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B17.3 Air Pollution

GCSE Combined Science · AQA · Chapter B17: Biodiversity and Ecosystems

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: smoke, acidic gases and carbon monoxide from burning fuels, their effects and how to reduce them. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Name the main air pollutants released when fuels are burned and where they come from
  • Explain how sulfur dioxide and nitrogen oxides form acid rain, and its effects on living organisms
  • Describe the dangers of carbon monoxide and of smoke particles (particulates)
  • Describe ways of reducing air pollution
  • Analyse data linking pollution to biodiversity and calculate percentage change

Key words

  • Sulfur dioxide: SO₂. An acidic gas made when fuels containing sulfur impurities (such as coal) are burned.
  • Nitrogen oxides: NOx. Acidic gases made when nitrogen and oxygen from the air react at the high temperatures in engines and furnaces.
  • Acid rain: Rain, snow or mist with a low pH, formed when acidic gases react with water in clouds.
  • Carbon monoxide: CO. A colourless, odourless, poisonous gas made by incomplete combustion of fuels.
  • Particulates: Tiny solid or liquid particles in the air, such as soot from smoke. They form smog over cities.

Where air pollution comes from

Burning fuels in power stations, factories, homes and vehicles releases smoke and acidic gases. As the population grows and uses more energy and more vehicles, more pollutants are released.

  • Sulfur dioxide; Main source Burning coal, oil and diesel that contain sulfur; Main effect Acid rain; breathing problems
  • Nitrogen oxides; Main source Car, lorry and aircraft engines; power stations; Main effect Acid rain; breathing problems
  • Carbon monoxide; Main source Incomplete combustion in engines, faulty gas boilers, coal and wood fires; Main effect Poisonous: stops the blood carrying oxygen; hard to detect
  • Smoke / particulates; Main source Diesel engines, coal and wood fires; Main effect Lung and heart disease; smog; less light reaches the ground
  • Carbon dioxide; Main source Complete combustion of any fossil fuel; Main effect Greenhouse gas: global warming (see B17.5)

Acid rain

  • Trees: acid rain damages leaves and lowers the pH of the soil. Minerals are washed out and roots are damaged, so whole forests can die.
  • Lakes and rivers: as the water becomes more acidic, fish eggs, insect larvae and snails cannot survive. The whole food web is affected, so biodiversity falls.
  • Buildings: acid rain wears away limestone statues and damages metal.

Exam tip: Polluted clouds can be carried hundreds of kilometres by the wind, so acid rain often falls in a different area, or even a different country, from where the gases were made.

Acid rain and biodiversity: using data

Worked example: percentage decrease in species

At pH 6.8 there are 16 species. At pH 4.8 there are 5. Calculate the percentage decrease.
Decrease = 16 − 5 = 11 species
Percentage decrease = 11 ÷ 16 × 100 = 68.75%

Answer: about 69% fewer species

This is a correlation: as pH falls, the number of species falls. A correlation does not prove cause. To be more confident that acidity is the cause, scientists compare lakes that are similar in other ways (size, depth, temperature) and look for a mechanism, e.g. acid kills larvae in laboratory tests.

Reducing air pollution

  • Low-sulfur fuels; How it helps Less sulfur dioxide is made when the fuel burns.
  • Scrubbers in power station chimneys; How it helps Remove sulfur dioxide and particles before the waste gases leave the chimney.
  • Catalytic converters on cars; How it helps Change nitrogen oxides into nitrogen and carbon monoxide into carbon dioxide.
  • Serviced boilers and CO alarms; How it helps Prevent incomplete combustion and warn people if carbon monoxide is present.
  • Renewable energy, electric vehicles, public transport; How it helps Less fuel is burned, so fewer acidic gases, less CO₂ and less smoke.

Worked example: cleaner city air

The mean nitrogen dioxide level on a high street was 56 µg/m³. After a clean-air zone was introduced it fell to 42 µg/m³.
Percentage decrease = (56 − 42) ÷ 56 × 100 = 14 ÷ 56 × 100 = 25%

Answer: 25% decrease

Common mistakes

  • Saying carbon dioxide causes acid rain. At GCSE, acid rain is caused by sulfur dioxide and nitrogen oxides; carbon dioxide is linked to global warming.
  • Mixing up carbon monoxide (poisonous, from incomplete combustion) with carbon dioxide (from complete combustion).
  • Writing that acid rain “kills fish” with no link to biodiversity. Say that fewer species can survive, so biodiversity falls.
  • Thinking acid rain only affects the area where the gases are released. Wind carries polluted clouds a long way.

Revision questions

Try these, then check your answers below each one.

Question 1. Which two gases are the main causes of acid rain? (Choose from: Carbon dioxide and methane, Sulfur dioxide and nitrogen oxides, Oxygen and nitrogen, Carbon monoxide and water vapour.)

Answer: Sulfur dioxide and nitrogen oxides

Working: Sulfur dioxide and nitrogen oxides react with water in clouds to form acids that fall as acid rain.

Question 2. The diagram shows how acid rain forms. At which labelled point do the acidic gases react with water droplets to form acids? (Choose from: P, Q, R, S.)

Answer: Q

Working: At Q, in the cloud, sulfur dioxide and nitrogen oxides react with water droplets. P shows the gases being released, R the acid rain falling and S the lake that becomes acidic.

Question 3. True or false? Carbon monoxide is made by incomplete combustion and is hard to detect because it is colourless and has no smell.

Answer: True

Working: True. Carbon monoxide is poisonous, colourless and odourless, so faulty boilers need servicing and CO alarms.

Related lessons

Continue in the full course

This lesson is section B17.3 (AGCSC1703) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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