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Which of the following anions produces a reddish-brown gas when treated with concentrated H2SO4?

1. Br-
2. NO3-
3. SO3²-
4. I-

Correct answer: Br-

Solution

When Br- ions react with concentrated H2SO4, the acid first forms HBr, which is then partially oxidised by the concentrated H2SO4 to give Br2 (reddish-brown gas) along with SO2. NO3- gives colourless NO2 (actually brown NO2), SO3²- gives SO2 (colourless pungent), and I- gives violet I2 vapour.

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