# Air Quality, Climate and Fertilisers

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u10-air-quality-climate-and-fertilisers/

This guide covers Core and Extended content for CIE IGCSE Chemistry 0620 Unit 10.2 and 10.3, including air composition, pollutants, climate change, photosynthesis, and fertilisers, aligned to the 2026-2028 syllabus.

**Prerequisites:** [Basic understanding of chemical reactions and non-metal element groups](https://www.owlsprep.com/study/cie-0620-u4-non-metals/); [Familiarity with basic environmental chemistry concepts](https://www.owlsprep.com/study/cie-0620-u10-environmental-chemistry-intro/)

## Learning objectives

- Describe the percentage composition of clean dry air
- Identify common air pollutants, their sources, and health/environmental effects
- Explain the natural and enhanced greenhouse effect and links to climate change
- Describe photosynthesis and state its word equation (and, for Extended, its balanced symbol equation)
- Describe the use of NPK fertilisers, and state that nitrate/phosphate runoff causes deoxygenation of water

## Composition of Air and Common Air Pollutants (Core)

Clean dry air has a consistent percentage composition: 78% nitrogen ($N_2$), 21% oxygen ($O_2$), ~1% argon (Ar), and ~0.04% carbon dioxide ($CO_2$). Air pollutants are substances present at harmful concentrations, almost all generated by human industrial or domestic activity.

**Air Pollutant** — A harmful substance released into the atmosphere that negatively impacts human health, ecosystems, or the climate

**Worked example:** List three common air pollutants, their main source, and one negative effect of each.

1. 1. Pollutant: Carbon monoxide (CO). Source: Incomplete combustion of carbon-containing fuels (e.g., car engines, faulty gas boilers). Effect: Binds irreversibly to haemoglobin in blood, reducing oxygen transport, which can cause unconsciousness or death.
2. 2. Pollutant: Sulfur dioxide ($SO_2$). Source: Combustion of fossil fuels with sulfur impurities (e.g., coal in power stations). Effect: Dissolves in rainwater to form acid rain, which erodes buildings, kills aquatic life, and damages plant leaves.
3. 3. Pollutant: Nitrogen oxides ($NO_x$). Source: High-temperature combustion in car engines where atmospheric nitrogen and oxygen react. Effect: Causes respiratory irritation in humans, and contributes to acid rain and photochemical smog.

> **Extended only: catalytic converters**
>
> Oxides of nitrogen form in car engines when the high temperature makes atmospheric nitrogen and oxygen react. Catalytic converters remove carbon monoxide and oxides of nitrogen from exhaust gases, e.g. $2CO + 2NO \rightarrow 2CO_2 + N_2$.

> **Exam tip:** Always link pollutant sources to specific human activities for full marks; avoid vague answers like 'factories' without explaining the associated process.

## Greenhouse Effect and Climate Change (Core)

The natural greenhouse effect keeps Earth at a habitable average temperature of ~15°C. Radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, which re-emits it as longer-wavelength thermal (infrared) energy. The greenhouse gases carbon dioxide and methane absorb, reflect and re-emit this thermal energy, reducing the loss of thermal energy to space and so warming the lower atmosphere.

**Greenhouse Gas** — An atmospheric gas that absorbs infrared radiation, contributing to the greenhouse effect. Key examples include carbon dioxide, methane, water vapour, and nitrous oxide.

**Worked example:** Explain how rising atmospheric carbon dioxide levels contribute to climate change.

1. 1. Human activities including fossil fuel combustion and deforestation release excess $CO_2$ into the atmosphere, raising concentrations above natural levels.
2. 2. Higher $CO_2$ levels mean more infrared radiation emitted from the Earth's surface is absorbed, rather than escaping into space.
3. 3. This enhanced greenhouse effect raises average global surface temperatures, leading to impacts including melting polar ice, sea level rise, more frequent extreme weather, and disrupted food production.

> **warning**
>
> Do not describe the greenhouse effect as entirely harmful: the natural effect is essential for life on Earth, only the human-caused enhanced effect drives dangerous climate change.

## Fertilisers (Core)

Ammonium salts and nitrates are used as fertilisers. NPK fertilisers supply the three elements nitrogen (N), phosphorus (P) and potassium (K), which plants need for improved growth (for example, nitrogen is used to make proteins).

**NPK fertiliser** — A fertiliser that provides the elements nitrogen, phosphorus and potassium for improved plant growth

> **warning**
>
> Environmental impact: if excess nitrates and phosphates are washed off farmland into rivers and lakes, they lead to deoxygenation of the water and damage to aquatic life. (This water-pollution outcome is covered in detail in the Water subtopic; the detailed steps of the eutrophication process are not required by the 0620 syllabus.)

> **Exam tip:** For fertilisers you only need to state that ammonium salts and nitrates are used, and that NPK fertilisers supply nitrogen, phosphorus and potassium. You do not need the step-by-step eutrophication mechanism.

## Photosynthesis

Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll (the green pigment in plants) and using energy from light. Because it removes carbon dioxide from the air, photosynthesis is an important part of the carbon cycle.

**Photosynthesis** — The reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll and using energy from light

$$\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$$

> **Extended only: symbol equation**
>
> Extended candidates must also state the balanced symbol equation for photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$.

**Worked example:** State the word equation for photosynthesis and name the two conditions required.

1. 1. Word equation: carbon dioxide + water → glucose + oxygen.
2. 2. Conditions: chlorophyll must be present, and energy from light is required.

> **Exam tip:** Learn the word equation for Core; for Extended also learn the balanced symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Do not forget the two conditions: chlorophyll and light energy.

## Common pitfalls

- **Wrong:** Stating air is ~70% oxygen
  - Why it fails: Learners mix up nitrogen and oxygen percentages in clean air
  - Correct: Recall clean dry air is 78% nitrogen, 21% oxygen, <1% other gases
- **Wrong:** Claiming carbon monoxide causes acid rain
  - Why it fails: Confusing CO effects with sulfur dioxide and nitrogen oxides
  - Correct: Link acid rain only to $SO_2$ and $NO_x$; CO reduces oxygen transport in blood
- **Wrong:** Describing the greenhouse effect as entirely harmful
  - Why it fails: Forgetting the natural greenhouse effect is essential to keep Earth habitable
  - Correct: Explicitly distinguish the natural beneficial effect from the human-caused enhanced effect driving climate change
- **Wrong:** Writing the full step-by-step eutrophication mechanism in a 0620 answer
  - Why it fails: The 0620 syllabus states that details of the eutrophication process are not required
  - Correct: State only that nitrates and phosphates lead to deoxygenation of water and damage to aquatic life
- **Wrong:** Forgetting the conditions in the photosynthesis equation
  - Why it fails: Photosynthesis only occurs in the presence of chlorophyll and using energy from light
  - Correct: Include the conditions; for Extended, balance the symbol equation as $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

## Cheatsheet

| Concept | Core Key Fact | Extended Key Fact |
| --- | --- | --- |
| Clean Air Composition | 78% N₂, 21% O₂, remainder noble gases and CO₂ | Same as Core |
| Acid Rain Causes | Sulfur dioxide ($SO_2$) and oxides of nitrogen ($NO_x$) | Same as Core |
| Greenhouse Effect | Natural = essential for life; enhanced = drives climate change | CO₂ and CH₄ absorb, reflect and re-emit thermal energy, reducing loss to space |
| Catalytic converters | Reduce CO and oxides of nitrogen from car exhausts | $2CO + 2NO \rightarrow 2CO_2 + N_2$ |
| Photosynthesis | carbon dioxide + water → glucose + oxygen (chlorophyll, light) | $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ |
| Fertilisers | Ammonium salts and nitrates used; NPK provides N, P and K for growth | Same as Core |
| Fertiliser runoff | Nitrates/phosphates cause deoxygenation of water and harm aquatic life (Water topic) | Same as Core |

## What's next

You have now covered the Core and Extended content for CIE IGCSE Chemistry 0620 Unit 10.2 and 10.3 on air quality, climate, photosynthesis, and fertilisers. Next, practice structured exam questions on this topic to reinforce your knowledge, particularly air pollutant sources and effects, the greenhouse effect, and the photosynthesis equations. You should also revise the Water subtopic in the Chemistry of the Environment unit, before completing full unit revision for your mock and final exams. Use the included cheatsheet for quick, last-minute revision in the days before your exam.

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