# Gases in the Atmosphere

> Chemistry · Edexcel IGCSE
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-gases-in-the-atmosphere/

This guide covers Edexcel IGCSE Chemistry specification points 2.9–2.14 on atmospheric gases, including air composition, oxygen percentage practicals, combustion reactions, thermal decomposition of carbonates, and CO₂'s climate links.

**Prerequisites:** [Balancing chemical equations with state symbols](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-balancing-equations/); [Basic chemical reaction types](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-reaction-types/)

## Learning objectives

- Recall the approximate volume percentages of the four most abundant gases in dry air
- Describe and calculate the percentage by volume of oxygen in air using metal/non-metal reaction experiments
- Write balanced state symbol equations for the combustion of magnesium, hydrogen and sulfur in oxygen
- Explain the formation of carbon dioxide via thermal decomposition of metal carbonates, including copper(II) carbonate
- Describe the qualitative link between atmospheric carbon dioxide levels and climate change
- Carry out the practical investigation to find the approximate percentage of oxygen in air

## Composition of Dry Air

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)

> **tip**
>
> Mnemonic: Nitrogen makes up ~4/5 of air, oxygen ~1/5, argon is almost 1%, CO₂ is a tiny ~0.04% fraction.

**Worked example:** 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

## Determining Percentage of Oxygen in Air

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.

**Worked example:** 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

> **Exam tip**
>
> Always

*Calculator:* allowed

## Combustion of Elements in Oxygen

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

**Worked example:** 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.

> **warning**
>
> Sulfur dioxide is a gas, so sulfur combustion cannot be used in the oxygen percentage experiment, as it replaces the oxygen used up and no measurable volume decrease occurs.

## Formation of Carbon Dioxide via Thermal Decomposition

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.

**Worked example:** 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).

## Carbon Dioxide as a Greenhouse Gas

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.

> **warning**
>
> You only need a qualitative link between increasing CO₂ levels and climate change for this topic. Detailed radiative forcing mechanisms or carbon cycle calculations are out of scope.

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.

## Common pitfalls

- **Wrong:** Swapping nitrogen and oxygen percentages, stating air is 21% nitrogen and 78% oxygen
  - Why it fails: Students often mix up the order of the two most abundant gases
  - Correct: Remember nitrogen makes up ~4/5 of air (78%), oxygen ~1/5 (21%)
- **Wrong:** Using sulfur combustion for the oxygen percentage experiment and expecting a volume decrease
  - Why it fails: Sulfur dioxide is a gas that replaces the oxygen used up, so no volume change occurs
  - Correct: Only use reactions that produce solid/liquid products (e.g. iron rusting, phosphorus combustion) for this experiment
- **Wrong:** Taking the final volume reading in the oxygen experiment before the apparatus cools to room temperature
  - Why it fails: Heated gases expand, leading to an artificially high final volume and underestimation of oxygen percentage
  - Correct: Leave the apparatus to cool fully before recording the final gas volume
- **Wrong:** Writing the product of hydrogen combustion as H₂O(g) instead of H₂O(l)
  - Why it fails: Reactions are written at standard room temperature and pressure, so water is a liquid
  - Correct: Include the (l) state symbol for water in the hydrogen combustion equation
- **Wrong:** Adding oxygen as a reactant to the thermal decomposition of copper(II) carbonate equation
  - Why it fails: Thermal decomposition only requires heat, no other reactants are present
  - Correct: The only reactant is the metal carbonate: CuCO₃(s) → CuO(s) + CO₂(g)

## 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 |

## 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.

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