This guide covers IGCSE Chemistry topic 11: air and water. You will revise composition of air (N2, O2, CO2, noble gases), pollution (CO2, SO2, NOx; acid rain; greenhouse effect), water treatment (sedimentation, filtration, chlorination), and rusting (iron + oxygen + water; prevention).

Key concepts to revise

• Air – Nitrogen ~78%; oxygen ~21%; carbon dioxide ~0.04%; noble gases ~1%; variable water vapour.
• Pollution – Burning fossil fuels: CO2 (greenhouse); SO2, NOx (acid rain). Describe sources and effects.
• Water treatment – Sedimentation (heavy particles); filtration (remove solids); chlorination (kill bacteria); sometimes fluoridation.
• Rusting – Iron + oxygen + water → hydrated iron(III) oxide (rust); prevent by coating (paint, grease), galvanising, or sacrificial metal.

Exam tips and command words

• State – The approximate percentage of N2 and O2 in air; the conditions needed for rusting (oxygen, water).
• Describe – The steps in water treatment (sedimentation, filtration, chlorination); how to prevent rusting (one method).
• Explain – Why SO2 causes acid rain (forms sulfuric acid in atmosphere); why CO2 is linked to global warming (greenhouse gas).
• Suggest – One way to reduce air pollution (e.g. catalytic converter; renewable energy).

Common mistakes

• Saying air is “mostly oxygen” (air is mostly nitrogen; oxygen ~21%).
• Confusing rusting (chemical reaction; iron oxide) with corrosion (general term; rusting is corrosion of iron).
• Not stating that chlorination kills bacteria (not just “cleans” water).

Revision checklist

• State the approximate composition of air (N2, O2, CO2 percentages).
• Describe the main steps in water treatment and the purpose of each.
• State the conditions for rusting and one method of prevention.
• Describe the cause of acid rain (SO2, NOx) and link CO2 to greenhouse effect.

Next steps

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

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