Correct Answer - C
On heating with alkalies, `K_(2)Cr_(2)O_(7)` is converted to chromate and the color changes form orange to yellow. On acidifying, yellow color agin chnages to oranege
`K_(2)Cr_(2)O_(7) + 2KOH rarr 2K_(2) CrO_(4) + H_(2)O`
or `Cr_(2)O_(7)^(2-) + 2OH^(-) rarr 2CrO_(4)^(2-) + H_(2)O`
`2CrO_(4)^(2-) + 2H^(+) rarr Cr_(2)O_(7)^(2-) + H_(2)O`
Thus, in alkaline solution chromates chromate ions are present whilst in acidic medium dichromate ions are present. Both the ions are in euilbrium at pH = 4.
`Cr_(2)O_(7)^(2-) + H_(2)O overset(pH=4)hArr2CrO_(4)^(2-) + 2H^(+)`
