accompanies the reduction of chromium. In acidic medium the orange dichromate ion Cr_2O_7^2-, in which chromium is in the +6 state, is reduced by gaining electrons to give the green chromium(III) ion Cr^3+: Cr_2O_7^2- + 14H^+ + 6e^- → 2Cr^3+ + 7H_2O. Therefore the solution changes from orange to green as the oxidation proceeds, providing a clear visual indicator in titrations. The option of green to orange reverses the direction of the change. The option of purple to colourless describes permanganate, not dichromate. The option of yellow to blue does not correspond to chromium chemistry; the yellow chromate exists only in alkaline solution. This characteristic colour change is a standard NCERT d-block observation. Plausibility check: the appearance of green Cr^3+ from orange dichromate matches the known six-electron reduction of chromium from plus six to plus three, confirming the colour change.