Air and Water

What you'll learn

Air and Water is a core topic in CIE IGCSE Chemistry that examines the composition of the atmosphere, environmental chemistry, and water quality. This topic regularly appears in both Paper 2 (Extended) and Paper 4 (Alternative to Practical), with questions testing knowledge of pollutants, the carbon cycle, water treatment processes, and experimental techniques for analysing air and water samples. Understanding this topic is essential for 8-12 marks in typical examination papers.

Key terms and definitions

Fractional composition of air — dry air contains approximately 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and 1% noble gases (mainly argon), with variable water vapour content.

Greenhouse gases — atmospheric gases that absorb and re-emit infrared radiation, trapping heat in the atmosphere; includes carbon dioxide, methane, and water vapour.

Carbon cycle — the natural circulation of carbon between the atmosphere, living organisms, oceans, and the Earth's crust through processes including photosynthesis, respiration, combustion, and decomposition.

Potable water — water that is safe to drink, containing sufficiently low levels of dissolved salts and microbes to be suitable for human consumption.

Desalination — the removal of dissolved salts from sea water, typically by distillation or reverse osmosis, to produce fresh water.

Eutrophication — the excessive growth of algae in water bodies caused by high concentrations of nitrate and phosphate fertilisers, leading to oxygen depletion and death of aquatic organisms.

Thermal decomposition — the breakdown of a chemical compound by heating, such as the decomposition of metal carbonates or hydrogencarbonates.

Hard water — water containing dissolved calcium or magnesium ions (typically as hydrogencarbonates, chlorides, or sulfates) that reacts with soap to form scum.

Core concepts

Composition and properties of air

The composition of clean, dry air remains relatively constant due to natural cycles. The main components are:

• Nitrogen (N₂): approximately 78% by volume
• Oxygen (O₂): approximately 21% by volume
• Noble gases (primarily argon): approximately 1% by volume
• Carbon dioxide (CO₂): approximately 0.04% by volume (currently rising due to human activities)
• Water vapour: variable percentage depending on temperature and location

Experimentally determining oxygen percentage can be achieved using copper or iron. When copper turnings are heated in a sealed tube containing air and passed back and forth, the copper reacts with oxygen to form copper(II) oxide:

2Cu(s) + O₂(g) → 2CuO(s)

The volume decrease in the sealed apparatus indicates the percentage of oxygen consumed. Rust formation using iron filings over water produces similar results but takes several days.

Pollutants in air and their effects

Combustion of fossil fuels releases several pollutants with significant environmental and health impacts:

Carbon monoxide (CO)

• Formed by incomplete combustion of carbon-containing fuels when oxygen supply is limited
• Equation: 2C(s) + O₂(g) → 2CO(g)
• Toxic gas that binds irreversibly to haemoglobin, preventing oxygen transport in blood
• Colourless and odourless, making it particularly dangerous

Carbon dioxide (CO₂)

• Produced by complete combustion of carbon-containing fuels
• Equation: C(s) + O₂(g) → CO₂(g)
• Principal greenhouse gas contributing to global warming and climate change
• Dissolves in rainwater forming weak carbonic acid: CO₂(g) + H₂O(l) → H₂CO₃(aq)

Sulfur dioxide (SO₂)

• Formed when sulfur impurities in fossil fuels burn
• Equation: S(s) + O₂(g) → SO₂(g)
• Causes acid rain when dissolved in atmospheric moisture: SO₂(g) + H₂O(l) → H₂SO₃(aq), which oxidises to sulfuric acid
• Triggers respiratory problems and damages stonework (especially limestone and marble)

Nitrogen oxides (NOₓ)

• Produced when nitrogen and oxygen from air react at high temperatures in vehicle engines
• Equation: N₂(g) + O₂(g) → 2NO(g), followed by 2NO(g) + O₂(g) → 2NO₂(g)
• Contribute to acid rain and photochemical smog
• Nitrogen dioxide is a brown toxic gas causing respiratory irritation

Unburnt hydrocarbons and particulates

• Released from incomplete combustion in vehicle engines
• Particulate carbon (soot) causes respiratory diseases and global dimming
• Contributes to smog formation in urban areas

Methane (CH₄)

• Released from decomposition of organic matter, livestock digestion, and rice paddy fields
• Greenhouse gas approximately 25 times more potent than carbon dioxide over 100 years

Reducing air pollution

Several strategies reduce atmospheric pollution:

Catalytic converters in vehicle exhaust systems convert harmful gases into less harmful products:

• 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)
• Unburnt hydrocarbons are also oxidised to carbon dioxide and water
• Use platinum, palladium, and rhodium as catalysts

Flue gas desulfurisation removes sulfur dioxide from power station emissions:

• Calcium oxide or calcium carbonate (limestone) reacts with sulfur dioxide
• CaO(s) + SO₂(g) → CaSO₃(s)
• CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)
• Calcium sulfite can be oxidised to calcium sulfate (gypsum) for construction materials

Alternative energy sources reduce fossil fuel dependence: solar, wind, hydroelectric, nuclear, and biofuels.

The carbon cycle and greenhouse effect

The carbon cycle maintains atmospheric carbon dioxide balance through:

Processes removing CO₂ from atmosphere:

• Photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
• Dissolution in oceans forming hydrogencarbonates
• Formation of carbonate shells and limestone deposits

Processes adding CO₂ to atmosphere:

• Respiration in plants and animals: C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l)
• Combustion of fossil fuels and biomass
• Decomposition of dead organic matter by bacteria and fungi
• Volcanic eruptions

The greenhouse effect is a natural process where greenhouse gases absorb infrared radiation emitted by Earth's surface, re-radiating heat and warming the atmosphere. Enhanced greenhouse effect occurs when increased concentrations of CO₂, methane, and other gases intensify this warming, leading to climate change consequences: ice cap melting, sea level rise, extreme weather events, and ecosystem disruption.

Water treatment and purification

Potable water production requires removing harmful microbes and excessive dissolved substances:

1. Sedimentation: water stands in tanks allowing large particles to settle
2. Filtration: passing through filter beds (sand and gravel) removes smaller suspended particles
3. Chlorination: adding chlorine gas or chlorine compounds kills harmful bacteria and microorganisms

Alternatively, distillation produces pure water by:

• Heating water to boiling point (100°C at standard pressure)
• Collecting and condensing steam, leaving dissolved salts and microbes behind
• Energy-intensive but produces very pure water

Desalination of sea water uses:

• Distillation: evaporating sea water and condensing the steam
• Reverse osmosis: forcing sea water through semi-permeable membranes under high pressure

Both methods are expensive but necessary in water-scarce regions.

Testing water purity

Several tests identify water purity:

Anhydrous copper(II) sulfate test:

• White anhydrous copper(II) sulfate turns blue in the presence of water
• CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)
• Tests for presence of water but not purity

Boiling point test:

• Pure water boils at exactly 100°C at standard atmospheric pressure
• Dissolved substances elevate boiling point
• Indicates purity level

Anhydrous cobalt(II) chloride paper:

• Blue paper turns pink in the presence of water
• Sensitive indicator for detecting moisture

Hard water and softening

Hard water contains dissolved Ca²⁺ or Mg²⁺ ions. Two types exist:

Temporary hardness:

• Caused by dissolved calcium hydrogencarbonate or magnesium hydrogencarbonate
• Removed by boiling: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
• Forms scale (calcium carbonate) in kettles and pipes

Permanent hardness:

• Caused by dissolved calcium sulfate or magnesium sulfate
• Cannot be removed by boiling
• Requires chemical treatment

Disadvantages of hard water:

• Wastes soap, forming scum: 2C₁₇H₃₅COO⁻Na⁺(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2Na⁺(aq)
• Scale formation reduces heating efficiency and blocks pipes

Advantages of hard water:

• Provides dietary calcium for healthy teeth and bones
• Better taste than soft water
• Reduces lead dissolution from old pipes

Water softening methods:

Adding sodium carbonate (washing soda):

• Na₂CO₃(aq) + Ca²⁺(aq) → CaCO₃(s) + 2Na⁺(aq)
• Precipitates calcium ions as insoluble calcium carbonate

Ion exchange columns:

• Hard water passes through resin containing sodium ions
• Ca²⁺ and Mg²⁺ ions exchange with Na⁺ ions
• Sodium ions do not form scum with soap

Using soapless detergents:

• Do not form scum with calcium or magnesium ions
• Work effectively in hard water

Water pollution

Major water pollutants include:

Fertiliser run-off (nitrates and phosphates):

• Causes eutrophication in rivers and lakes
• Algal blooms block light, preventing photosynthesis by aquatic plants
• Decomposition of dead algae depletes oxygen, killing fish

Sewage and organic waste:

• Bacterial decomposition consumes dissolved oxygen
• Increases biological oxygen demand (BOD)
• Spreads waterborne diseases

Industrial waste:

• Heavy metals (lead, mercury, cadmium) bioaccumulate in food chains
• Toxic chemicals harm aquatic life and contaminate drinking water

Pesticide run-off:

• Non-biodegradable pesticides persist in ecosystems
• Accumulate through food chains, reaching harmful concentrations

Worked examples

Example 1: Calculating oxygen percentage

Question: A student heated 100 cm³ of air in a sealed tube with excess copper turnings. After several minutes, the gas cooled to room temperature and measured 79 cm³. Calculate the percentage of oxygen in the air sample. [2 marks]

Solution:

• Volume of oxygen consumed = 100 cm³ - 79 cm³ = 21 cm³ [1 mark]
• Percentage of oxygen = (21/100) × 100 = 21% [1 mark]

Example 2: Hard water reactions

Question: Explain why adding sodium carbonate solution softens temporarily hard water. Include a balanced symbol equation in your answer. [3 marks]

Solution:

• Temporarily hard water contains dissolved calcium hydrogencarbonate (or calcium ions) [1 mark]
• Sodium carbonate reacts with calcium ions to form a precipitate [1 mark]
• Na₂CO₃(aq) + Ca²⁺(aq) → CaCO₃(s) + 2Na⁺(aq) OR Na₂CO₃(aq) + Ca(HCO₃)₂(aq) → CaCO₃(s) + 2NaHCO₃(aq) [1 mark]

Example 3: Environmental impact

Question: A coal-fired power station releases sulfur dioxide into the atmosphere.

(a) Write a balanced symbol equation for the formation of sulfur dioxide when coal burns. [1 mark]

(b) Describe how sulfur dioxide contributes to acid rain formation. [2 marks]

(c) State one method the power station could use to reduce sulfur dioxide emissions. [1 mark]

Solution:

(a) S(s) + O₂(g) → SO₂(g) [1 mark]

(b) Sulfur dioxide dissolves in atmospheric water/rain [1 mark] forming sulfurous acid which oxidises to sulfuric acid [1 mark]

(c) Use flue gas desulfurisation / react with calcium carbonate or calcium oxide / use low-sulfur coal [1 mark]

Common mistakes and how to avoid them

• Mistake: Stating that carbon dioxide is the only greenhouse gas. Correction: Methane and water vapour are also significant greenhouse gases; carbon dioxide is the main contributor from human activities but not the only greenhouse gas.

• Mistake: Confusing temporary and permanent hardness removal methods. Correction: Only temporary hardness (caused by hydrogencarbonates) can be removed by boiling; permanent hardness (caused by sulfates or chlorides) requires sodium carbonate or ion exchange.

• Mistake: Writing that distillation removes only microbes from water. Correction: Distillation removes both dissolved salts and microbes, producing pure water; simple filtration and chlorination remove microbes but leave dissolved salts.

• Mistake: Stating the anhydrous copper(II) sulfate test proves water is pure. Correction: This test only confirms the presence of water; testing the boiling point (exactly 100°C) indicates purity.

• Mistake: Claiming photosynthesis removes all carbon dioxide from the atmosphere. Correction: Photosynthesis and respiration are balanced processes; increased CO₂ from combustion exceeds the removal capacity of photosynthesis, causing atmospheric accumulation.

• Mistake: Describing carbon monoxide as a greenhouse gas. Correction: Carbon monoxide is a toxic gas from incomplete combustion that prevents oxygen transport in blood; it is not a significant greenhouse gas (carbon dioxide is the greenhouse gas from combustion).

Exam technique for Air and Water

• Command word awareness: "Explain" questions require reasons and mechanisms (e.g., explaining how acid rain forms requires stating SO₂ dissolves in water and naming the acid formed). "Describe" questions need observations or steps without detailed explanations. "State" requires brief factual answers.

• Equation precision: Balance all symbol equations correctly and include state symbols when specifically requested. For combustion reactions, identify whether complete (producing CO₂) or incomplete (producing CO) based on oxygen availability mentioned in the question.

• Environmental questions structure: Address both the source/cause and the consequence/effect. For pollution questions, name the specific pollutant, its source, and its environmental or health impact for full marks.

• Practical question approach: When describing experiments to determine oxygen percentage or water purity, include apparatus details, measurements taken, safety considerations, and expected observations. Typical questions award 1 mark per valid point up to 4-6 marks total.

Quick revision summary

Air contains 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and 1% noble gases. Combustion produces pollutants: CO (toxic, incomplete combustion), CO₂ (greenhouse gas), SO₂ (acid rain from sulfur impurities), and NOₓ (from high temperatures). The carbon cycle balances CO₂ through photosynthesis, respiration, combustion, and dissolution. Potable water requires sedimentation, filtration, and chlorination. Hard water contains Ca²⁺ or Mg²⁺ ions; temporary hardness (hydrogencarbonates) removes by boiling, permanent hardness (sulfates) needs sodium carbonate or ion exchange. Fertiliser run-off causes eutrophication, depleting oxygen and killing aquatic life.