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Chemistry · Practice book · Senior 1 · Unit 10 of 11

Air composition and pollution

Senior 1 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 10 Air composition and pollution

Key unit competence: Assess the composition, role and pollution of air to recommend effective actions for maintaining a clean and sustainable environment.

Key ideas

  • Air is a mixture of gases: nitrogen about 78%, oxygen about 21%, argon about 0.9%, carbon dioxide about 0.04%, plus water vapour and traces of other noble gases.
  • Oxygen is the active part of air: it supports burning (combustion), respiration and rusting. About one fifth (21%) of air is used up when copper or iron filings react with it in a closed volume.
  • Importance of air: oxygen for respiration and burning; carbon dioxide for photosynthesis; nitrogen for making fertilisers (ammonia); noble gases for lamps and welding; moving air for wind energy and drying.
  • Air pollution is the presence in air of substances that harm living things or the environment. Major pollutants: carbon monoxide (incomplete burning), sulfur dioxide and nitrogen oxides (fuels, vehicles: cause acid rain), smoke and dust (particulates), CFCs (damage the ozone layer) and excess carbon dioxide (global warming).
  • Sources: vehicle exhausts, burning charcoal, firewood and rubbish, factories, mining, quarrying and aerosol sprays.
  • Prevention: good vehicle maintenance, catalytic converters, using cleaner fuels (gas, electricity), improved cook stoves, planting trees, not burning plastics, filtering factory smoke and public transport.

Common misconceptions

  • "Air is mostly oxygen." No: Air is mostly nitrogen (about 78%); oxygen is about 21%.
  • "Carbon dioxide is poisonous like carbon monoxide." No: CO2 is not poisonous in normal amounts; CO is very poisonous.
  • "Air pollution only happens in big cities with factories." No: Burning charcoal, firewood and rubbish pollutes air in homes and villages too.

Worked examples

Example 1. Describe an experiment to find the percentage of oxygen in air using copper.

Solution

Pass a known volume of air (e.g. 100 cm3) between two gas syringes over heated copper turnings.

Push the air back and forth until the volume stops decreasing; copper turns black as it reacts with oxygen.

Let the apparatus cool and read the final volume (about 79 cm3).

% oxygen = (initial − final) ÷ initial × 100 ≈ 21%.

Example 2. Give three major air pollutants, their sources and one effect of each.

Solution

Carbon monoxide: incomplete burning of charcoal or petrol; poisonous, reduces oxygen in blood.

Sulfur dioxide: burning fuels containing sulfur; causes acid rain and breathing problems.

Smoke/particulates: burning wood, rubbish; lung diseases and dirty buildings.

Practice questions

  1. Name the two main gases in air and their percentages.

    Show answer

    Nitrogen about 78%, oxygen about 21%.

    Common mistake: Giving oxygen as 78%.

  2. Why is oxygen called the active part of air?

    Show answer

    It takes part in burning, respiration and rusting; nitrogen is unreactive.

    Common mistake: Saying it is the largest part.

  3. Name the gas produced by respiration and burning.

    Show answer

    Carbon dioxide.

    Common mistake: Writing oxygen.

  4. Give two uses of nitrogen gas.

    Show answer

    Making ammonia/fertilisers; food packaging; refrigeration (liquid nitrogen) (any two).

    Common mistake: Saying it helps burning.

  5. Define air pollution.

    Show answer

    The presence in air of substances in amounts that harm living things or the environment.

    Common mistake: Saying 'dirty smell'.

  6. Name two pollutants from vehicle exhausts.

    Show answer

    Carbon monoxide, nitrogen oxides, sulfur dioxide, soot, unburnt hydrocarbons (any two).

    Common mistake: Naming oxygen.

  7. What is the effect of CFCs on the atmosphere?

    Show answer

    They destroy the ozone layer, letting more harmful UV rays reach the Earth.

    Common mistake: Confusing it with acid rain.

  8. Give two ways to reduce air pollution at school.

    Show answer

    Do not burn rubbish; plant trees; use improved stoves or gas in the kitchen (any two).

    Common mistake: Saying 'close windows'.

  9. Explain why the copper experiment does not give exactly 21% sometimes.

    Show answer

    Not all oxygen may react, the apparatus may leak, or volume read while hot.

    Common mistake: Saying air has changed composition.

  10. Explain how carbon dioxide causes global warming.

    Show answer

    It absorbs heat radiated from the Earth and traps it in the atmosphere (greenhouse effect), raising temperatures.

    Common mistake: Saying it makes a hole in the ozone layer.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which activity releases smoke and particulates into the air?a. CFCs (chlorofluorocarbons)
    2. Which noble gas is most abundant in air?b. Nitrogen
    3. Which gas makes up the largest part of air?c. Argon
    4. Which gas damages the ozone layer?d. Burning rubbish in the open
    Show answer

    1 → d (Burning rubbish in the open) · 2 → c (Argon) · 3 → b (Nitrogen) · 4 → a (CFCs (chlorofluorocarbons))

    • Open burning releases smoke and harmful particles.
    • Argon is about 0.9% of air.
    • Nitrogen is about 78% of air.
    • CFCs release chlorine that breaks down ozone.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    increases because oxygen combines with it  ·  79 cm3  ·  incomplete burning of fuels  ·  respiration
    1. In an experiment, 100 cm3 of air is passed over hot copper until no further change. The volume left is about .
    2. Copper turns black when heated in air. The mass of copper .
    3. Carbon monoxide is produced by .
    Show answer

    1: 79 cm3 · 2: increases because oxygen combines with it · 3: incomplete burning of fuels

    • Copper removes oxygen (21%), leaving about 79 cm3.
    • 2Cu + O2 → 2CuO: the oxide is heavier than the copper.
    • When oxygen is limited, carbon burns to CO.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which gas in air is chemically very unreactive and used in light bulbs?a. 21%
    2. Which effect is caused by acid rain?b. Argon
    3. Which gas supports burning?c. Oxygen
    4. What percentage of air is oxygen, approximately?d. Damage to buildings, trees and lakes
    Show answer

    1 → b (Argon) · 2 → d (Damage to buildings, trees and lakes) · 3 → c (Oxygen) · 4 → a (21%)

    • Argon does not react with the hot filament.
    • Acid rain corrodes metals and limestone and harms plants and aquatic life.
    • Burning is a reaction with oxygen.
    • About one fifth of air is oxygen.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    global warming  ·  Carbon dioxide  ·  acid soils only  ·  ammonia for fertilisers
    1. Nitrogen from air is used in industry to make .
    2. The increase of carbon dioxide in the atmosphere is linked to .
    3. Which gas in air is used by plants for photosynthesis? →
    Show answer

    1: ammonia for fertilisers · 2: global warming · 3: Carbon dioxide

    • The Haber process combines nitrogen and hydrogen to make ammonia.
    • CO2 traps heat (greenhouse effect).
    • Plants take in CO2 and release oxygen.

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