Which of the following oxide causes acid rain?

Analyze the question: We need the oxide among the options that causes acid rain. Acid rain forms when gases dissolve in rainwater to make acidic solutions.

Concept: The main acid-rain-causing oxides are:

• Sulphur dioxide (SO₂) and sulphur trioxide (SO₃), which form sulphurous and sulphuric acids
• Nitrogen dioxide (NO₂), which forms nitric acid (and nitrous acid)

Key reaction: 4NO₂ + O₂ + 2H₂O → 4HNO₃

Or: 2NO₂ + H₂O → HNO₃ + HNO₂

Checking each option:

• A. CO is carbon monoxide. It is a neutral oxide and a toxic gas (it binds to haemoglobin), but it does not form an acid in water. ✗
• B. NO is nitric oxide. It is also a neutral oxide and does not react directly with water to form an acid. It can be oxidised in air to NO₂, which then causes acid rain, but NO itself is not the direct cause. ✗
• C. H₂O₂ is hydrogen peroxide. It is not an oxide gas pollutant that causes acid rain (it is a peroxide). ✗
• D. NO₂ is nitrogen dioxide. It is an acidic oxide that dissolves in rainwater to form nitric acid, so it directly causes acid rain. ✓

Answer: D (NO₂)

Tip: Remember that acid rain comes from the acidic oxides of sulphur and nitrogen (SO₂, SO₃, NO₂). Neutral oxides such as CO, NO, and N₂O do not make acids with water. Think "Sulphur and Nitrogen = Sour Nature."