Question

find the anion of a salt which on reacting with a dilute acid gives a gas that turns acidified potassium dichromate green

Answer 1

The anion of the salt is Sulphate which on reacting with acid gives sulphur dioxide gas which turns acidified potassium dichromate paper green
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Answer 2

It's a sulphite ion.
When dilute acids react with sulphite salts, they release sulphur dioxide. This reacts with potassium dichromate, leaving chromium oxide behind, which is green in color.