Air quality and climate

O Level / IGCSE Chemistry · Chemistry of the environment. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Clean, dry air is a mixture: about 78% nitrogen, 21% oxygen, and a small remainder of noble gases and carbon dioxide. Human activity adds pollutants, and you need to know where each one comes from and what harm it does.

Carbon dioxide and methane are greenhouse gases. The Earth's surface emits thermal energy; these gases absorb it and emit some back towards the Earth, so less is lost to space. Higher levels cause global warming, which leads to climate change.

Sulfur dioxide and oxides of nitrogen cause acid rain. Oxides of nitrogen form in car engines, where the high temperature makes nitrogen and oxygen from the air react. Plants remove carbon dioxide from the air by photosynthesis, so planting trees helps.

Rules to remember

Common mistake

Mixing up the effects of the pollutants. Carbon dioxide and methane cause global warming; sulfur dioxide and oxides of nitrogen cause acid rain; carbon monoxide is toxic but does not cause acid rain.

Worked example

Calcium oxide removes sulfur dioxide from flue gases: CaO + SO2 → CaSO3. What mass of calcium oxide is needed to remove 6.4 kg of sulfur dioxide? (O 16, S 32, Ca 40)

  1. Mr of SO2 = 32 + (2 × 16) = 64; Mr of CaO = 40 + 16 = 56.
  2. 1 mol of CaO reacts with 1 mol of SO2, so 64 g of SO2 needs 56 g of CaO.
  3. Mass of CaO = 6.4 × 56 ÷ 64 = 5.6 kg.

Answer: 5.6 kg of calcium oxide.

Practice questions

Try each one, then open the answer.

1. Which two gases are the main greenhouse gases contributing to climate change?

  1. A
    carbon dioxide and methane, which both absorb infrared radiation
  2. B
    carbon monoxide and hydrogen, both produced by fires
  3. C
    sulfur dioxide and nitrogen oxides, which cause acid rain
  4. D
    nitrogen and oxygen, the two main gases in air
Show answer

Answer: A. CO2 (fossil fuels, deforestation) and methane (cattle, rice fields, landfill) absorb infrared and warm the atmosphere.

2. Which gas is the main cause of acid rain from burning coal?

  1. A
    sulfur dioxide
  2. B
    carbon dioxide
  3. C
    argon
  4. D
    oxygen
Show answer

Answer: A. Sulfur impurities in coal burn to sulfur dioxide, which dissolves in rain to form acids that damage buildings, trees and lakes.

3. Why is carbon monoxide dangerous?

  1. A
    it absorbs infrared radiation and warms the atmosphere
  2. B
    it is acidic and dissolves in rain to form acid rain
  3. C
    it reacts with water in the lungs and destroys their lining
  4. D
    it binds to haemoglobin so the blood cannot carry oxygen
Show answer

Answer: D. CO, from incomplete combustion, attaches to haemoglobin more strongly than oxygen does, so the blood cannot carry oxygen.

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