JEE Main 2023 · 31 January · evening · Single correct · +4 / −1
The normal rain water is slightly acidic and its pH value is 5.6 because of which one of the following?
- (a) N₂ O₅+H₂ O → 2 HNO₃
- (b) 2 SO₂+O₂+2 H₂ O → 2 H₂ SO₄
- (c) 4 NO₂+O₂+2 H₂ O → 4 HNO₃
- (d) CO₂+H₂ O → H₂ CO₃
Answer and solution
Answer (d) CO₂+H₂ O → H₂ CO₃
Solution
Chemistry ma’am · 4 steps
Clean air everywhere has CO₂, and CO₂ dissolving in rain is what pulls the pH down to 5.6.
- Normal versus acid. Normal rain falls through unpolluted air. So the cause must be a gas that is always present in the atmosphere, not one released by pollution.
- Carbon dioxide dissolves. Air always contains CO₂. It dissolves in rain drops and forms carbonic acid, a weak acid that releases a little H⁺.
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- Why only 5.6. H₂CO₃ is weak and only slightly ionised, so the pH falls just a little below 7, to about 5.6. The oxides of sulphur and nitrogen form strong acids and give acid rain, with pH below 5.6.
- So. The pH of 5.6 of normal rain water comes from CO₂ + H₂O → H₂CO₃, which is option (d).
The idea to keep. pH 5.6 is the boundary. Above it, the acidity comes only from natural CO₂ forming weak H₂CO₃. Below it, polluting oxides of S and N make strong acids, and that is acid rain.
Answer from the NTA final answer key. Solution by PrepWiser.