Pearson Edexcel International GCSE Chemistry 4CH1: Gases in the atmosphere – Revision Notes

Condensed for the final weeks. For full explanations and five worked examples, use the Gases in the atmosphere study guide.

These notes cover sub-topic 2(c), Gases in the atmosphere, points 2.9–2.14, of the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024), including required practical 2.14. The qualification is untiered and none of these points has a “C” reference, so all of them can appear on Paper 1 or Paper 2. The one molar-volume item in the self-test uses point 1.35C and is marked Paper 2 only.

Links: 4CH1 course hub · printable 4CH1 checklist · free 4CH1 all-topics diagnostic · Reactivity revision notes (the rest of Topic 2) · Reactivity practice questions · Groups, acids, salts and tests practice set.

Key definitions

• Dry air: air with the water vapour removed; the fixed percentages refer to this.
• Percentage by volume: the volume of one gas as a percentage of the total volume of the sample.
• Combustion: a reaction of a substance with oxygen that releases heat (and usually light).
• Oxide: a compound of an element with oxygen, e.g. MgO, SO₂, CuO.
• Thermal decomposition: breaking down a compound using heat alone.
• Greenhouse gas: a gas that absorbs infrared radiation given out by the Earth’s surface, e.g. carbon dioxide.

2.9 Dry air: the four numbers

Rank	Gas	Approx. % by volume
1	Nitrogen	78
2	Oxygen	21
3	Argon	0.9
4	Carbon dioxide	0.04

• “Dry” = water vapour excluded (it varies).
• Oxygen ≈ one-fifth; nitrogen ≈ four-fifths.
• Argon beats CO₂ by more than twenty times. CO₂ is 0.04%, not 4% and not 0.4%.

2.10 and 2.14 Oxygen in air: the practical

Principle. A substance removes the oxygen from a trapped volume of air. The volume falls by the volume of oxygen.

Method in steps

1. Measure the starting volume of air.
2. Let the metal or non-metal react with the oxygen.
3. Repeat or wait until the volume stops changing (all oxygen used).
4. Let the apparatus return to room temperature.
5. Measure the final volume.
6. % oxygen = (start − final) ÷ start × 100.

The three set-ups

Method	Substance	What happens	Key point
Iron wool in an inverted measuring cylinder over water	Metal (iron)	Iron rusts; water rises	Iron wool must be damp; takes about a week; no heating
Phosphorus burned over water in a bell jar or tube	Non-metal (phosphorus)	4P + 5O₂ → P₄O₁₀, white smoke that dissolves; water rises	Hazardous, usually a demonstration; wait to cool
Air pushed between two gas syringes over hot copper	Metal (copper)	2Cu + O₂ → 2CuO, pink-brown to black	Pass air over many times; cool before reading

Worked reminder

Start 90.0 cm³, end 71.5 cm³ → oxygen = 18.5 cm³ → 18.5 ÷ 90.0 × 100 = 20.6%.

Practical write-up checklist (2.14)

• Name the substance used and say whether it is a metal or a non-metal.
• Record the starting volume and the final volume, with units (cm³).
• State how you knew the reaction had finished: the volume stopped changing.
• State that you let the apparatus return to room temperature first.
• Show the subtraction and the division by the starting volume.
• Compare your answer with about 21% and give a reason for any difference.

Errors that make the answer too low

• Not enough metal, or not enough time, so some oxygen is left.
• Reading the volume while the gas is still warm (it has expanded).
• A leak lets fresh air in.

2.11 Combustion in oxygen

Element	Observation	Product	Equation
Magnesium	Very bright white flame; white powder	Magnesium oxide	2Mg(s) + O₂(g) → 2MgO(s)
Hydrogen	Pale blue flame; with air, a squeaky pop	Water	2H₂(g) + O₂(g) → 2H₂O(l)
Sulfur	Blue flame	Sulfur dioxide (colourless, choking)	S(s) + O₂(g) → SO₂(g)

• Elements burn more brightly in pure oxygen than in air (air is only about 21% oxygen).
• An element + oxygen → its oxide. Combustion releases heat (exothermic).

2.12 Thermal decomposition of carbonates

Definition. Thermal decomposition: breaking down a compound using heat alone.

General word equation: metal carbonate → metal oxide + carbon dioxide

Carbonate	Colour change	Equation
Copper(II) carbonate	Green → black	CuCO₃(s) → CuO(s) + CO₂(g)
Zinc carbonate	White → yellow when hot → white when cool	ZnCO₃(s) → ZnO(s) + CO₂(g)
Calcium carbonate	White stays white; needs strong heat	CaCO₃(s) → CaO(s) + CO₂(g)

• Show the CO₂: delivery tube into limewater, which turns milky.
• Remove the delivery tube from the limewater before removing the heat (prevents suck-back).
• Mass lost on heating = mass of CO₂ given off.
• Sodium and potassium carbonates do not decompose at Bunsen temperatures.

Reacting-mass reminder (Ar: Cu = 63.5, C = 12, O = 16)

Mr CuCO₃ = 123.5; Mr CuO = 79.5; Mr CO₂ = 44. Ratio CuCO₃ : CuO : CO₂ = 1 : 1 : 1 by moles.

2.13 CO₂ and climate

Method in steps: explaining the greenhouse effect

1. Sunlight warms the Earth’s surface.
2. The surface gives out infrared (heat) radiation.
3. CO₂ absorbs some infrared and re-emits some back towards the surface.
4. More CO₂ (burning fossil fuels, deforestation) → more heat kept in.
5. This may contribute to climate change.

Write “may contribute”: the specification uses that wording.

Must-know distinctions

• Combustion vs thermal decomposition. Combustion adds oxygen to an element (Mg → MgO). Thermal decomposition breaks a compound apart with heat and needs no oxygen (CuCO₃ → CuO + CO₂).
• Burning in oxygen vs burning in air. Same product, but brighter and faster in pure oxygen.
• Starting volume vs final volume. The percentage always uses the starting volume as the denominator.
• Greenhouse effect vs ozone depletion. CO₂ is linked to the greenhouse effect, not to the ozone layer.
• Hydrogen test vs hydrogen combustion. The squeaky pop is a small hydrogen–air explosion; both make water.
• Sulfur dioxide vs sulfur trioxide. Burning sulfur gives SO₂.
• Iron method vs copper method. Iron wool rusts slowly at room temperature and needs water; copper needs strong heating but works in minutes. Both are metals; phosphorus is the non-metal example.
• Observation vs product. “White powder” is an observation; “magnesium oxide” is a product name. A question asking what you see wants observations.

Quick self-test

1. Name the four most abundant gases in dry air in order, with approximate percentages.
2. 200 cm³ of air is left over damp iron wool until the volume is constant at 158 cm³. Calculate the percentage of oxygen.
3. Why must the iron wool be damp?
4. State the flame colour when magnesium burns in oxygen and name the product.
5. Write the balanced equation, with state symbols, for sulfur burning in oxygen.
6. Name the product when hydrogen burns in oxygen.
7. State the colour change when copper(II) carbonate is heated, and write the equation.
8. 2.47 g of copper(II) carbonate is heated to constant mass. Calculate the mass of copper(II) oxide formed and the mass of carbon dioxide lost.
9. Paper 2 only. Calculate the volume, in cm³ at rtp, of CO₂ given off in question 8.
10. In the gas-syringe method, why must the apparatus cool before the final reading?
11. Name the product when phosphorus burns in air.
12. Give two human activities that increase the amount of carbon dioxide in the atmosphere.

Answers

1. Nitrogen 78%, oxygen 21%, argon 0.9%, carbon dioxide 0.04%.
2. 200 − 158 = 42 cm³; 42 ÷ 200 × 100 = 21.0%.
3. Iron needs both water and oxygen to rust.
4. Very bright white flame; magnesium oxide (white solid).
5. S(s) + O₂(g) → SO₂(g).
6. Water.
7. Green to black; CuCO₃(s) → CuO(s) + CO₂(g).
8. Moles = 2.47 ÷ 123.5 = 0.0200 mol. CuO = 0.0200 × 79.5 = 1.59 g; CO₂ = 0.0200 × 44 = 0.88 g.
9. 0.0200 × 24 000 = 480 cm³.
10. Hot gas has expanded, so reading it warm gives a volume that is too large and a percentage of oxygen that is too low.
11. Phosphorus(V) oxide, P₄O₁₀.
12. Burning fossil fuels; deforestation (cutting down or burning forests).

Where marks are usually lost

• Giving CO₂ as 4% or 0.4% of air, or placing it above argon.
• Using the final gas volume as the denominator in the % oxygen calculation.
• Giving a final volume read before the apparatus cooled, then not spotting why the answer is low.
• Writing “the air is used up” instead of “the oxygen is used up / reacts with the iron”.
• Leaving out water when explaining why iron wool rusts in the measuring cylinder.
• “It burns brightly” with no colour and no product when two observations are needed.
• Writing SO₃ for burning sulfur, or forgetting to balance 2Mg + O₂ → 2MgO and 2H₂ + O₂ → 2H₂O.
• Saying copper(II) carbonate turns “brown” or starts “blue”.
• Missing the suck-back precaution in the carbonate experiment.
• Linking CO₂ to “holes in the ozone layer” instead of the greenhouse effect and climate change.

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1), Specification, Issue 3, September 2024, published by Pearson Education Limited. Topic 2: Inorganic chemistry, sub-topic (c) Gases in the atmosphere, points 2.9–2.14.