Why is "Giving a vague limewater result like 'it changes colour'" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: Students remember 'something happens to the limewater' but not the exact result. How to avoid it: Write the precise result: the limewater turns **milky / cloudy / white**. This is the mark-scoring phrase. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Saying a carbonate decomposes to a metal oxide and water" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: Confusion with metal hydroxides, which DO give a metal oxide and water on heating. How to avoid it: Carbonates give **metal oxide + carbon dioxide**. The carbonate group (CO₃) becomes CO₂, not water. Source: Edexcel Chemistry examiner reports

Why is "Writing CO instead of CO₂ for the gas" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: Students muddle carbon monoxide (CO) with carbon dioxide (CO₂). How to avoid it: Decomposing carbonates give **carbon DIoxide, CO₂** (two oxygens). Carbon monoxide comes from incomplete combustion, not from carbonates. Source: Edexcel Chemistry examiner reports

Why is "Writing the wrong carbonate formula, e.g. CaCO₂ or CuCO instead of CaCO₃ / CuCO₃" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: The carbonate ion (CO₃²⁻) and its three oxygens are forgotten. How to avoid it: The carbonate group is always **CO₃**. So calcium carbonate is **CaCO₃** and copper(II) carbonate is **CuCO₃**. Source: Edexcel Chemistry examiner reports

Why is "Giving the copper(II) carbonate colour change the wrong way round (black → green)" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: Students recall the two colours but not which is the starting solid. How to avoid it: Start = **green** copper(II) carbonate; after heating = **black** copper(II) oxide. Green → black. Source: Edexcel Chemistry examiner reports

Why is "Saying mass is created or lost overall when a carbonate is heated" a common mistake in Carbon Dioxide from Thermal Decomposition?

Why it happens: The solid clearly gets lighter, so students think mass disappears. How to avoid it: Mass is **conserved** — the solid gets lighter only because **CO₂ gas escapes**. carbonate mass = oxide mass + CO₂ mass. Source: Edexcel Chemistry examiner reports

Gases in the atmosphereEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Carbon Dioxide from Thermal Decomposition

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Carbon Dioxide from Thermal Decomposition, ready to print or save offline.

Carbon Dioxide from Thermal Decomposition — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

How heating metal carbonates breaks them down to release carbon dioxide — the meaning of thermal decomposition, word and balanced equations for calcium carbonate and copper(II) carbonate, the green-to-black colour change, and the limewater test that turns milky. Covers Double Award Chemistry spec 2.9, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

2.9 — Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates (e.g. calcium carbonate, copper(II) carbonate), and the limewater test for CO2.

What is thermal decomposition? (spec 2.9)

One compound is broken down into simpler substances using heat — nothing else is added.

Thermal decomposition is the breaking down of a single compound into two or more simpler substances by heating it. The word gives you the meaning: thermal = heat, decomposition = breaking down.

The key idea examiners want is that heat is the only thing supplied — you do not add another chemical. This makes it different from a reaction like neutralisation, where an acid and an alkali react together.

For this topic, the compounds that matter are metal carbonates. When you heat a metal carbonate strongly, it breaks down to give a metal oxide and carbon dioxide gas:

metal carbonate → metal oxide + carbon dioxide

The general pattern is worth memorising, because every example you meet follows it.

Thermal decomposition = one compound broken down BY HEAT.
Only heat is supplied — no second chemical is added.
Metal carbonate → metal oxide + carbon dioxide.
It is a chemical change — new substances are made.

See the full worked example for carbon dioxide from thermal decomposition →

Equations for decomposing carbonates (spec 2.9)

Learn the word and balanced equations for calcium carbonate and copper(II) carbonate.

Two carbonates appear again and again in exams: calcium carbonate and copper(II) carbonate. You should be able to write both the word equation and the balanced symbol equation for each.

Calcium carbonate (found in limestone, chalk and marble) decomposes when heated strongly:

word: calcium carbonate → calcium oxide + carbon dioxide

symbol: CaCO₃ → CaO + CO₂

Calcium carbonate needs a high temperature (above about 800 °C — a roaring Bunsen flame for a long time) before it decomposes. The calcium oxide left behind is also called quicklime.

Copper(II) carbonate decomposes much more easily, and gives a clear colour change you can use to spot the reaction:

word: copper(II) carbonate → copper(II) oxide + carbon dioxide

symbol: CuCO₃ → CuO + CO₂

Here the green copper(II) carbonate turns into black copper(II) oxide. That green → black colour change is a favourite observation mark.

Why are these equations already balanced? Count the atoms on each side: in CaCO₃ → CaO + CO₂ there is 1 Ca, 1 C and 3 O on both sides. The numbers match, so no big numbers are needed in front of the formulae.

CaCO₃ → CaO + CO₂ (calcium carbonate → calcium oxide + CO₂).
CuCO₃ → CuO + CO₂ (copper(II) carbonate → copper(II) oxide + CO₂).
Copper(II) carbonate: GREEN solid → BLACK solid (visible change).
Both equations are already balanced — check atoms on each side.

See the full worked example for carbon dioxide from thermal decomposition →

The limewater test for carbon dioxide (spec 2.9)

Bubble the gas through limewater — CO₂ turns it milky / cloudy white.

Once a carbonate decomposes, how do you prove the gas given off is carbon dioxide? You use the limewater test — the standard chemical test for CO₂.

Limewater is a solution of calcium hydroxide (Ca(OH)₂) in water. It is clear and colourless to start with.

The test: bubble the gas through the limewater.

Positive result: the limewater turns milky / cloudy white. That cloudiness is the result; if you see it, the gas is carbon dioxide.

The cloudiness happens because the carbon dioxide reacts with the calcium hydroxide to make an insoluble white solid, calcium carbonate, which forms a fine cloud of tiny particles (a precipitate):

carbon dioxide + calcium hydroxide → calcium carbonate + water

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

The apparatus heats the carbonate in a tube and carries the gas through a delivery tube into a second tube of limewater.

Heat the carbonate; the CO₂ given off travels along the delivery tube and turns the limewater milky.

Exam tip: the result you must write is "the limewater turns milky / cloudy / white". Saying it "changes colour" is too vague to score — name the milky/cloudy result.

Limewater = calcium hydroxide solution, Ca(OH)₂ (clear and colourless).
Bubble the gas through the limewater.
Positive result: limewater turns MILKY / CLOUDY WHITE.
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (white CaCO₃ causes the cloudiness).

See the full worked example for carbon dioxide from thermal decomposition →

Simple mass calculations for the CO₂ given off

Use conservation of mass and Mr to work out how much CO₂ is produced.

Because no atoms are created or destroyed, the total mass is conserved in a thermal decomposition. The mass of carbonate you start with equals the mass of metal oxide left plus the mass of carbon dioxide that escaped.

mass of carbonate = mass of metal oxide + mass of carbon dioxide

This lets you find the mass of CO₂ given off without weighing the gas — you just weigh the solid before and after heating, and the loss in mass is the CO₂ that escaped.

Using relative formula masses (Mr). For a calculation you may be given the Mr values:

Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100
Mr of CaO = 40 + 16 = 56
Mr of CO₂ = 12 + (2 × 16) = 44

So from the equation CaCO₃ → CaO + CO₂, every 100 g of calcium carbonate gives 56 g of calcium oxide and 44 g of carbon dioxide. You can scale these ratios up or down for any starting mass.

Quick sense-check: 56 + 44 = 100, which equals the mass you started with — mass is conserved, exactly as expected.

Mass is conserved: carbonate mass = oxide mass + CO₂ mass.
Loss in mass on heating = mass of CO₂ given off.
Mr: CaCO₃ = 100, CaO = 56, CO₂ = 44.
100 g CaCO₃ → 56 g CaO + 44 g CO₂ (scale as needed).

See the full worked example for carbon dioxide from thermal decomposition →

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). This subtopic appears as: writing the word and balanced equation for a decomposing carbonate (2–3 marks); describing the limewater test and its milky result (1–2 marks); stating the green → black observation for copper(II) carbonate (1 mark); and short conservation-of-mass / Mr calculations for the CO₂ produced (2–3 marks). The biggest, easiest marks lost are vague test results — always write 'milky / cloudy'.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Carbon Dioxide from Thermal Decomposition, ready to print or save as PDF.

Step-by-step worked examples — Carbon Dioxide from Thermal Decomposition

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

1Define thermal decomposition (Getting started)
Getting started• 4WSD0/1C style — short answer• thermal-decomposition, spec-2.9
Question
What is meant by the term thermal decomposition? (2 marks)
Step-by-step solution
1. Step 1
  Breaking down (1). It is the breaking down of a compound into two or more simpler substances.
2. Step 2
  By heat (1). This breakdown is caused by heating — no other chemical is added.
3. Step 3
  Watch the wording. Don't just say 'it breaks down' — you must say it is heated, and that ONE compound splits into simpler ones.
Answer
Thermal decomposition is the breaking down of a compound into two or more simpler substances by heating it.
Examiner tip
Both ideas are needed: 'breaks down into simpler substances' AND 'by heating'. Leaving out 'heat' or describing a reaction between two chemicals scores zero.
2Test for carbon dioxide (Getting started)
Getting started• 4WSD0/1C style — short answer• limewater, gas-test, spec-2.9
Question
Describe the chemical test for carbon dioxide gas, including the result of a positive test. (2 marks)
Step-by-step solution
1. Step 1
  The test (1). Bubble the gas through limewater (calcium hydroxide solution).
2. Step 2
  The result (1). The limewater turns milky / cloudy white.
3. Step 3
  Be specific. 'It changes colour' is not enough — name the milky/cloudy result.
Answer
Bubble the gas through limewater; if carbon dioxide is present the limewater turns milky (cloudy white).
Examiner tip
One mark for 'limewater', one for 'turns milky/cloudy'. The single most common lost mark here is a vague result like 'goes a different colour'.
3Write the decomposition equation (Building confidence)
Building confidence• 4WSD0/1C style — structured• equations, calcium-carbonate, spec-2.9
Question
Calcium carbonate is heated strongly. Write the word equation and the balanced symbol equation for the reaction. (3 marks)
Step-by-step solution
1. Step 1
  Apply the pattern. Metal carbonate → metal oxide + carbon dioxide.
2. Step 2
  Word equation (1). calcium carbonate → calcium oxide + carbon dioxide.
3. Step 3
  Formulae (1). Calcium carbonate is CaCO₃, calcium oxide is CaO, carbon dioxide is CO₂.
4. Step 4
  Balance (1). CaCO₃ → CaO + CO₂. Check: 1 Ca, 1 C, 3 O on each side — already balanced.
Answer
Word: calcium carbonate → calcium oxide + carbon dioxide. Symbol: CaCO₃ → CaO + CO₂.
Examiner tip
The symbol equation must have correct formulae AND balance. The most common error is writing CaCO₂ or CaCO instead of CaCO₃, or CO instead of CO₂.
4Explain the observations for copper(II) carbonate (Building confidence)
Building confidence• 4WSD0/1C style — structured• copper-carbonate, observations, spec-2.9
Question
A green solid is heated in a tube and a gas is given off. The solid turns black. The gas is bubbled through limewater. Name the green solid, the black solid and the gas, and state what is seen in the limewater. (4 marks)
Step-by-step solution
1. Step 1
  Green solid (1). The green solid is copper(II) carbonate (CuCO₃).
2. Step 2
  Black solid (1). Heating decomposes it to copper(II) oxide (CuO), which is black.
3. Step 3
  The gas (1). The gas given off is carbon dioxide (CO₂).
4. Step 4
  Limewater (1). The limewater turns milky / cloudy, confirming the gas is CO₂.
Answer
Green solid = copper(II) carbonate; black solid = copper(II) oxide; gas = carbon dioxide; the limewater turns milky/cloudy.
Examiner tip
The green → black colour change and the milky limewater are independent observation marks. Students who only name the carbonate and oxide but forget the limewater result drop an easy mark.
5Calculate the mass of CO₂ released (Stretch)
Stretch• 4WSD0/1C style — extended• calculation, conservation-of-mass, spec-2.9
Question
25.0 g of calcium carbonate (CaCO₃) is heated until it has fully decomposed. (Mr: CaCO₃ = 100, CaO = 56, CO₂ = 44.) Calculate the mass of carbon dioxide given off. (3 marks)
Step-by-step solution
1. Step 1
  Ratio from the equation (1). CaCO₃ → CaO + CO₂, so 100 g of CaCO₃ gives 44 g of CO₂.
2. Step 2
  Scale to 25.0 g (1). mass of CO₂ = (44 ÷ 100) × 25.0 = 0.44 × 25.0.
3. Step 3
  Final answer (1). mass of CO₂ = 11.0 g.
4. Step 4
  Check by mass conservation. CaO left = (56 ÷ 100) × 25.0 = 14.0 g; 14.0 + 11.0 = 25.0 g ✓.
Answer
11.0 g of carbon dioxide.
Examiner tip
Method marks are for the 44/100 ratio and the scaling. Show the working — even with a calculator, a wrong final number can still earn the method marks if the steps are visible.
6Use loss in mass to find the oxide (Stretch)
Stretch• 4WSD0/1C style — extended• calculation, conservation-of-mass, spec-2.9
Question
A student heats some copper(II) carbonate until no more gas is given off. Before heating the solid weighs 12.4 g; after heating it weighs 8.0 g. (a) Explain why the solid loses mass. (b) Calculate the mass of carbon dioxide given off. (4 marks)
Step-by-step solution
1. Step 1
  (a) Reason (2). On heating, the copper(II) carbonate decomposes (CuCO₃ → CuO + CO₂). Carbon dioxide gas escapes from the tube, so the solid that remains weighs less.
2. Step 2
  (b) Use conservation of mass (1). mass of CO₂ lost = mass before − mass after = 12.4 − 8.0.
3. Step 3
  Final answer (1). mass of CO₂ = 4.4 g.
Answer
(a) CO₂ gas escapes as the carbonate decomposes, so the remaining solid (copper(II) oxide) weighs less. (b) 12.4 − 8.0 = 4.4 g of carbon dioxide.
Examiner tip
The explanation mark needs the word 'gas escapes', not just 'it decomposes'. The calculation is simply the loss in mass — no Mr needed because mass is conserved.

Model Answers — Carbon Dioxide from Thermal Decomposition

High-scoring sample answers for carbon dioxide from thermal decomposition on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style2 marks
Complete the general word equation for the thermal decomposition of a metal carbonate: metal carbonate → __________ + __________. (2 marks)
Model answer
metal carbonate → metal oxide + carbon dioxide

metal oxide (1)

carbon dioxide (1)
Why this scores
1 mark each. Both products are required; 'metal oxide and water' is a common wrong answer — carbonates give CO₂, not water.
Question 2
4WSD0/1C style2 marks
A gas turns limewater milky. (a) Name the gas. (b) Name the chemical in limewater. (2 marks)
Model answer
(a) The gas is carbon dioxide (CO₂). (1)

(b) Limewater is a solution of calcium hydroxide (Ca(OH)₂). (1)
Why this scores
Straightforward recall. A 'milky limewater' result is the definitive test for carbon dioxide — make sure you can name both the gas and the calcium hydroxide.
Question 3
4WSD0/1C style4 marks
Copper(II) carbonate is heated. (a) State the colour change seen. (b) Write the balanced symbol equation for the reaction. (4 marks)
Model answer
(a) Colour change.

The solid changes from green (copper(II) carbonate)... (1)

...to black (copper(II) oxide). (1)

(b) Symbol equation.

Correct formulae: CuCO₃, CuO and CO₂. (1)

Balanced equation: CuCO₃ → CuO + CO₂ (1 Cu, 1 C, 3 O on each side). (1)
Why this scores
Marks for both colours in the right order (green then black) and for a correctly balanced equation. Writing 'CuCO₃ → CuO + CO₂' as already balanced is fine — check the atoms to be safe.
Question 4
4WSD0/1C style4 marks
Explain, including an equation, why limewater turns milky when carbon dioxide is bubbled through it. (4 marks)
Model answer
The carbon dioxide reacts with the calcium hydroxide in the limewater. (1)

This forms calcium carbonate, which is insoluble in water. (1)

The insoluble calcium carbonate forms a fine white precipitate, which makes the limewater look milky / cloudy. (1)

Equation: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (1)
Why this scores
The 'why' marks are for an insoluble white solid (calcium carbonate) forming a precipitate. Many students describe the milky result but cannot explain that the cloudiness IS solid calcium carbonate.
Question 5
4WSD0/1C style — extended6 marks
Describe how you would use a sample of green copper(II) carbonate to show that heating a metal carbonate produces carbon dioxide. Include the apparatus, what you would do, and the observations that confirm carbon dioxide is produced. (6 marks)
Model answer
Apparatus and setup.

Place the copper(II) carbonate in a boiling tube / test tube fitted with a bung and delivery tube. (1)

Lead the delivery tube into a second tube containing limewater. (1)

Method.

Heat the carbonate strongly with a Bunsen burner. (1)

The gas given off travels along the delivery tube and bubbles through the limewater. (1)

Observations confirming CO₂.

The green solid turns black (copper(II) oxide forms). (1)

The limewater turns milky / cloudy, which confirms the gas is carbon dioxide. (1)
Why this scores
Top-band answers cover apparatus (delivery tube into limewater), the action (heat strongly), and BOTH observations (green→black solid; milky limewater). Forgetting the delivery tube or the limewater result caps the mark.
Question 6
4WSD0/1C style — extended8 marks
A sample of calcium carbonate is heated until it has fully decomposed (CaCO₃ → CaO + CO₂). The mass falls from 50.0 g to 28.0 g. (a) Explain why the mass falls. (b) Calculate the mass of carbon dioxide lost. (c) Using Mr values (CaCO₃ = 100, CO₂ = 44), calculate the mass of CO₂ expected from 50.0 g of pure calcium carbonate, and comment on whether the sample was pure. (8 marks)
Model answer
(a) Why the mass falls (2).

The calcium carbonate decomposes and carbon dioxide gas is given off. (1)

The gas escapes from the tube, so the remaining solid (calcium oxide) weighs less than the carbonate. (1)

(b) Mass of CO₂ lost (2).

Mass lost = mass before − mass after = 50.0 − 28.0. (1)

= 22.0 g of carbon dioxide. (1)

(c) Expected mass and comment (4).

From the equation, 100 g of CaCO₃ gives 44 g of CO₂. (1)

For 50.0 g: expected CO₂ = (44 ÷ 100) × 50.0 = 22.0 g. (1)

The actual mass lost (22.0 g) equals the expected mass (22.0 g). (1)

So the sample was (effectively) pure calcium carbonate — all 50.0 g decomposed as expected. (1)
Why this scores
This combines conservation of mass with an Mr ratio and a 'comment' mark. The comparison must be explicit (actual = expected → pure). If the actual loss were less than expected, the answer would be 'impure / contains other substances that do not decompose'.

Key Definitions and Keywords — Carbon Dioxide from Thermal Decomposition

Definitions to memorise and the exact keywords mark schemes credit for carbon dioxide from thermal decomposition answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Thermal decomposition
Examiner keyword
The breaking down of a single compound into two or more simpler substances by heating it. No other chemical is added.
Metal carbonate
Examiner keyword
A compound containing a metal combined with the carbonate group (CO₃²⁻), e.g. calcium carbonate (CaCO₃) or copper(II) carbonate (CuCO₃). Metal carbonates decompose on heating to a metal oxide and carbon dioxide.
Metal oxide
A compound of a metal with oxygen, e.g. calcium oxide (CaO) or copper(II) oxide (CuO). It is the solid product left when a metal carbonate is decomposed.
Carbon dioxide (CO₂)
Examiner keyword
A colourless gas given off when metal carbonates are heated; it turns limewater milky.
Limewater
Examiner keyword
A solution of calcium hydroxide, Ca(OH)₂, used to test for carbon dioxide. It is clear and colourless, and turns milky/cloudy when CO₂ is bubbled through it.
Milky (cloudy)
Examiner keyword
The positive result of the limewater test — the limewater becomes a cloudy white because insoluble calcium carbonate forms.
Precipitate
An insoluble solid formed in a solution during a reaction. In the limewater test the precipitate is white calcium carbonate, which causes the milkiness.
Conservation of mass
Examiner keyword
The total mass of the products equals the total mass of the reactants, because atoms are not created or destroyed. The loss in mass on heating a carbonate equals the mass of CO₂ that escaped.
Calcium oxide (quicklime)
The white solid (CaO) left when calcium carbonate is decomposed by strong heating; also called quicklime.
Balanced symbol equation
An equation with the same number of each type of atom on both sides, e.g. CaCO₃ → CaO + CO₂ (1 Ca, 1 C, 3 O each side).

Common Mistakes and Misconceptions — Carbon Dioxide from Thermal Decomposition

The traps other students keep falling into on carbon dioxide from thermal decomposition questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Giving a vague limewater result like 'it changes colour'
Edexcel Chemistry examiner reports — recurring error
Why it happens
Students remember 'something happens to the limewater' but not the exact result.
How to avoid it
Write the precise result: the limewater turns milky / cloudy / white. This is the mark-scoring phrase.
✕Saying a carbonate decomposes to a metal oxide and water
Edexcel Chemistry examiner reports
Why it happens
Confusion with metal hydroxides, which DO give a metal oxide and water on heating.
How to avoid it
Carbonates give metal oxide + carbon dioxide. The carbonate group (CO₃) becomes CO₂, not water.
✕Writing CO instead of CO₂ for the gas
Edexcel Chemistry examiner reports
Why it happens
Students muddle carbon monoxide (CO) with carbon dioxide (CO₂).
How to avoid it
Decomposing carbonates give carbon DIoxide, CO₂ (two oxygens). Carbon monoxide comes from incomplete combustion, not from carbonates.
✕Writing the wrong carbonate formula, e.g. CaCO₂ or CuCO instead of CaCO₃ / CuCO₃
Edexcel Chemistry examiner reports
Why it happens
The carbonate ion (CO₃²⁻) and its three oxygens are forgotten.
How to avoid it
The carbonate group is always CO₃. So calcium carbonate is CaCO₃ and copper(II) carbonate is CuCO₃.
✕Giving the copper(II) carbonate colour change the wrong way round (black → green)
Edexcel Chemistry examiner reports
Why it happens
Students recall the two colours but not which is the starting solid.
How to avoid it
Start = green copper(II) carbonate; after heating = black copper(II) oxide. Green → black.
✕Saying mass is created or lost overall when a carbonate is heated
Edexcel Chemistry examiner reports
Why it happens
The solid clearly gets lighter, so students think mass disappears.
How to avoid it
Mass is conserved — the solid gets lighter only because CO₂ gas escapes. carbonate mass = oxide mass + CO₂ mass.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Carbon Dioxide from Thermal Decomposition — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

Calcium carbonate (found in limestone, chalk and marble) decomposes when heated strongly:

word: calcium carbonate → calcium oxide + carbon dioxide

symbol: CaCO₃ → CaO + CO₂

Calcium carbonate needs a high temperature (above about 800 °C — a roaring Bunsen flame for a long time) before it decomposes. The calcium oxide left behind is also called quicklime.

Copper(II) carbonate decomposes much more easily, and gives a clear colour change you can use to spot the reaction:

word: copper(II) carbonate → copper(II) oxide + carbon dioxide

symbol: CuCO₃ → CuO + CO₂

Here the green copper(II) carbonate turns into black copper(II) oxide. That green → black colour change is a favourite observation mark.

Carbon Dioxide from Thermal Decomposition

Detailed Notes

What you’ll learn

How it’s examined

1Define thermal decomposition (Getting started)

2Test for carbon dioxide (Getting started)

3Write the decomposition equation (Building confidence)

4Explain the observations for copper(II) carbonate (Building confidence)

5Calculate the mass of CO₂ released (Stretch)

6Use loss in mass to find the oxide (Stretch)

Thermal decomposition

Metal carbonate

Metal oxide

Carbon dioxide (CO₂)

Limewater

Milky (cloudy)

Precipitate

Conservation of mass

Calcium oxide (quicklime)

Balanced symbol equation

✕Giving a vague limewater result like 'it changes colour'

✕Saying a carbonate decomposes to a metal oxide and water

✕Writing CO instead of CO₂ for the gas

✕Writing the wrong carbonate formula, e.g. CaCO₂ or CuCO instead of CaCO₃ / CuCO₃

✕Giving the copper(II) carbonate colour change the wrong way round (black → green)

✕Saying mass is created or lost overall when a carbonate is heated

Past paper style quiz

Carbon Dioxide from Thermal Decomposition

Detailed Notes

What you’ll learn

How it’s examined

1Define thermal decomposition (Getting started)

2Test for carbon dioxide (Getting started)

3Write the decomposition equation (Building confidence)

4Explain the observations for copper(II) carbonate (Building confidence)

5Calculate the mass of CO₂ released (Stretch)

6Use loss in mass to find the oxide (Stretch)

Thermal decomposition

Metal carbonate

Metal oxide

Carbon dioxide (CO₂)

Limewater

Milky (cloudy)

Precipitate

Conservation of mass

Calcium oxide (quicklime)

Balanced symbol equation

✕Giving a vague limewater result like 'it changes colour'

✕Saying a carbonate decomposes to a metal oxide and water

✕Writing CO instead of CO₂ for the gas

✕Writing the wrong carbonate formula, e.g. CaCO₂ or CuCO instead of CaCO₃ / CuCO₃

✕Giving the copper(II) carbonate colour change the wrong way round (black → green)

✕Saying mass is created or lost overall when a carbonate is heated

Past paper style quiz