Why is SO₃ not directly absorbed in water?

Sulfur trioxide (SO₃) is the anhydride of sulfuric acid (H₂SO₄) and reacts readily with water to form sulfuric acid:

SO3+H2O→H2SO4SO₃ + H₂O → H₂SO₄SO3+H2O→H2SO4

This reaction is extremely exothermic, releasing a large amount of heat. If SO₃ is added directly to water, the heat released can cause the water to boil instantly, forming dense sulfuric acid mist, which is hazardous and difficult to handle. This violent reaction is why SO₃ is not absorbed directly in water in industrial processes.

Instead, in the contact process used to manufacture sulfuric acid, SO₃ is first absorbed in concentrated H₂SO₄ to form oleum (H₂S₂O₇):

SO3+H2SO4→H2S2O7SO₃ + H₂SO₄ → H₂S₂O₇SO3+H2SO4→H2S2O7

Oleum can then be carefully diluted with water to produce concentrated sulfuric acid safely:

H2S2O7+H2O→2H2SO4H₂S₂O₇ + H₂O → 2H₂SO₄H2S2O7+H2O→2H2SO4

This method prevents the violent exothermic reaction and allows for safe handling and production of concentrated H₂SO₄.

Other options are incorrect because:

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  B) It is insoluble in water — SO₃ is actually highly soluble, but the problem is the reaction energy, not solubility.

  
  


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  C) It forms dilute acid — Direct reaction would produce concentrated acid, not dilute.

  
  


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  D) None of the above — Incorrect, because option A is correct.

  
  

Thus, the primary reason SO₃ is not absorbed directly in water is its highly exothermic reaction, which makes direct contact dangerous and impractical in industrial chemistry.