Find the missing substance in the given reaction. Sulphur dioxide (SO₂) + ______ → Sulphurous acid (H₂SO₃).

• The correct answer is H2O.
• When sulphur dioxide is dissolved in water it forms sulfurous acid, the chemical equation is as follow:
• SO2 + H2O ⟶ H2SO3

Key Points 

• Sulfurous acid 
  • Sulfurous acid is a good reducing agent.
  • Sulfurous acid is a colourless liquid.
  • Sulfurous acid is unstable and has never been isolated in its pure state. It decomposes readily into water and sulfur dioxide.
  • Sulfurous acid being powerful reducing agents, it is used as a disinfecting agent.
  • It is also used as a mild bleaching agent for applications having chlorine sensitive elements.

Additional Information 

•  Sulphur Trioxide
  • It reacts violently with water to form sulfuric acid with the release of heat.
• Sulphur Pentaoxide
  • No direct reaction.
• Sulphuric Acid
  • Water is less dense than sulfuric acid and will tend to float above the acid.
  • The reaction is best thought of as forming hydronium ions.