What are carbonates and how are they relevant in the Cambridge IGCSE Chemistry syllabus?

Carbonates are salts of carbonic acid containing the carbonate ion, CO3^2-. They are significant in the Cambridge IGCSE Chemistry syllabus as they play a crucial role in various chemical reactions and environmental processes. Understanding carbonates helps students grasp concepts such as the carbon cycle, acid-base reactions, and the formation of sedimentary rocks.

How do carbonates react with acids, and why is this reaction important?

Carbonates react with acids to produce carbon dioxide, water, and a salt. This reaction is important because it is a fundamental chemical process that illustrates acid-base reactions, which are a key part of the Cambridge IGCSE Chemistry curriculum. For example, when calcium carbonate reacts with hydrochloric acid, it forms calcium chloride, water, and carbon dioxide gas.

What role do carbonates play in the carbon cycle?

Carbonates are integral to the carbon cycle, a crucial environmental process covered in the Chemistry of the Environment. They help regulate carbon dioxide levels in the atmosphere. Carbonates in rocks and oceans act as carbon sinks, storing carbon for long periods. This process helps balance the Earth's carbon levels, which is essential for maintaining climate stability.

Why is the thermal decomposition of carbonates significant in chemistry?

The thermal decomposition of carbonates is significant because it demonstrates how heat can break down compounds into simpler substances. For example, heating calcium carbonate results in the formation of calcium oxide and carbon dioxide. This reaction is important in industrial processes like cement production and is a key concept in the Cambridge IGCSE Chemistry syllabus.

How are carbonates used in everyday life and industry?

Carbonates have various applications in everyday life and industry. They are used in the production of glass, ceramics, and cement. In agriculture, calcium carbonate is used to neutralize acidic soil. Additionally, sodium carbonate is a common ingredient in household cleaning products. Understanding these applications helps students appreciate the practical relevance of chemistry in daily life.

Why is "Forgetting water in the carbonate + acid equation" a common mistake in Carbonates?

Why it happens: Speed. How to avoid it: Carbonate + acid → salt + water + CO₂. Three products. Source: 0620/42 — recurring

Why is "Saying decomposition gives Ca + O₂" a common mistake in Carbonates?

Why it happens: Misremembering products. How to avoid it: CaCO_3 CaO + CO_2 — calcium OXIDE plus CO₂, not the metal alone.

Why is "Saying limewater turns blue with CO₂" a common mistake in Carbonates?

Why it happens: Mixing tests. How to avoid it: Limewater turns CLOUDY/MILKY with CO₂. (Cobalt chloride goes blue→pink with water.)

Chemistry of the EnvironmentCambridge IGCSE Chemistry (0620)

Carbonates

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Carbonates — Cambridge IGCSE 0620 Chemistry Extended (2026)

Calcium carbonate (limestone) — its uses, thermal decomposition to make quicklime, and reactions with acids and water in the lime cycle.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

12.6 — Describe thermal decomposition of carbonates.
12.6 — Describe the reactions of calcium oxide with water and the role of limewater.
12.6 — State uses of limestone and lime.

Thermal decomposition of carbonates

Heat carbonate → metal oxide + CO₂. Energy-IN reaction.

General equation. $\text{metal carbonate} \xrightarrow{\text{heat}} \text{metal oxide} + \text{CO}_{2}.$

Calcium carbonate decomposes at about $800\degree\text{C}$ : $\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}.$

This is endothermic — needs continuous heat input. The product CaO is "quicklime", a white, alkaline solid.

Industrial scale: limestone is heated in lime kilns at $\sim 1000\degree\text{C}$ . Continuous flow of air removes CO₂; otherwise high CO₂ partial pressure shifts equilibrium back.

Reactivity affects ease of decomposition.

Group 1 carbonates (Na₂CO₃, K₂CO₃) — STABLE, do NOT decompose easily on heating.
Group 2 carbonates (Mg, Ca, Sr) — decompose with heating.
Less reactive metal carbonates (Cu) — decompose easily; CuCO₃ goes from green to black on gentle heating: $\text{CuCO}_{3} \to \text{CuO} + \text{CO}_{2}$ .

Pattern. The MORE REACTIVE the metal, the MORE STABLE its carbonate.

Worked qualitative. Sodium carbonate doesn't decompose even at very high temperatures (above $1000\degree\text{C}$ ). Why? Group 1 metal — strongly basic — strongly stabilises the carbonate ion.

Carbonate + heat → oxide + CO₂.
Group 1: stable; Group 2: decomposes with heat; Cu: easy.
More reactive metal → more stable carbonate.
$\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}$ at $\sim 800\degree\text{C}$ .

The lime cycle

Limestone → quicklime → slaked lime → limewater → carbonate (back to start).

Step 1: Make quicklime (CaO). $\text{CaCO}_{3}(s) \xrightarrow{\text{heat}} \text{CaO}(s) + \text{CO}_{2}(g).$

Step 2: Slake quicklime. Add water to CaO: $\text{CaO}(s) + \text{H}_{2}\text{O}(l) \to \text{Ca(OH)}_{2}(s).$

This is HIGHLY exothermic — the lump cracks, swells, and turns into white powder.

Step 3: Make limewater. Dissolve a little Ca(OH)₂ in water — it's only sparingly soluble. The clear solution is limewater.

Step 4: Test for CO₂. Bubble CO₂ through limewater: $\text{Ca(OH)}_{2}(aq) + \text{CO}_{2}(g) \to \text{CaCO}_{3}(s) + \text{H}_{2}\text{O}(l).$

CaCO₃ is INSOLUBLE → forms as a white precipitate → limewater turns CLOUDY.

This closes the cycle: limestone → CaO → Ca(OH)₂ → CaCO₃ (back to start).

Worked qualitative. Why does limewater go cloudy when you breathe into it? Your breath contains $\sim 4\%$ CO₂. CO₂ reacts with limewater Ca(OH)₂ to form insoluble CaCO₃ — the cloudiness.

Limestone $\to$ quicklime (heat).
Quicklime $\to$ slaked lime (water).
Slaked lime $\to$ limewater (more water).
Limewater + CO₂ $\to$ CaCO₃ (cloudy).
Test for CO₂ uses this last step.

Uses of limestone and lime products

Cement, glass, soil neutraliser, blast furnace flux. The biggest non-metallic mineral by volume.

Limestone (CaCO₃) and its products are used in many places.

1. Cement and concrete. Limestone heated with clay → cement clinker. Mixed with water and aggregate → concrete. The most-used artificial material in the world by mass.

2. Glass. Limestone + sand + sodium carbonate → glass.

3. Neutralising acidic soils. Slaked lime (Ca(OH)₂) added to acidic soil neutralises excess acid → improves crop growth. $\text{Ca(OH)}_{2} + 2\text{HX} \to \text{CaX}_{2} + 2\text{H}_{2}\text{O}.$

4. Blast furnace flux. Limestone removes acidic silica impurity (SiO₂) from iron ore as slag (CaSiO₃). See the Extraction of Metals study note.

5. Reducing acid rain damage. Some lakes are treated with crushed limestone to neutralise acidity caused by acid rain.

6. Building stone. Limestone blocks used directly in construction.

Worked qualitative. Why is limestone used in the blast furnace, NOT the metal oxide directly? Limestone is cheap and widely available. It decomposes in the furnace heat to form CaO (basic), which then reacts with SiO₂ (acidic) to form slag — neatly removing impurities while generating extra heat.

Cement and concrete.
Glass making.
Neutralising acidic soils.
Blast furnace flux (forms slag).
Building stone.

How it’s examined

Carbonates appear most years on Paper 4 (5-7 marks): thermal decomposition equation, lime cycle, uses. Examiner reports flag students forgetting limewater is Ca(OH)₂ specifically (not just "alkali"), and confusing the directions in the lime cycle.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (12.6); 0620/22 May/Jun 2024 — Q18 (limestone, lime cycle); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Carbonates

Step-by-step solutions to past-paper-style questions on carbonates, written exactly the way a tutor would explain them at the board.

1Carbonate + acid
Core• acid
Question
Write the equation for calcium carbonate reacting with dilute hydrochloric acid.
Step-by-step solution
1. Step 1
  Carbonate + acid → salt + water + CO₂.
  $\text{CaCO}_{3} + 2\text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$
Answer
$\text{CaCO}_{3} + 2\text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$
2Thermal decomposition of CaCO₃
Extended• Adapted from 0620/42 May/Jun 2024 Q17• decomposition
Question
Write the equation for the thermal decomposition of calcium carbonate, and state two industrial uses of the products.
Step-by-step solution
1. Step 1
  Equation:
  $\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}$
2. Step 2
  CaO (quicklime): used to make slaked lime Ca(OH)₂; neutralises acidic soil; in steelmaking.
3. Step 3
  CO₂: used in fizzy drinks, fire extinguishers.
Answer
$\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}$ . Uses include lime for agriculture/steel; CO₂ in drinks/extinguishers.
3Uses of calcium carbonate
Core• uses
Question
List three uses of calcium carbonate.
Step-by-step solution
1. Step 1
  Cement and concrete production.
2. Step 2
  Glass making.
3. Step 3
  Iron extraction (limestone in the blast furnace).
Answer
Cement, glass, blast furnace flux.

Key Definitions and Keywords — Carbonates

Definitions to memorise and the exact keywords mark schemes credit for carbonates answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Carbonate
Examiner keyword
Compound containing the $\text{CO}_{3}^{2-}$ ion (e.g. CaCO₃, Na₂CO₃).
Thermal decomposition
Examiner keyword
Breakdown of a compound into simpler substances by heat.
Quicklime (CaO)
Examiner keyword
Calcium oxide; produced by heating limestone. Used in agriculture and steelmaking.

Common Mistakes and Misconceptions — Carbonates

The traps other students keep falling into on carbonates questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Forgetting water in the carbonate + acid equation
0620/42 — recurring
Why it happens
Speed.
How to avoid it
Carbonate + acid → salt + water + CO₂. Three products.
✕Saying decomposition gives Ca + O₂
Why it happens
Misremembering products.
How to avoid it
$\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}$ — calcium OXIDE plus CO₂, not the metal alone.
✕Saying limewater turns blue with CO₂
Why it happens
Mixing tests.
How to avoid it
Limewater turns CLOUDY/MILKY with CO₂. (Cobalt chloride goes blue→pink with water.)

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Carbonates — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Carbonates

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Carbonate + acid

2Thermal decomposition of CaCO₃

3Uses of calcium carbonate

Carbonate

Thermal decomposition

Quicklime (CaO)

✕Forgetting water in the carbonate + acid equation

✕Saying decomposition gives Ca + O₂

✕Saying limewater turns blue with CO₂

Practice questions

Past paper style quiz

Assess your understanding

Carbonates — frequently asked questions