In a test tube containing a salt, a few drops of dilute H_2SO_4 was...

When a salt containing the acetate anion ($\text{CH}_3\text{COO}^{-}$) is treated with dilute $\text{H}_2\text{SO}_4$, acetic acid ($\text{CH}_3\text{COOH}$) is formed.

$2\text{CH}_3\text{COO}^{-} + \text{H}_2\text{SO}_4 \rightarrow 2\text{CH}_3\text{COOH} + \text{SO}_4^{2-}$

Acetic acid vapours are colourless and have a characteristic smell of vinegar. Being acidic in nature, these vapours turn blue litmus paper red.

Sulphide ($\text{S}^{2-}$) gives $\text{H}_2\text{S}$ gas which has a rotten egg smell.

Sulphate ($\text{SO}_4^{2-}$) does not react with dilute $\text{H}_2\text{SO}_4$.

Carbonate ($\text{CO}_3^{2-}$) gives $\text{CO}_2$ gas which is odourless.

Answer: Acetate, $\text{CH}_3\text{COO}^{-}$