Oleum, also known as fuming sulfuric acid, is a highly reactive compound consisting of dissolved sulfur trioxide gas (SO₃) in 100% sulfuric acid.

What do you mean by fuming?

When we say “fuming” (like fuming sulfuric acid = oleum), we mean:

• The liquid gives off visible vapors (“fumes”) when exposed to air.
• In the case of oleum, those vapors are actually SO₃ gas, which escapes easily because SO₃ is volatile.
• When this SO₃ gas meets moisture in the air, it reacts immediately to form tiny droplets of H₂SO₄, which look like white, smoky fumes.

That’s why:

• Concentrated H₂SO₄ does not “fume” much in air.
• Oleum does, because of the escaping SO₃.

So fuming = releasing visible vapors (SO₃ + H₂SO₄ mist) into the air

Fumes of H₂SO₄ dangerous?

Yes ✅ the fumes of H₂SO₄ (sulfuric acid) are very dangerous.

Here’s why:

• What the fumes are:
  Pure H₂SO₄ itself doesn’t evaporate much, but when it’s very concentrated (or as oleum) it releases SO₃ vapors.
  These vapors react with moisture in the air → form fine droplets of H₂SO₄ mist.
• Effects on health:
  • Extremely corrosive to skin, eyes, and mucous membranes.
  • Causes severe burns on contact.
  • If inhaled, can damage the nose, throat, lungs (leading to breathing difficulty, pulmonary edema).
  • Even short exposure to strong fumes is harmful.

That’s why handling concentrated H₂SO₄ / oleum requires fume hoods, protective gloves, goggles, and masks.

H₂S₂O₇ is the chemical formula of oleum?

Oleum is also called fuming sulfuric acid.

It is not a single pure compound but rather a solution of SO₃ dissolved in H₂SO₄.

H₂S₂O₇ = pyrosulfuric acid, which represents one particular composition of oleum (when there is exactly one mole of SO₃ combined with one mole of H₂SO₄).

Oleum in general can have different amounts of SO₃, not just the 1:1 case.

If it is mixture, so why it is represented by molecular formula H₂S₂O₇?

Oleum is really a mixture (H₂SO₄ + SO₃), but in chemistry we often like to represent it in a molecular way. That’s why one of its possible compositions is written as H₂S₂O₇ (pyrosulfuric acid).

Convenience in Representation

• Writing H₂S₂O₇ makes it easier to handle in equations and discussions, instead of always writing “H₂SO₄·SO₃”.
• For example, when balancing reactions in textbooks, H₂S₂O₇ is simpler to use.

But in Reality

• Oleum is not just H₂S₂O₇ — it may have more (or less) SO₃ dissolved in H₂SO₄.
• So H₂S₂O₇ is only one specific case of oleum.

Why Sulphonation of OF BENZENE IS Carried out with Oleum not pure Conc. H₂SO₄?

leum already contains a large amount of free SO₃ dissolved in H₂SO₄.

That SO₃ directly acts as the electrophile in the sulphonation of benzene.

The H₂SO₄ in oleum helps by protonating SO₃ to form the stronger electrophile