Question

An inorganic salt is lemon yellow in colour. It becomes orange in colour like methyl orange when it is acidic and again becomes yellow when it is alkaline. The inorganic salt will be :

• A. copper nitrate
• B. ferric chloride
• C. potassium chromate
• D. potassium ferricyanide

Answer 1

The correct option is C potassium chromate

2CrO₄^2-(aq) (yellow) + 2H⁺(aq) $\to$ Cr₂O₇^2-(aq) (orange) + H₂O(l)

The addition of acid encourages the equilibrium towards the right, producing more orange-coloured dichromate(VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease, and this brings the equilibrium back to the left-hand side, regenerating yellow chromate(VI) ions.