What is environmental chemistry? The air we breathe
Environmental chemistry is the study of chemicals in air, water, soil and food: where they come from, how they react and what they do to living things.
Air is a mixture
Dry air is mostly nitrogen (about 78%) and oxygen (about 21%). These are the major parts. The minor parts include argon (0.93%), carbon dioxide (about 0.04%), neon, helium, methane and a little ozone. Air also holds water vapour (0–4%), which changes with weather.
Useful words
- Pollutant: a substance in the wrong place or in too large an amount, which causes harm.
- Primary pollutant: comes straight from a source (SO₂ from a chimney).
- Secondary pollutant: forms in the air from other pollutants (ozone in smog, acid in rain).
- ppm: parts per million; 1 ppm = 1 mg per kg (or 1 mL of gas in 1000 L of air).
Gases and particles that pollute the air
Most air pollution comes from burning carbon fuels (coal, petrol, diesel, wood).
- Carbon dioxide (CO₂): from complete burning. Not toxic in small amounts, but it traps heat (greenhouse gas). CH₄ + 2O₂ → CO₂ + 2H₂O
- Carbon monoxide (CO): from incomplete burning when oxygen is short. It stops blood carrying oxygen. 2C + O₂ → 2CO
- Sulfur dioxide (SO₂): sulfur in coal and diesel burns. S + O₂ → SO₂
- Nitrogen oxides (NOₓ): in hot engines nitrogen and oxygen of the air join. N₂ + O₂ → 2NO; 2NO + O₂ → 2NO₂
- Particulates (PM10, PM2.5): soot and dust smaller than 10 or 2.5 micrometres. They go deep into the lungs.
Smog
Smoky smog = smoke + fog + SO₂ (cold, damp places). Photochemical smog = NOₓ + unburnt fuel + sunlight → ozone and other irritating gases (sunny, busy cities).
Acids, bases and acid rain
Arrhenius said: an acid gives H⁺ ions in water; a base gives OH⁻ ions in water.
Strong vs weak
A strong acid (HCl, H₂SO₄, HNO₃) ionises almost 100%. A weak acid (ethanoic acid CH₃COOH, carbonic acid H₂CO₃) ionises only a few per cent.
Strength is not concentration
Concentration = how many moles of acid are in 1 L (mol/L). Strength = what fraction of the acid breaks into ions. So you can have a dilute strong acid or a concentrated weak acid.
Acid rain
Clean rain is slightly acidic (pH about 5.6) because CO₂ dissolves: CO₂ + H₂O ⇌ H₂CO₃. Rain below pH 5.6 is acid rain:
- SO₂ + H₂O → H₂SO₃; 2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄
- 4NO₂ + O₂ + 2H₂O → 4HNO₃
Effects: lakes too acidic for fish, leaves damaged, metals rust faster, and limestone and marble wear away: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.
Water, soil and food: fertilisers, metals and POPs
Natural water is always a solution: it carries dissolved oxygen, CO₂ and salts (hard water has Ca²⁺ and Mg²⁺).
- Fertilisers (nitrates, phosphates) wash into rivers. Algae grow fast, die and rot; bacteria use up the oxygen; fish die. This is eutrophication.
- Heavy metals (lead, mercury, cadmium, arsenic) do not break down and build up in the body.
- Pesticides: some organophosphorus insecticides are very toxic to nerves. Persistent organic pollutants (POPs) such as DDT last for years and build up along food chains (biomagnification).
- Food additives (preservatives, colours) have code numbers (E-numbers or INS numbers) and are allowed only up to safe limits.
Extraction
To test a pollutant, chemists often first extract it: shake the water with a solvent in which the pollutant dissolves better, then separate the layers. Water treatment uses filtering, settling, activated carbon and chlorine.
Measuring pollution: titration, standards and laws
Quantitative analysis means measuring how much. Without numbers we cannot compare with a limit or prove that a rule works.
Acid–base titration
- Put a known volume of acid sample in a flask with an indicator (phenolphthalein).
- Fill a burette with a base of known concentration (e.g. 0.100 mol/L NaOH).
- Add the base slowly until the colour just changes (end point). Read the volume.
- Use moles: n(base) = c × V, then the mole ratio from the balanced equation, then c(acid) = n ÷ V.
Standards and risk
Governments and the WHO set standards, for example limits for PM2.5 in air, nitrate in drinking water (about 50 mg/L) or lead in food. A risk inventory lists each hazard, how likely it is and how serious. Rules such as catalytic converters in cars, low-sulfur fuel, scrubbers on power stations (CaCO₃ removes SO₂) and bans on lead in petrol have clearly lowered pollution. To judge a new factory, we weigh its products and jobs against its emissions, health effects and long-term sustainability.
Try it at home
Chop some red cabbage, soak it in hot water and pour off the purple liquid: this is your indicator. Add a little to rainwater, tap water, lemon juice and soap water. Red or pink means acidic, purple neutral, green or yellow basic. Is your rainwater slightly acidic?
Key formulas and definitions
- Dry air: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.04%
- Acid rain: SO₂ + H₂O → H₂SO₃; 2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄
- 4NO₂ + O₂ + 2H₂O → 4HNO₃
- Marble damage: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
- Neutralisation: HCl + NaOH → NaCl + H₂O
- Moles: n = c × V (V in litres); c = n ÷ V
- pH = −log[H⁺]; rain below pH 5.6 is acid rain
Worked examples
1. Write balanced equations to show how sulfur in coal becomes sulfuric acid in rain.
S + O₂ → SO₂ (burning). 2SO₂ + O₂ → 2SO₃ (in air). SO₃ + H₂O → H₂SO₄ (in cloud water).
2. 25.0 mL of a rainwater-acid sample (HCl) needs 12.5 mL of 0.0100 mol/L NaOH to reach the end point. Find the acid concentration.
n(NaOH) = 0.0100 × 0.0125 = 1.25 × 10⁻⁴ mol. Ratio HCl : NaOH = 1 : 1, so n(HCl) = 1.25 × 10⁻⁴ mol. c = 1.25 × 10⁻⁴ ÷ 0.0250 = 5.0 × 10⁻³ mol/L.
3. 20.0 mL of H₂SO₄ needs 30.0 mL of 0.100 mol/L NaOH. Find c(H₂SO₄).
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. n(NaOH) = 0.100 × 0.0300 = 3.00 × 10⁻³ mol. n(H₂SO₄) = half = 1.50 × 10⁻³ mol. c = 1.50 × 10⁻³ ÷ 0.0200 = 0.0750 mol/L.
4. Rain in town A has pH 4 and in town B pH 5. How many times more H⁺ is in A?
Each pH unit is ×10, so A has 10 times more H⁺ ions than B.
Common mistakes
- Thinking a strong acid is always concentrated. Strength = how fully it ionises; concentration = how much is dissolved.
- Saying clean rain has pH 7. It is about 5.6 because CO₂ dissolves in it; only below 5.6 is acid rain.
- Forgetting to change mL to L in n = c × V (12.5 mL = 0.0125 L).
- Mixing up CO and CO₂: CO comes from incomplete burning and is poisonous; CO₂ is a greenhouse gas.