Question

What is the change in color of potassium dichromate when aq. KOH is added in it.

Answer 1

Hint: When KOH solution is added to potassium dichromate, the color of the solution becomes yellow because dichromate ions react with KOH and convert into chromate ions, which have yellow color.

Complete step by step answer:
In this question we have asked what happens when KOH reacts with potassium dichromate. The formula of potassium dichromate is \[\mathop K\nolimits_2 C\mathop r\nolimits_2 \mathop O\nolimits_7 \] . The color of potassium dichromate is orange and it is a strong oxidizing agent. When dichromate dissolves in water, the following reaction takes place:
$
  KOH + \mathop K\nolimits_2 C\mathop r\nolimits_2 \mathop O\nolimits_7 \to 2\mathop K\nolimits_2 Cr\mathop O\nolimits_4 (yellow) + \mathop H\nolimits_2 O \\
    \\
 $
When aq. KOH is added in potassium dichromate, the OH- ion of KOH will react with H+ and the equilibrium will shift towards right according to Le Chateliar's principle. So, more chromate ions will form with H+ ion. This chromate ion is yellow in color. So, the color of the solution will change yellow from orange. We can also say that the color of the potassium dichromate depends on the pH of the solution. At acidic pH the color of the solution is orange and at basic pH, the color of the solution is yellow. When we add aq. KOH in the solution, the pH will increase and the color of the solution changes from orange to yellow.

Note:
Acidified potassium dichromate is used for testing drunkenness. If the person has consumed alcohol, the orange color of the solution will change into green color due to oxidation of alcohol and reduction of acidified potassium dichromate to Cr(lll) ion.