Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want carbonates — limestone, thermal decomposition and the test for carbon dioxide — to become reliable marks instead of half-remembered fizzing reactions.
What query it owns: how to understand and revise carbonates in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the carbonates revision-guide angle, while Tutopiya’s Carbonates subtopic page owns the learning resource and the free Carbonates quiz owns the practice.

Carbonates are ionic compounds containing the carbonate ion (CO₃²⁻). Cambridge IGCSE Coordinated Science (0654) focuses on calcium carbonate in limestone and marble, its thermal decomposition, reaction with acids, and the standard test for carbon dioxide. These ideas appear in chemistry papers every year — often linked to the lime cycle and environmental chemistry. This guide covers the syllabus definitions, equations examiners expect, and the question types that reward full marks.

Key takeaways

• Calcium carbonate (CaCO₃) is the main component of limestone, chalk and marble.
• Thermal decomposition: CaCO₃ → CaO + CO₂ (limestone → quicklime + carbon dioxide).
• Carbonates react with acids to produce salt + water + carbon dioxide.
• Test for CO₂: bubble gas through limewater — turns cloudy/milky.
• The lime cycle: CaCO₃ ⇌ CaO ⇌ Ca(OH)₂ through heating, slaking and reacting with CO₂.

What are carbonates in Cambridge IGCSE Coordinated Science?

Carbonates are compounds of metal ions bonded to the carbonate ion. Calcium carbonate is the most tested example — it decomposes on heating in kilns to make quicklime (calcium oxide), which is slaked with water to form calcium hydroxide. When carbon dioxide is bubbled through calcium hydroxide solution (limewater), a white precipitate confirms the gas. Examiners also test the reaction of carbonates with dilute acids.

Read the full notes on Tutopiya’s Carbonates subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Thermal decomposition	Breakdown by heating	”Write the equation for heating CaCO₃.”
Reaction with acid	CO₂ produced	”Describe the reaction of limestone with HCl.”
Test for CO₂	Limewater turns milky	”Describe the test for carbon dioxide.”
Lime cycle	CaCO₃ → CaO → Ca(OH)₂ → CaCO₃	”Describe the lime cycle.”
Uses of limestone	Building, cement, neutralising acid soil	”State a use of limestone.”

Key reactions — equations to know

Reaction	Equation	Conditions
Thermal decomposition	CaCO₃ → CaO + CO₂	Strong heat in kiln
With dilute acid	CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂	Room temperature
Slaking quicklime	CaO + H₂O → Ca(OH)₂	Water added to CaO
Test for CO₂	CO₂ + Ca(OH)₂ → CaCO₃ + H₂O	Gas bubbled through limewater

The lime cycle — step by step

1. Limestone (CaCO₃) heated strongly in a kiln → quicklime (CaO) + CO₂.
2. Quicklime reacts with water (slaking) → slaked lime (Ca(OH)₂).
3. Slaked lime used to neutralise acidic soil or make mortar.
4. Over time, CO₂ from air reacts with Ca(OH)₂ → CaCO₃ again (lime cycle complete).

Carbonates in past-paper wording

Command word	What the question wants	Typical stem
Write equation	Balanced symbol equation	”Write the equation for the decomposition of CaCO₃.”
Describe	Test or process	”Describe the test for carbon dioxide.”
Explain	Why limewater turns milky	”Explain the result of the CO₂ test.”
State	Use of limestone	”State a use of calcium carbonate.”
Complete	Lime cycle steps	”Complete the lime cycle diagram.”

Worked exam-style stems

1. “Write a word equation for the thermal decomposition of calcium carbonate.” Calcium carbonate → calcium oxide + carbon dioxide. Reward: correct three substances.
2. “Describe the test for carbon dioxide.” Bubble the gas through limewater (calcium hydroxide solution); limewater turns cloudy/milky white. Reward: limewater + cloudy/milky result.
3. “Explain why limewater turns milky when carbon dioxide is bubbled through it.” CO₂ reacts with Ca(OH)₂ to form insoluble CaCO₃ (a white precipitate), making the solution appear milky. Reward: CO₂ + Ca(OH)₂ → CaCO₃ precipitate.

Practise on the Carbonates quiz.

How carbonates connect to the syllabus

Carbonates link to Air and Water (CO₂ as a product and greenhouse gas) and Sulfur (acid rain erodes limestone). The Cambridge IGCSE Coordinated Science resource hub links every Chemistry subtopic.

Common mistakes students make

• Confusing quicklime (CaO) with slaked lime (Ca(OH)₂).
• Saying limewater turns milky because of CO₂ gas bubbles (it is a precipitate of CaCO₃).
• Forgetting CO₂ is produced in both thermal decomposition and acid reactions.
• Writing CaCO₃ + HCl → CaCl + … (correct salt is CaCl₂ — calcium is Group 2).
• Mixing up the lime cycle direction (heating makes CaO, not Ca(OH)₂).

When you need more support

If carbonate and CO₂-test questions keep costing marks, work through the Carbonates quiz, then get help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

What is thermal decomposition of a carbonate? Heating a carbonate to break it down into a metal oxide and carbon dioxide, e.g. CaCO₃ → CaO + CO₂.

How do you test for carbon dioxide? Bubble the gas through limewater; it turns cloudy/milky due to a precipitate of calcium carbonate.

What is the lime cycle? The cycle in which CaCO₃ is converted to CaO, then Ca(OH)₂, and back to CaCO₃ by reaction with CO₂.

How do I revise carbonates effectively? Learn the key equations, the CO₂ test, the lime cycle and uses of limestone, then take the quiz.

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