This guide covers IGCSE Chemistry topic 12: sulfur. You will revise sources of sulfur (natural deposits; from fossil fuels), sulfur dioxide (formation; acid rain; use in bleaching), contact process (SO2 → SO3 → H2SO4; catalyst; conditions), and uses of sulfuric acid.

Key concepts to revise

• Sulfur – Yellow solid; used in sulfuric acid manufacture; found in natural deposits and in fossil fuels (removed to reduce SO2 emissions).
• Sulfur dioxide – Formed when sulfur burns (S + O2 → SO2); when fossil fuels containing sulfur burn; causes acid rain; can bleach.
• Contact process – SO2 + O2 ⇌ SO3 (V2O5 catalyst; 450°C; 2 atm); SO3 + H2O → H2SO4 (sulfuric acid).
• Uses of H2SO4 – Fertilizers (e.g. (NH4)2SO4); detergents; batteries; dyes.

Exam tips and command words

• State – How SO2 is formed (sulfur or sulfide burns in air); the catalyst in the contact process (V2O5).
• Describe – The contact process (SO2 to SO3; conditions; SO3 to H2SO4); the environmental effect of SO2 (acid rain).
• Explain – Why a catalyst is used (speed up reaction; lower temperature); why SO2 is removed from flue gases (reduce acid rain).
• Write – Equations for burning sulfur; for the contact process (SO2 + O2 → SO3; SO3 + H2O → H2SO4).

Common mistakes

• Writing SO3 + H2O → H2SO4 as “direct” in industry (often SO3 is dissolved in concentrated H2SO4 first to form oleum, then diluted; check syllabus).
• Saying SO2 “is” acid rain (SO2 forms acid rain when it reacts with water/oxygen in the atmosphere).
• Not stating the catalyst (V2O5) and temperature (around 450°C) for the contact process.

Revision checklist

• Write the equation for the combustion of sulfur (S + O2 → SO2).
• Describe the contact process (SO2 to SO3; catalyst; SO3 to H2SO4).
• State the environmental impact of SO2 (acid rain) and one way to reduce it.
• State one use of sulfuric acid.

Next steps

Book a free trial with an IGCSE Chemistry tutor to practise sulfur and contact process questions, or explore Tutopiya’s learning portal for more revision resources.

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