Gases in the Atmosphere
Chemistry· 2.9–2.14 (2017 4CH1 specification)· 12 min read
1. Composition of Dry Air★☆☆☆☆⏱ 2 min
Dry air is a mixture of gases with consistent approximate volume percentages across the lower atmosphere. You are required to recall these four most abundant gases for your exam.
Dry air composition
The four main gases in dry air by volume are: ~78% nitrogen (N₂), ~21% oxygen (O₂), ~1% argon (Ar), ~0.04% carbon dioxide (CO₂)
A sample of 500 cm³ of dry air is collected. Calculate the approximate volume of nitrogen and oxygen in this sample.
- 1
Recall nitrogen makes up 78% of dry air: Volume of N₂ = (78/100) × 500 cm³ = 390 cm³
- 2
Recall oxygen makes up 21% of dry air: Volume of O₂ = (21/100) × 500 cm³ = 105 cm³
2. Determining Percentage of Oxygen in Air★★★☆☆⏱ 3 min
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You can calculate the percentage of oxygen in air by reacting a substance that only reacts with oxygen, forming a non-gaseous product. The decrease in gas volume equals the volume of oxygen used up.
Oxygen percentage calculation
% O₂ in air = (volume decrease of air ÷ initial volume of air) × 100, expected result ~21% (~1/5 of air)
Common methods use either iron rusting (metal oxidation) or phosphorus combustion (non-metal oxidation). Both reactions consume only oxygen and produce solid products, so no extra gas is added to the system.
A student sets up an experiment with 120 cm³ of air in a syringe, heated with excess iron wool. After the reaction finishes and cools to room temperature, the final gas volume is 95 cm³. Calculate the percentage of oxygen in the sample.
- 1
Calculate volume decrease = Initial volume - Final volume = 120 cm³ - 95 cm³ = 25 cm³
- 2
Apply the formula: % O₂ = (25 / 120) × 100 = 20.8%, which matches the expected ~21% value
3. Combustion of Elements in Oxygen★★☆☆☆⏱ 3 min
When elements burn in excess oxygen, they form oxides. You must recall the products and write balanced equations with state symbols for three elements: magnesium, hydrogen and sulfur.
Combustion reaction
An exothermic reaction where a substance reacts with oxygen to form one or more oxide products
Write the full balanced symbol equation with state symbols for the combustion of magnesium in oxygen.
- 1
Identify reactants and products: Mg(s) + O₂(g) → MgO(s)
- 2
Balance the equation: 2Mg(s) + O₂(g) → 2MgO(s)
For hydrogen combustion, the product is liquid water: 2H₂(g) + O₂(g) → 2H₂O(l). For sulfur combustion, the product is gaseous sulfur dioxide: S(s) + O₂(g) → SO₂(g). You must recall all three equations for the exam.
4. Formation of Carbon Dioxide via Thermal Decomposition★★☆☆☆⏱ 2 min
Carbon dioxide is produced when metal carbonates undergo thermal decomposition: heating breaks the carbonate down into a metal oxide and carbon dioxide gas, with no other reactants required.
Write the balanced symbol equation with state symbols for the thermal decomposition of copper(II) carbonate.
- 1
Identify reactant and products: CuCO₃(s) → CuO(s) + CO₂(g)
- 2
Check balancing: 1 Cu, 1 C, 3 O on both sides, so no further balancing is needed
Copper(II) carbonate is a green solid that turns black when heated, as copper(II) oxide is black. The carbon dioxide produced can be tested using limewater, which turns milky (this test is covered in a separate sub-topic on gas tests).
5. Carbon Dioxide as a Greenhouse Gas★☆☆☆☆⏱ 2 min
Carbon dioxide is a greenhouse gas: it absorbs infrared radiation reflected from the Earth's surface, trapping heat in the lower atmosphere, which keeps the planet warm enough to support life.
Human activities including burning fossil fuels and deforestation have increased atmospheric CO₂ concentrations since the industrial revolution, which is contributing to global warming and associated climate change effects like rising sea levels and extreme weather events.
6. Common Pitfalls
Wrong move:
Swapping nitrogen and oxygen percentages, stating air is 21% nitrogen and 78% oxygen
Why:
Students often mix up the order of the two most abundant gases
Correct move:
Remember nitrogen makes up ~4/5 of air (78%), oxygen ~1/5 (21%)
Wrong move:
Using sulfur combustion for the oxygen percentage experiment and expecting a volume decrease
Why:
Sulfur dioxide is a gas that replaces the oxygen used up, so no volume change occurs
Correct move:
Only use reactions that produce solid/liquid products (e.g. iron rusting, phosphorus combustion) for this experiment
Wrong move:
Taking the final volume reading in the oxygen experiment before the apparatus cools to room temperature
Why:
Heated gases expand, leading to an artificially high final volume and underestimation of oxygen percentage
Correct move:
Leave the apparatus to cool fully before recording the final gas volume
Wrong move:
Writing the product of hydrogen combustion as H₂O(g) instead of H₂O(l)
Why:
Reactions are written at standard room temperature and pressure, so water is a liquid
Correct move:
Include the (l) state symbol for water in the hydrogen combustion equation
Wrong move:
Adding oxygen as a reactant to the thermal decomposition of copper(II) carbonate equation
Why:
Thermal decomposition only requires heat, no other reactants are present
Correct move:
The only reactant is the metal carbonate: CuCO₃(s) → CuO(s) + CO₂(g)
7. Quick Reference Cheatsheet
Concept | Key Recall Information |
|---|---|
Dry air composition | N₂ ~78%, O₂ ~21%, Ar ~1%, CO₂ ~0.04% |
% O₂ calculation | (Volume decrease ÷ initial air volume) × 100 = ~21% |
Mg combustion | 2Mg(s) + O₂(g) → 2MgO(s) |
H₂ combustion | 2H₂(g) + O₂(g) → 2H₂O(l) |
S combustion | S(s) + O₂(g) → SO₂(g) |
CuCO₃ decomposition | CuCO₃(s) → CuO(s) + CO₂(g) |
CO₂ climate link | Increasing CO₂ levels contribute to climate change via the greenhouse effect |
8. Frequently Asked
Do I need to remember the exact percentages of gases in dry air for the exam?
Yes, you must recall: ~78% nitrogen, ~21% oxygen, ~1% argon, ~0.04% carbon dioxide. These values are not provided in the exam paper.
Why does the volume of air decrease in the oxygen percentage experiment?
Oxygen is a reactant in the combustion/oxidation reaction, so it is used up to form solid or liquid products that do not contribute to gas volume. The total gas volume decreases by the volume of oxygen originally present.
Do I need to include state symbols in combustion equations for this topic?
Yes, Edexcel expects full balanced equations with state symbols for the combustion of magnesium, hydrogen and sulfur, as specified in the 4CH1 syllabus.
Going deeper
What's Next
Now that you have mastered the core content on gases in the atmosphere, you can move on to related topics that build on this foundational knowledge. Next, you will learn about tests for common gases including oxygen and carbon dioxide, which are used to identify products of the reactions covered in this guide. You will also study atmospheric pollution from the combustion of hydrocarbon fuels, including the formation of carbon monoxide, sulfur dioxide and oxides of nitrogen, and their impacts on human health and the environment. This topic also forms the basis for understanding the carbon cycle, which you will encounter in later units of the Edexcel IGCSE Chemistry specification. Practice the oxygen percentage practical and past paper questions on this sub-topic to consolidate your learning before moving on.