ChemistryICSE Class 8 · Carbon and its Compounds

Carbon Dioxide and Carbon Monoxide: Preparation, Tests, Uses and Hazards (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

Carbon dioxide is made in the laboratory from marble chips and dilute hydrochloric acid, collected by upward displacement of air; it turns lime water milky, is acidic and does not support combustion. Carbon monoxide, formed in limited air, is poisonous, neutral, burns with a blue flame and is a reducing agent.

Understand Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses

Carbon dioxide: laboratory preparation.

$C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$

Marble chips (calcium carbonate) and dilute hydrochloric acid in a flask with a thistle funnel dipping into the acid and a delivery tube.

Collection: upward displacement of air (the gas is about 1.5 times heavier than air); not over water because it is fairly soluble.
Test that the jar is full: a burning splinter at the mouth goes out.
Dilute sulphuric acid is not used: calcium sulphate, only slightly soluble, coats the marble and stops the reaction.

Properties of carbon dioxide.

Colourless, odourless; about 1.5 times heavier than air; fairly soluble in water (more under pressure); solid CO₂ (dry ice) sublimes.
Acidic oxide: $C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$ (weak carbonic acid); moist blue litmus turns light red.
Lime water test: turns milky: $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ . With excess CO₂ the milkiness disappears: $C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$ .
With alkalis: $2 N a O H + C O_{2} \to N a_{2} C O_{3} + H_{2} O$ .
Does not burn or support combustion of most substances; burning magnesium continues: $2 M g + C O_{2} \to 2 M g O + C$ .

Uses: fire extinguishers (heavier than air, does not support burning), aerated drinks, dry ice, photosynthesis.

Carbon monoxide.

Formation in limited air: $2 C + O_{2} \to 2 C O$ (car exhaust, charcoal stoves in closed rooms).
Colourless, odourless, almost insoluble in water; neutral to litmus.
Burns with a blue flame: $2 C O + O_{2} \to 2 C O_{2}$ .
Reducing agent: in extraction of iron, $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ .
Poisonous: combines with haemoglobin to form carboxyhaemoglobin, so the blood carries less oxygen (headache, dizziness, unconsciousness, asphyxia).

	CO₂	CO
Litmus	acidic (blue turns light red)	neutral
Lime water	milky	no change
Burns?	no	yes, blue flame
Effect on breathing	not poisonous but displaces air	poisonous

Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses: formulas and key facts

Preparation of CO₂: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$; Upward displacement of air.
Lime water test: $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$; Excess: Ca(HCO₃)₂, clear again.
Carbonic acid: $C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$; Weak acid; litmus light red.
Iron extraction: $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$; CO is the reducing agent.
Limited air: $2 C + O_{2} \to 2 C O$; Poisonous gas.

How to solve Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses questions

For gas questions, give the reagents, the collection method with its reason, and the test.

1Write the balanced equation for the preparation.
2Choose the collection method from density and solubility.
3Give the identity test: lime water for CO₂; blue flame for CO.
4For mass questions use the formula masses given in the question (here CaCO₃ 100, CO₂ 44, HCl 36.5, CaCl₂ 111, H₂O 18).

Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Fizzy drink

Suppose a bottle of soda water is opened and fizzes, and a teacher holds a lit match above the fizz.

Explain the fizz and what happens to the flame.

Try it first, then show the solutionSolution

Carbon dioxide dissolved under pressure escapes as the pressure falls, forming bubbles.
CO₂ does not support burning and is heavier than air, so the flame may go out when the gas collects around it.

Answer: Dissolved CO₂ escapes; it does not support combustion.

Memory hook: Pressure off, gas out.

Coal stove at night

Imagine a family lights a coal stove in a closed room and falls asleep.

Why is this dangerous?

Try it first, then show the solutionSolution

In limited air, carbon forms carbon monoxide, which is colourless and odourless.
It combines with haemoglobin and prevents the blood from carrying oxygen; asphyxia can follow.

Answer: Carbon monoxide poisoning; keep rooms ventilated.

Memory hook: You cannot see or smell it.

Tricks to remember Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses

Short memory aids. Say each one aloud once, then use it on a question the same day.

CaCO₃ and HCl
Marble + hydrochloric acid gives salt, water and CO₂.
Use it when: Preparation.
Milky then clear
Lime water: milky with CO₂, clear again with excess.
Use it when: Test.
CO is silent
Colourless, odourless, poisonous, neutral, blue flame.
Use it when: Carbon monoxide.

Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Identifying a gas · 1 mark · MCQBoard level

Which pair of observations shows that a colourless gas is carbon dioxide?
(a) burns with a blue flame; lime water unchanged
(b) turns lime water milky; turns moist blue litmus light red
(c) burns with a pop; turns lime water milky
(d) relights a glowing splinter; no effect on litmus

Try it first, then show the solutionSolution

CO₂ forms insoluble calcium carbonate with lime water and is acidic (carbonic acid) on moist blue litmus.

Answer: (b)

Example 2: Why not sulphuric acid? · 2 marks · Short answerBoard level

In the laboratory preparation of carbon dioxide, why is dilute sulphuric acid not used with marble chips?

Try it first, then show the solutionSolution

Calcium sulphate forms and is only slightly soluble. (1)
It coats the marble chips and stops the reaction after a short time. (1)

Answer: Calcium sulphate coating stops the reaction.

Example 3: Milky and clear · 3 marks · Short answerHarder board level

Carbon dioxide is passed through lime water for a long time. (a) What is seen first? (b) What is seen later? (c) Write balanced equations for both stages.

Try it first, then show the solutionSolution

(a) The lime water turns milky. (½)
(b) The milkiness disappears. (½)
(c) $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ (1); $C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$ (1).

Answer: See the working.

Example 4: Mass of gas · 4 marks · Long answerHarder board level

10 g of calcium carbonate reacts completely with excess dilute hydrochloric acid. Formula masses: CaCO₃ = 100, HCl = 36.5, CaCl₂ = 111, CO₂ = 44, H₂O = 18. Find (a) the mass of CO₂ formed, (b) the mass of HCl used, (c) the masses of CaCl₂ and water, and (d) verify the law of conservation of mass.

Try it first, then show the solutionSolution

(a) 100 g gives 44 g, so 10 g gives 4.4 g. (1)
(b) $C a C O_{3} + 2 H C l$ : 73 g HCl for 100 g, so 7.3 g. (1)
(c) CaCl₂: 11.1 g; H₂O: 1.8 g. (1)
(d) Reactants $10 + 7.3 = 17.3$ g; products $4.4 + 11.1 + 1.8 = 17.3$ g. (1)

Answer: (a) 4.4 g (b) 7.3 g (c) 11.1 g and 1.8 g (d) 17.3 g = 17.3 g

Example 5: The closed room · 4 marks · Case-basedHOTS / competency

A charcoal stove burns in a closed room. (a) Name the gas formed that is dangerous and write the equation. (b) State its effect on the blood. (c) Give two properties that make it hard to detect. (d) State two precautions.

Try it first, then show the solutionSolution

(a) Carbon monoxide: $2 C + O_{2} \to 2 C O$ . (1)
(b) It combines with haemoglobin to form carboxyhaemoglobin, so the blood carries less oxygen. (1)
(c) Colourless and odourless. (1)
(d) Ventilate the room; never burn charcoal or run an engine in a closed space. (1)

Answer: (a) CO (b) carboxyhaemoglobin (c) colourless, odourless (d) ventilation

Where students lose marks in Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses

The slips examiners see most often, and what to write instead.

Mistake: Collecting CO₂ over water
CO₂ is fairly soluble.
Wrong working
"CO₂ is collected over water."
Right working
"CO₂ is collected by upward displacement of air."
Do this: Give the reason: density and solubility.
Mistake: Wrong lime water observation
It turns milky, then clear with excess.
Wrong working
"Lime water becomes clear."
Right working
"It first turns milky; with excess CO₂ it becomes clear again."
Do this: Give both stages with equations.
Mistake: Confusing CO and CO₂
CO is poisonous and neutral; CO₂ is acidic.
Wrong working
"CO₂ is poisonous."
Right working
"CO is poisonous; CO₂ is acidic."
Do this: Learn the table.

Exam tips

Write the preparation equation first, then the diagram labels.
Always give the reason for the collection method.
In test questions, give observation and equation.
For CO, mention haemoglobin and asphyxia.

Practice questions on Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Name the method used to collect carbon dioxide and give the reason.
Hint
Compare with air; solubility.
Answer
Upward displacement of air, because CO₂ is heavier than air and fairly soluble in water.
Q2 · 3 marks · Short answerHarder board level
28 g of carbon monoxide reduces iron(III) oxide: $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ (CO = 28, CO₂ = 44). Find the mass of CO₂ formed. Name the substance oxidised.
Hint
Mole ratio 3 : 3.
Answer
3 CO (84 g) gives 3 CO₂ (132 g); so 28 g gives $\frac{132}{3} = 44$ g. Carbon monoxide is oxidised (it gains oxygen).
Q3 · 3 marks · Short answerHarder board level
Why is carbon dioxide used in fire extinguishers? Why does magnesium burn in it though? Write the equation.
Hint
Heavier than air; magnesium is very reactive.
Answer
It is heavier than air and does not support burning, so it blankets the fire. Magnesium is so reactive that it takes oxygen from CO₂: $2 M g + C O_{2} \to 2 M g O + C$ .
Q4 · 2 marks · Short answerHarder board level
Why does soda water turn blue litmus light red? Write the equation.
Hint
Weak acid.
Answer
CO₂ dissolves to form carbonic acid, a weak acid: $C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$ .
Q5 · 4 marks · Long answerHOTS / competency
Compare carbon dioxide and carbon monoxide in four points: litmus, lime water, burning and effect on breathing.
Hint
Make a table.
Answer
CO₂: turns moist blue litmus light red; turns lime water milky; does not burn; displaces air and is not poisonous in small amounts. CO: neutral to litmus; no effect on lime water; burns with a blue flame; poisonous because it binds haemoglobin.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Reactants for CO₂ preparation?Show answer
Marble chips and dilute hydrochloric acid.
2.CO₂ collected by?Show answer
Upward displacement of air.
3.Test for CO₂?Show answer
Lime water turns milky.
4.CO burns with?Show answer
A blue flame.
5.CO is poisonous because?Show answer
It combines with haemoglobin.

Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses: frequently asked questions

How is carbon dioxide prepared in the laboratory?

By reacting marble chips (calcium carbonate) with dilute hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. It is collected by upward displacement of air.

Why does lime water turn milky and then clear?

CO₂ forms insoluble calcium carbonate, making it milky; with excess CO₂ soluble calcium hydrogencarbonate forms and the milkiness disappears.

Why is carbon monoxide poisonous?

It combines with haemoglobin in the blood far more readily than oxygen, so the blood cannot carry enough oxygen and asphyxia results.