How Does Sulphur Dioxide React with Water?

What Happens When SO₂ Meets Water?

Sulphur dioxide (SO₂) is a sharp-smelling, colorless gas with wide industrial relevance. But what happens when it dissolves in water? The answer: it forms a weak acid known as sulfurous acid (H₂SO₃). Here’s the reaction:

SO₂ + H₂O → H₂SO₃

This process is exothermic, meaning it releases energy (about -1.1 kJ of free energy). However, sulfurous acid is unstable and tends to exist in equilibrium with its dissociation products.

Dissociation of Sulfurous Acid

In water, sulfurous acid doesn’t completely dissociate like sulfuric acid does. It partially ionizes into two steps:

H₂SO₃ ⇌ H⁺ + HSO₃⁻
      HSO₃⁻ ⇌ H⁺ + SO₃²⁻

Because the acid is weak, most of it remains as H₂SO₃ molecules. The dissociation constants (Ka) back this up:

Why This Reaction Matters

This reversible reaction is important in environmental science and industry. In nature, SO₂ from volcanic eruptions or fossil fuel combustion dissolves in atmospheric moisture, creating acidic droplets that can eventually lead to acid rain.

In fact, on Venus, the thick clouds contain sulfur dioxide that reacts with water vapor to form sulfuric acid, contributing to the planet’s infamous acid rain.

From SO₂ to H₂SO₄: The Next Step

Sulfurous acid can also be further oxidized by oxygen to form sulfuric acid:

H₂SO₃ + ½ O₂ → H₂SO₄

This reaction is central to the Contact Process, the industrial method of producing sulfuric acid from sulfur dioxide and oxygen. The transformation is essential for fertilizer production, metal processing, and more.

Industrial Uses of Sulfurous Acid

Though sulfurous acid isn’t available in pure form—it decomposes easily—it’s used in:

• Textile and paper industries (for bleaching wool and straw)
• Water treatment (as a mild reducing agent)
• Intermediate in sulfuric acid production

Final Thoughts

When sulphur dioxide reacts with water, it gives rise to a weak, unstable acid—sulfurous acid. While it doesn’t fully ionize or exist in pure form, its role in both environmental chemistry and industrial production is undeniable. From air pollution to acid rain and fertilizer factories, this simple reaction echoes across science and industry.