SO$_3$ is not directly dissolved in water in the industrial preparation of H$_2$SO$_4$ by the contact process because

Analyze the question: The question asks why SO₃ is not dissolved directly in water in the contact process. In the final stage, SO₃ is first absorbed in concentrated H₂SO₄ to form oleum, which is then diluted with water.

Concept: The reaction SO₃ + H₂O → H₂SO₄ is highly exothermic. If SO₃ is passed directly into water, so much heat is released that the acid forms as a fine, corrosive mist of sulphuric acid droplets. This mist is hard to condense and collect, and it is dangerous and wasteful.

Step-by-step solution:

1. Stage 1: S + O₂ → SO₂
2. Stage 2 (catalytic, V₂O₅): 2SO₂ + O₂ ⇌ 2SO₃
3. Stage 3 (absorption): SO₃ + H₂SO₄ → H₂S₂O₇ (oleum)
4. Stage 4 (dilution): H₂S₂O₇ + H₂O → 2H₂SO₄

Checking each option:

• A. SO₃ is an acidic gas. This is true, but it is not the reason for avoiding water. Acidic character does not stop it from dissolving.
• B. Acid is added to water, not water to acid. This is a laboratory safety rule for diluting concentrated acid. It does not explain why SO₃ is not dissolved directly in water.
• C. The reaction between SO₃ and water is violently exothermic. This is correct. The large heat release creates an acid mist, which is the actual industrial reason.
• D. SO₃ does not dissolve readily in water. This is false. SO₃ reacts vigorously with water.

Answer: C

Tip: Remember "Oleum, not water": SO₃ goes into concentrated H₂SO₄ first, because the direct reaction with water is too violent and exothermic and forms a mist. Whenever a JAMB question asks why something is done indirectly in industry, look for the safety or heat-related reason.