Oxygen compounds of sulfur
Sulfur has two important oxides: SO₂ (S +4) and SO₃ (S +6). SO₂ is a colourless, sharp-smelling, poisonous gas and an acidic oxide. It is made by burning sulfides in air: 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂; in nature it escapes with volcanic gases. It dissolves in water giving unstable sulfurous acid: SO₂ + H₂O ⇌ H₂SO₃. With alkalis and basic oxides it forms sulfites: SO₂ + 2NaOH → Na₂SO₃ + H₂O. Because sulfur is in an intermediate state, it can be an oxidizing agent (2H₂S + SO₂ → 3S + 2H₂O) and a reducing agent (2SO₂ + O₂ ⇌ 2SO₃, with a catalyst). SO₂ bleaches dyes, kills microbes and is used in small amounts to preserve dried fruit. SO₃ is a colourless liquid boiling at 45 °C that reacts violently with water to give sulfuric acid: SO₃ + H₂O → H₂SO₄; it is the key intermediate of sulfuric acid production. If SO₂ escapes into the air, moisture turns it into sulfurous and sulfuric acid, causing acid rain that harms plants and buildings.
“Acid rain diagram”: draw a factory chimney → SO₂ → cloud → H₂SO₃ / H₂SO₄ → rain → trees and buildings with arrows and write the equation at each step. Then think of 3 ways to clean it (filters, cleaner fuel, washing with alkali). Drawing only; nothing is burned.