Question

The colour of $ {K_2}C{r_2}{O_7} $ changes from red orange to lemon yellow on treatment with aqueous $ KOH $ because of
(A) The reduction of $ C{r^{VI}} $ to $ C{r^{III}} $
(B) The formation of chromium hydroxide
(C) The conversion of dichromate to chromate
(D) The oxidation of potassium hydroxide to potassium peroxide

Answer 1

Hint: Potassium dichromate is found as red-orange crystalline solid. Potassium chromate is found as yellow crystals in nature. The colour of a solution depends on the colour exhibited by the products formed.

Complete answer:
Let’s check what happens when $ {K_2}C{r_2}{O_7} $ reacts with aqueous $ KOH $ :
 $ {K_2}C{r_2}{O_7}_{(aq)} + 2KO{H_{(aq)}} \to 2{K_2}Cr{O_4}_{(aq)} + {H_2}{O_{(l)}} $
As shown above, when potassium dichromate reacts with aqueous potassium hydroxide, it produces potassium chromate and water. Potassium chromate has a yellow colour in nature and hence it changes the colour of the solution.
Therefore, option C is correct.

Additional Information:
 $ {K_2}C{r_2}{O_7} $ or Potassium dichromate has a red-orange crystalline solid. It is also used as a strong oxidizing agent and is used to convert $ 1^\circ $ alcohol directly to carboxylic acid as well as $ 2^\circ $ alcohol to ketones. It is odourless and soluble in water. It is used as a precursor to potassium chrome alum $ (KCr{(S{O_4})_2}) $ and in leather tanning.
 $ KOH $ or Potassium hydroxide is also known as caustic potash. It has white solid appearance. It is odourless in nature. It is found as white or slightly yellow lumps, rods, flakes, sticks or pellets. Aqueous $ KOH $ is used in manufacturing alcohols from haloalkanes.
 $ {K_2}Cr{O_4} $ or Potassium chromate has a yellow colour. It is found as yellow crystals or yellow powder in nature. It is used in precipitation titration as an indicator. It is also used to manufacture dyes.

Note:
Simply remembering the characteristic colour of potassium dichromate and potassium chromate can help you in solving the question quickly if you know the chemical reaction. Potassium permanganate $ (KMn{O_4}) $ is also equally powerful oxidising agent as $ {K_2}C{r_2}{O_7} $ and can also be used for the synthesis of carboxylic acid and ketones.