Chemistry of the Environment: Revision Notes

Condensed for the final weeks. For the full explanation, use the Chemistry of the Environment study guide.

Tier note: items marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus: 10.3 outcome 7 (how carbon dioxide and methane cause global warming), 10.3 outcome 8 (how oxides of nitrogen form in car engines and their removal by catalytic converters, with the equation) and 10.3 outcome 9 (the symbol equation for photosynthesis). Water (10.1) and fertilisers (10.2) are entirely Core, and everything else is 0620 Core. 5070 has no tiers, so O Level candidates need all of it.

Water

Test for water: anhydrous copper(II) sulfate white → blue, or anhydrous cobalt(II) chloride blue → pink. Test for pure water: boils at exactly 100 °C and freezes at 0 °C. Dissolved substances lower the melting point and raise the boiling point, so a sample melting or boiling at the “wrong” temperature is impure. The chemical tests above show water is present, not that it is pure — a distinction examiners test. This is also why distilled water, not tap water, is used in practical chemistry: tap water’s dissolved impurities could interfere with a reaction or a measurement.

Treatment of domestic water supply:

1. Sedimentation/filtration — removes solids.
2. Carbon — removes tastes and odours.
3. Chlorination — kills bacteria.

Fertilisers

NPK — nitrogen, phosphorus, potassium.

Element	Role in the plant
Nitrogen	Protein synthesis, leaf growth
Phosphorus	Root growth
Potassium	Flowering, fruit, disease resistance

Ammonium salts and nitrates supply nitrogen, applied together in a single NPK application. Excess fertiliser washes into rivers causing eutrophication — aquatic life dies from oxygen depletion. The syllabus does not require the detailed mechanism, only that excess fertiliser causes eutrophication and its outcome; the step-by-step process below is included as background only:

(background, not required) fertiliser runoff -> algal bloom -> blocks light -> plants die
-> bacteria decompose them -> oxygen used up -> fish die

Air pollutants — source and effect

Pollutant	Source	Effect
Carbon monoxide CO	Incomplete combustion	Toxic — binds to haemoglobin, reduces oxygen transport
Sulfur dioxide SO₂	Sulfur impurities in fossil fuels	Acid rain
Nitrogen oxides NOₓ	Car engines (how they form — N₂ and O₂ reacting at the high temperature inside the engine — is (0620 Extended, 5070 required))	Acid rain, photochemical smog, respiratory problems
Particulates	Incomplete combustion	Increased risk of respiratory problems and cancer
Methane CH₄	Livestock, decomposition, rice paddies	Greenhouse gas
Carbon dioxide CO₂	Complete combustion, respiration	Enhanced greenhouse effect

Acid rain effects: damages trees and crops, acidifies lakes killing fish, erodes limestone buildings, corrodes metals.

Reducing acid rain: catalytic converters, low-sulfur fuels, and flue gas desulfurisation — passing waste gases through calcium oxide (or calcium carbonate), which reacts with the sulfur dioxide before it reaches the atmosphere.

0620 Core (10.3 outcome 4) needs you to state and explain the use of catalytic converters in vehicles to reduce acid rain: they cut the oxides of nitrogen released in exhaust gases. How the converter removes them, with the equation, is Extended.

(0620 Extended, 5070 required) Catalytic converters remove CO and NOₓ: 2CO + 2NO → 2CO₂ + N₂

Clean, dry air is approximately 78% nitrogen and 21% oxygen, with the small remainder made up of noble gases and carbon dioxide.

The greenhouse effect

(0620 Extended, 5070 required) Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth. The Earth re-radiates longer-wavelength infrared, which greenhouse gases absorb and re-emit, some of it back downward rather than letting it escape to space, warming the atmosphere.

The enhanced greenhouse effect is the extra warming from raised CO₂ and CH₄ levels — climate change, rising sea levels, extreme weather. See the Chemistry of the Environment study guide for the full syllabus coverage and worked reasoning behind every table above.

Reduction strategies (the syllabus’s closed list of five): planting trees, reduction in livestock farming, decreasing use of fossil fuels, increasing use of hydrogen, and increasing use of renewable energy.

Photosynthesis removes CO₂ from the atmosphere, converting it (with water, using light energy and chlorophyll) into glucose and oxygen.

• Word equation (0620 Core): carbon dioxide + water → glucose + oxygen
• (0620 Extended, 5070 required) Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Photosynthesis is the reverse of respiration (and of burning glucose), and it is why deforestation makes the enhanced greenhouse effect worse: fewer trees means less CO₂ removed from the atmosphere by this route.

Exam traps

• CO is toxic; CO₂ is a greenhouse gas. Don’t swap their effects.
• SO₂ causes acid rain, not the greenhouse effect.
• The greenhouse effect is natural and necessary — it is the enhanced effect that is the problem.
• Anhydrous copper(II) sulfate tests for the presence of water, not purity.
• Eutrophication kills fish through oxygen depletion, not poisoning.
• Forgetting that dissolved substances lower the melting point and raise the boiling point of water — a sample that doesn’t melt/boil at exactly 0 °C/100 °C is impure, even if the copper sulfate test confirms water is present.
• Confusing flue gas desulfurisation (removes SO₂ using calcium oxide, tackling acid rain) with a catalytic converter in a vehicle (a different method; (0620 Extended, 5070 required) it removes CO and NOₓ together, a different pair of pollutants).

Self-test

1. Give the test for water and the test for pure water.
2. Name the three stages of water treatment.
3. State what causes eutrophication and its effect on aquatic life. (The detailed four-step mechanism is background, not required by the syllabus.)
4. Which pollutant causes acid rain, and where does it come from?
5. (0620 Extended, 5070 required) Why does a catalytic converter reduce two pollutants at once?
6. Why is distilled water, rather than tap water, used in practical chemistry?
7. (0620 Extended, 5070 required) Give the symbol equation for photosynthesis, and explain why deforestation worsens the enhanced greenhouse effect.

Answers: 1. Anhydrous copper(II) sulfate turns white → blue (or cobalt chloride blue → pink); pure water boils at 100 °C and freezes at 0 °C. 2. Filtration/sedimentation, carbon treatment, chlorination. 3. Excess fertiliser washes into waterways, ultimately depleting the oxygen dissolved in the water so that fish and other aquatic life suffocate. (Background detail, not required: fertiliser runoff → algal bloom blocks light → water plants die → bacteria decompose them, consuming oxygen → fish suffocate.) 4. Sulfur dioxide, from sulfur impurities burned in fossil fuels (nitrogen oxides also contribute). 5. It converts CO and NO together into CO₂ and N₂ — one reaction removes both a toxic gas and an acid-rain precursor. 6. Tap water contains dissolved impurities that could interfere with a reaction or a measurement, unlike distilled water. 7. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂; fewer trees means less CO₂ is removed from the atmosphere by photosynthesis, so more remains to enhance the greenhouse effect.