This guide covers IGCSE Chemistry topic 13: carbonates. You will revise reaction with acid (carbonate + acid → salt + water + CO2), thermal decomposition (e.g. CaCO3 → CaO + CO2), lime cycle (CaCO3 → CaO → Ca(OH)2 → CaCO3), and test for CO2 (limewater turns cloudy).

Key concepts to revise

• Carbonate + acid – Salt + water + CO2 (e.g. CaCO3 + 2HCl → CaCl2 + H2O + CO2). Used to test for carbonates (effervescence; CO2 turns limewater cloudy).
• Thermal decomposition – Some carbonates decompose on heating: e.g. CaCO3 → CaO + CO2; CuCO3 → CuO + CO2. Group I carbonates more stable (harder to decompose).
• Lime cycle – CaCO3 (limestone) → heat → CaO (quicklime) + CO2; CaO + H2O → Ca(OH)2 (slaked lime); Ca(OH)2 + CO2 → CaCO3 + H2O (e.g. in mortar).
• Test for CO2 – Bubble gas through limewater; turns cloudy (white precipitate of CaCO3).

Exam tips and command words

• State – What is observed when carbonate reacts with acid (effervescence; CO2); what is observed when CO2 is passed through limewater (turns cloudy).
• Write – Equation for carbonate + acid (salt + water + CO2); equation for thermal decomposition of CaCO3.
• Describe – The lime cycle (limestone → quicklime → slaked lime; use in mortar/neutralising soil).
• Explain – Why thermal decomposition of CaCO3 is endothermic (bond breaking; heat needed).

Common mistakes

• Forgetting water as a product in carbonate + acid (salt + water + CO2).
• Saying limewater “turns white” (it turns cloudy; white precipitate of CaCO3 forms).
• Not stating that Group I carbonates (e.g. Na2CO3) do not decompose easily on heating (unlike CaCO3, CuCO3).

Revision checklist

• Write the general equation for carbonate + acid (salt + water + CO2).
• Describe the test for CO2 (limewater cloudy) and the test for carbonate (acid; effervescence; CO2).
• Write the equation for thermal decomposition of CaCO3 and of CuCO3.
• Describe the lime cycle (CaCO3 → CaO → Ca(OH)2) and one use (e.g. mortar; neutralising soil).

Next steps

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

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