CBSE 2025, Set 3

Question

While doing qualitative analysis in chemistry lab, Abhishek added yellow coloured potassium chromate solution into a test tube. He was surprised to see the colour of the solution changing immediately to orange. He realised that the test tube was not clean and contained a few drops of some liquid. Which of the following substances will be the most likely liquid to be present in the test tube before adding potassium chromate solution? A). Sodium hydrogen carbonate solution B). Methyl orange solution C). Sodium hydroxide solution D). HCl solution

Edvaya · Accepted Answer

The correct answer is HCl solution. Potassium chromate (K$_2$CrO$_4$) is **yellow** in alkaline/neutral medium. In **acidic medium**, chromate (CrO$_4^{2-}$) converts to dichromate (Cr$_2$O$_7^{2-}$) which is **orange**: $$2	ext{CrO}_4^{2-} + 2	ext{H}^+ ightleftharpoons 	ext{Cr}_2	ext{O}_7^{2-} + 	ext{H}_2	ext{O}$$ **HCl solution** (acidic) would cause this immediate yellow → orange colour change. - NaHCO$_3$: alkaline, would not cause this change - Methyl orange: indicator, would not convert chromate to dichromate - NaOH: alkaline, would maintain yellow colour

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Chemistry > Inorganic Chemistry > D And F Block Elements

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