Overview

What you'll learn

Describe the composition of air. 78% N₂, 21% O₂ and ~1% other gases. Separate the gases by fractional distillation of liquefied air, and measure the %O₂ using heated copper.

Identify the pollutants. Know the sources and effects of CO, NO, NO₂, SO₂, CH₄, unburnt hydrocarbons and ground-level ozone.

Explain acid rain. Write the formation equations for SO₂ and NO₂ dissolving in rain, the effects on lakes, buildings and forests, and control by liming.

Describe pollution controls. Catalytic converters (Pt / Rh / Pd) and flue-gas desulfurisation, plus the greenhouse effect, the ozone layer and CFCs.

Tutor's Insight

"The last topic — environment in the chemistry paper."
There are 7 pollutants worth memorising — CO, NO, NO₂, SO₂, CH₄, unburnt hydrocarbons and ground-level ozone — and questions cluster in Paper 2 structured form. Most questions ask the same three things: where does the pollutant come from, what does it do, and how is it removed? Master one row of the pollutants table at a time. The catalytic converter and flue-gas desulfurisation are the two named control technologies — learn the equations for both. This topic touches redox, combustion, acids and equilibrium, so it quietly rewards everything you already know.

12.1 Air & Pollutants

12.1 Air & Pollutants

What this subtopic asks of you
  • State the composition of clean dry air (about 78% nitrogen, 21% oxygen, 1% noble gases and 0.04% carbon dioxide), and describe how the gases are separated by fractional distillation.
  • Name the common atmospheric pollutants, their sources and their effects, including CO, NO, NO₂, SO₂, CH₄ and unburnt hydrocarbons.
  • Write balanced equations for the formation of oxides of nitrogen, sulfur dioxide and carbon monoxide.

12.1 Air & Pollutants

What dry air really is

Major Components
78% N₂ · 21% O₂
Nitrogen is unreactive at low temperatures — most of the air is essentially inert. Oxygen is the reactive part: combustion, respiration, rusting.
Minor Components
Noble gases & CO₂
About 1% noble gases (mostly Ar). 0.04% CO₂ — a small fraction but a big climate effect. Plus variable amounts of water vapour.
Measuring % O₂
The heated-copper experiment
Pass a known volume of air over heated copper. Cu + O₂ → CuO turns the bright Cu black. The remaining gas (~79%) is N₂ + noble gases.
LIQUEFY AIR → WARM SLOWLY → N₂ · Ar · O₂
Fractional distillation of liquefied air. Air is cooled below −200 °C until it liquefies, then warmed slowly. Nitrogen boils off first (b.p. −196 °C), argon next, then oxygen — giving pure N₂, O₂ and Ar for industry.

Free Notes · O-Level Pure Chemistry

Read the full chapter

The pollutants table, key equations, acid rain, catalytic converters, flue-gas desulfurisation, the greenhouse effect, the ozone layer and 4 worked exam questions.

12.1 Air & Pollutants

Six culprits worth memorising

Pollutant Source Effect
CO Incomplete combustion Binds to haemoglobin irreversibly
NO N₂ + O₂ in hot engines Forms NO₂ in air
NO₂ Oxidation of NO Acid rain; respiratory irritant
SO₂ Burning sulfur-containing fuels Acid rain; respiratory irritant
CH₄ Decomposition, livestock Potent greenhouse gas
Unburnt HC Incomplete combustion of petrol Photochemical smog
CO is the silent one. Colourless, odourless — and it binds to haemoglobin irreversibly, stopping the blood from carrying O₂. That's why incomplete combustion indoors kills.

12.1 Air & Pollutants

Three equations examiners love

Oxides of Nitrogen
From hot air in engines
N₂ + O₂ → 2 NO
2 NO + O₂ → 2 NO₂

Forms only at the high temperatures inside a car engine — not at room temperature.
Sulfur Dioxide
Burning sulfur-rich fuels
S + O₂ → SO₂

Sulfur is a trace impurity in coal and oil. Refining reduces the S content — giving low-sulfur fuels.
Incomplete Combustion
Carbon monoxide and soot
2 C + O₂ → 2 CO  (limited O₂)
C + O₂ → CO₂  (complete)

Less O₂ → more CO; very limited O₂ → soot (C) too.
Exam Habit

Balance the O₂. Every one of these equations is a favourite because it tests balancing. Count oxygen atoms last, and remember NO forms first and is then oxidised to NO₂ — never jump straight to NO₂ from N₂ and O₂.

12.2 Environmental Problems & Controls

12.2 Environmental Problems & Controls

What this subtopic asks of you
  • Explain how acid rain forms from SO₂ and NO₂, describe its effects on lakes, buildings and forests, and describe control by liming.
  • Describe how a catalytic converter and flue-gas desulfurisation reduce pollution, with balanced equations for each.
  • Explain the greenhouse effect and its consequences, and explain how CFCs deplete the stratospheric ozone layer.

12.2 Environmental Problems & Controls

When the sky turns acidic

What is it
Rain with pH below 5
Normal rainwater is slightly acidic (pH 5.6) because of dissolved CO₂. When the pH drops below ~5, we call it acid rain.
How it forms
SO₂ and NO₂ + rain
SO₂ + H₂O → H₂SO₃ (sulfurous)
2 SO₂ + O₂ + 2 H₂O → 2 H₂SO₄ (sulfuric)
4 NO₂ + O₂ + 2 H₂O → 4 HNO₃
What it does
Damages everything
• Kills fish in lakes (low pH).
• Erodes limestone buildings and statues.
• Damages forests and crops.
• Corrodes metal structures.
Liming the lakes. Adding CaCO₃ (or Ca(OH)₂) neutralises acidic soil and water and restores the pH — a slow, ongoing remedy. The real fix is reducing emissions in the first place.

12.2 Environmental Problems & Controls

Two technologies do most of the work

In every petrol exhaust
Catalytic converter
A honeycomb of Pt / Rh / Pd inside the exhaust pipe. It simultaneously oxidises CO and unburnt hydrocarbons and reduces NO / NO₂:

2 CO + 2 NO → 2 CO₂ + N₂

Three pollutants out → safer products in.
In every power-station chimney
Flue-gas desulfurisation
Spray a slurry of CaCO₃ / Ca(OH)₂ into the flue gas. The basic carbonate / hydroxide neutralises acidic SO₂:

CaCO₃ + SO₂ → CaSO₃ + CO₂
2 CaSO₃ + O₂ → 2 CaSO₄

The CaSO₄ (gypsum) is sold for plasterboard.
Exam Habit

Name the target pollutant. A catalytic converter removes CO, unburnt hydrocarbons and oxides of nitrogen from car exhausts; flue-gas desulfurisation removes SO₂ from power stations. State which technology tackles which pollutant before you quote the equation.

12.2 Environmental Problems & Controls

Trapped heat — the greenhouse effect

Greenhouse Gases
CO₂ · CH₄ · H₂O · N₂O
They let visible light in (warming the Earth) but trap infrared radiation on the way out — keeping the surface warm.
Sources
Where they come from
CO₂ — burning fossil fuels, deforestation.
CH₄ — livestock digestion, decay, rice fields.
N₂O — fertiliser use, combustion.
Effects
More than just warmer
• Melting ice caps → rising sea levels.
• Ocean acidification (CO₂ + H₂O).
• Disrupted weather patterns.
• Loss of biodiversity.
What helps

Switch to renewable energy (solar, wind, hydro), plant trees, use hydrogen as a fuel, and capture and store CO₂. Photosynthesis pulls CO₂ back out of the air — trees are the cheapest carbon-capture system on Earth.

12.2 Environmental Problems & Controls

A different gas, a different problem

Up There
Ozone protects us
The stratospheric ozone (O₃) layer absorbs UV radiation, shielding life on Earth from DNA damage. Don't confuse this with ground-level ozone (a pollutant).
Down Here
CFCs break it down
CFCs (chlorofluorocarbons) used to be in fridges and aerosols. UV releases a Cl atom that catalyses the decomposition of O₃.
One chlorine atom can destroy thousands of ozone molecules. Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂. The Cl is regenerated — which is why the Montreal Protocol (1987) phased CFCs out.

Practice

Exam-style questions

Question 01  ·  ACS(BR) 2024 Prelim
The Question
MCQ

When a scrubber doesn't help

To reduce atmospheric pollution, the waste gases from a coal-burning power station are passed through powdered calcium carbonate. Which waste gas will not be removed by the powdered calcium carbonate?

  • A  carbon monoxide
  • B  nitrogen dioxide
  • C  phosphorus(V) oxide
  • D  sulfur dioxide
Worked Answer
A — carbon monoxide
  1. CaCO₃ is basic — it neutralises acidic oxides only.
  2. B NO₂ is acidic → removed. ✓
  3. C P₂O₅ is acidic → removed. ✓
  4. D SO₂ is acidic → removed. ✓
  5. CO is a neutral oxide — it doesn't react with CaCO₃. Answer A.
Question 02  ·  CCHM 2024 Prelim
The Question
MCQ

The catalytic converter reaction

Oxides of nitrogen form in car engines and are removed by catalytic converters. Which equation represents a reaction that occurs in a catalytic converter?

  • A  CO + NO₂ → NO + CO₂
  • B  2 CO + 2 NO → N₂ + 2 CO₂
  • C  CO₂ + NO → NO₂ + CO
  • D  CO₂ + 2 NO₂ → N₂ + 3 O₂ + C
Worked Answer
B
  1. The converter must remove both pollutants — CO and NO — and leave safe products.
  2. B does exactly that: CO is oxidised to CO₂, NO is reduced to N₂. Safe outputs.
  3. A only partially reduces — it leaves NO behind.
  4. C and D run the wrong direction or produce nonsense (free C from CO₂). ✗
  5. Answer B.
Question 03  ·  Anderson 2024 Prelim
The Question
MCQ

Which statement about global warming is correct?

Which statement about global warming is correct?

  • A  Methane produced by digestion in animals has no effect on the rate of global warming.
  • B  The products of burning fossil fuels have no effect on the rate of global warming.
  • C  The products of decomposition of vegetation have no effect on the rate of global warming.
  • D  The products of photosynthesis have no effect on the rate of global warming.
Worked Answer
D
  1. A Animal digestion produces methane — a potent greenhouse gas. So it does affect warming. ✗
  2. B Burning fossil fuels → CO₂ — the major greenhouse gas. ✗
  3. C Decomposition releases CO₂ and CH₄ — both greenhouse gases. ✗
  4. D Photosynthesis produces O₂ and glucose; O₂ is not a greenhouse gas and the glucose stays in the plant. No effect on warming. ✓ Answer D.
Question 04
The Question
Structured

Sulfur dioxide from a power station

A power station burns coal that contains sulfur.

(a) Write an equation for the formation of sulfur dioxide when this fuel burns.

(b) Explain how sulfur dioxide leads to acid rain, with an equation.

(c) Name one method used to remove sulfur dioxide from the waste gases, and give an equation.

Worked Answer
Source → Acid Rain → Control
  1. (a) The sulfur impurity burns in oxygen: S + O₂ → SO₂.
  2. (b) SO₂ dissolves and is oxidised in rainwater to sulfuric acid: 2 SO₂ + O₂ + 2 H₂O → 2 H₂SO₄. This lowers the pH of rain below ~5, giving acid rain that kills fish, erodes limestone and damages forests.
  3. (c) Flue-gas desulfurisation — spray basic CaCO₃ (or Ca(OH)₂) into the flue gas to neutralise the acidic SO₂: CaCO₃ + SO₂ → CaSO₃ + CO₂.

Frequently Asked Questions

Maintaining Air Quality — FAQ

What is the composition of clean dry air?
Clean dry air is about 78% nitrogen and 21% oxygen, with roughly 1% noble gases (mostly argon) and about 0.04% carbon dioxide, plus variable water vapour. Nitrogen is unreactive at low temperatures, so most of the air is essentially inert; oxygen is the reactive part used in combustion, respiration and rusting. The gases are separated industrially by fractional distillation of liquefied air.
Why is carbon monoxide (CO) so dangerous?
Carbon monoxide is produced by the incomplete combustion of carbon-containing fuels in a limited supply of oxygen. It is colourless and odourless, so it gives no warning, and it binds to haemoglobin in red blood cells irreversibly. This stops the blood from carrying oxygen around the body, which is why incomplete combustion in an unventilated space can be fatal.
How does acid rain form and what does it do?
Acid rain forms when sulfur dioxide and nitrogen dioxide dissolve in rainwater: SO₂ + H₂O → H₂SO₃, 2 SO₂ + O₂ + 2 H₂O → 2 H₂SO₄, and 4 NO₂ + O₂ + 2 H₂O → 4 HNO₃. Normal rain is slightly acidic (pH 5.6) from dissolved CO₂; below about pH 5 it is called acid rain. It kills fish in lakes, erodes limestone buildings and statues, damages forests and crops, and corrodes metal structures. Adding limestone or slaked lime (liming) neutralises acidified lakes and soil.
How does a catalytic converter reduce pollution?
A catalytic converter is a honeycomb coated with platinum, rhodium and palladium fitted in a car's exhaust. It simultaneously oxidises carbon monoxide and unburnt hydrocarbons and reduces oxides of nitrogen, for example 2 CO + 2 NO → 2 CO₂ + N₂. This converts three harmful pollutants into safer products (carbon dioxide, nitrogen and water).
What is the difference between the greenhouse effect and the ozone layer problem?
They are two separate problems. The greenhouse effect is global warming caused by gases such as CO₂, CH₄, H₂O and N₂O trapping infrared radiation, leading to melting ice caps, rising sea levels and ocean acidification. Ozone-layer depletion is the breakdown of protective stratospheric ozone (O₃) by CFCs, which lets through harmful UV radiation. Do not confuse protective stratospheric ozone with harmful ground-level ozone, which is a pollutant.
How do CFCs destroy the ozone layer?
In the stratosphere, UV light breaks a chlorine atom off a CFC molecule. That chlorine atom catalyses the decomposition of ozone: Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂. The chlorine atom is regenerated at the end, so one chlorine atom can destroy thousands of ozone molecules. The Montreal Protocol (1987) phased out CFCs for this reason.

O-Level Pure Chemistry  ·  Syllabus 6092  ·  Topic 12 of 12  ·  © 2026 Overmugged. For personal study use only.