10. Chemistry of the environment
- Syllabus
- 0620–2026–2027
- Section
- 10
- Level
- —
Exam analysis
Most tested topics
10.1 Water
- Syllabus
- 0620–2026–2027
- Topic
- 10.1
- Level
- —
Test chemically for water
| Anhydrous reagent | Colour before | Positive result with water |
|---|---|---|
| cobalt(II) chloride | blue | turns pink |
| copper(II) sulfate | white | turns blue |
Add the suspected water to a small sample of the anhydrous solid, or expose the reagent to the liquid/vapour being tested. Record both the starting and final colour.
These colour changes show that water is present; they do not by themselves prove that a sample is pure water.
The reagents must be anhydrous. Keep the colour pairs separate: cobalt(II) chloride is blue → pink; copper(II) sulfate is white → blue.
Test whether water is pure
A pure substance has sharp, fixed melting and boiling points under stated pressure. Pure water melts/freezes at 0 °C and boils at 100 °C at standard atmospheric pressure.
| Measurement | Pure-water result | Sign of impurity |
|---|---|---|
| melting/freezing point | sharp value at 0 °C | shifted value or melting/freezing over a range |
| boiling point | sharp value at 100 °C | shifted value or boiling over a range |
Measure temperature while the sample changes state and compare with the expected value under the same pressure. Repeating the measurement improves confidence.
A positive cobalt(II)-chloride or copper(II)-sulfate test proves water is present, not that it is pure. Neutral pH alone also does not prove purity.
Choose distilled water for practical chemistry
Distilled water is used instead of tap water in practical chemistry because it contains fewer chemical impurities.
| Water source | Chemical contents | Experimental consequence |
|---|---|---|
| tap water | contains dissolved ions and other impurities | may form precipitates, alter colour/pH or leave residues |
| distilled water | contains fewer chemical impurities | is less likely to introduce an uncontrolled reactant |
Use distilled water when preparing solutions, rinsing apparatus for analysis or carrying out tests where trace dissolved ions could change the observation.
The required comparison is fewer chemical impurities, not that distilled water is guaranteed absolutely pure or sterile.
Identify substances in natural water
| Category | Substance found in natural water | Typical source or form |
|---|---|---|
| gas | dissolved oxygen | air and aquatic photosynthesis |
| dissolved compounds | metal compounds | rocks, soil and human activity |
| solid pollution | plastics | discarded material and fragments |
| biological/organic pollution | sewage | waste entering water |
| biological hazard | harmful microbes | contaminated water and sewage |
| nutrient pollution | nitrates | fertilisers |
| nutrient pollution | phosphates | fertilisers and detergents |
Natural water is a mixture. Its contents can be dissolved, suspended, biological, beneficial or harmful; 'natural' does not mean chemically pure.
State the named substance and, where specified, its source: nitrates from fertilisers; phosphates from fertilisers and detergents.
Do not treat every listed substance as harmful. Dissolved oxygen and some metal compounds can be beneficial, while the effect of other substances depends on type and amount.
Recognise beneficial substances in water
| Substance | Benefit |
|---|---|
| dissolved oxygen | supports respiration and survival of aquatic life |
| some metal compounds | supply essential mineral ions needed by living organisms |
Benefit depends on identity and concentration. The syllabus claim is that some metal compounds provide essential minerals, not that all metal compounds are safe.
The same natural-water inventory contains both useful and damaging substances, so classify by biological effect rather than simply by whether a substance is present.
Dissolved oxygen is beneficial; loss of dissolved oxygen damages aquatic life. Toxic metal compounds are harmful and must not be confused with essential mineral compounds.
Recognise harmful substances in water
| Harmful substance | Required harm |
|---|---|
| toxic metal compounds | poison living organisms |
| plastics | injure or otherwise harm aquatic life |
| sewage containing harmful microbes | spreads disease |
| nitrates and phosphates | deoxygenate water and damage aquatic life |
Name both pollutant and consequence. For example, fertiliser nitrates and detergent/fertiliser phosphates can reduce dissolved oxygen, leaving too little for aquatic organisms.
The required outcome is deoxygenation and harm to aquatic life. Detailed stages of eutrophication are not required.
Do not claim dissolved oxygen is a pollutant. It is beneficial; nitrates and phosphates are harmful here because they lead to its depletion.
Describe domestic water treatment
| Treatment | What it removes or does |
|---|---|
| sedimentation | allows suspended solids to settle |
| filtration | removes remaining insoluble solids |
| carbon treatment | removes unwanted tastes and odours |
| chlorination | kills harmful microbes |
Sedimentation and filtration handle solid particles first. Carbon improves taste and smell. Chlorine disinfects the water before supply.
Filtration removes insoluble solids, not dissolved salts. Chlorination kills microbes but does not remove all dissolved chemical substances.
Do not assign carbon the job of killing microbes or chlorine the job of removing solids. Each treatment stage has a distinct purpose.
10.2 Fertilisers
- Syllabus
- 0620–2026–2027
- Topic
- 10.2
- Level
- —
Recognise ammonium and nitrate fertilisers
Ammonium salts and nitrates are used as fertilisers. They supply nitrogen in a form that can support plant growth.
| Substance | Formula clue | Why it matches |
|---|---|---|
| ammonium nitrate | NH₄NO₃ | contains ammonium, NH₄⁺, and nitrate, NO₃⁻ |
| ammonium sulfate | (NH₄)₂SO₄ | is an ammonium salt |
| potassium nitrate | KNO₃ | is a nitrate |
To recognise a suitable example from a name or formula, look for the ammonium ion, NH₄⁺, or the nitrate ion, NO₃⁻. A compound may contain one of these ions or both.
Ammonia, NH₃, is not itself an ammonium salt. Also keep nitrate, NO₃⁻, distinct from nitrogen gas, N₂.
Choose compounds for an NPK fertiliser
NPK fertilisers provide the elements nitrogen (N), phosphorus (P) and potassium (K) for improved plant growth.
| Nutrient supplied | Symbol | Common formula clue |
|---|---|---|
| nitrogen | N | NH₄⁺ or NO₃⁻ |
| phosphorus | P | a phosphate ion such as PO₄³⁻ |
| potassium | K | K in the compound formula |
When choosing compounds, scan the complete formula of each substance and record which of N, P and K it supplies. The chosen compound or mixture must cover all three nutrients without counting the same symbol twice.
For example, ammonium phosphate supplies nitrogen and phosphorus, while potassium sulfate supplies potassium. Together they provide N, P and K.
An NPK fertiliser may be a mixture: every individual compound does not need to contain all three elements, but the full fertiliser must provide all three. P means phosphorus and K means potassium.
10.3 Air quality and climate
- Syllabus
- 0620–2026–2027
- Topic
- 10.3
- Level
- —
State the composition of clean, dry air
| Gas | Formula | Approximate percentage by volume |
|---|---|---|
| nitrogen | N₂ | 78% |
| oxygen | O₂ | 21% |
| noble gases and carbon dioxide | noble gases; CO₂ | the remaining about 1% |
Clean, dry air is a mixture, not a compound. 'Dry' means water vapour has been excluded, and 'clean' means pollutant amounts are not being included in this standard composition.
The two main percentages total about 99%, so noble gases and carbon dioxide together make up the small remainder.
Do not assign the whole remaining 1% to carbon dioxide: most of that remainder is noble gases. The figures are approximate, not exact integers for every sample.
Match each air pollutant to its source
| Pollutant | Required source |
|---|---|
| carbon dioxide, CO₂ | complete combustion of carbon-containing fuels |
| carbon monoxide, CO | incomplete combustion of carbon-containing fuels |
| particulates | incomplete combustion of carbon-containing fuels |
| methane, CH₄ | decomposition of vegetation and waste gases from digestion in animals |
| oxides of nitrogen, NOₓ | car engines |
| sulfur dioxide, SO₂ | combustion of fossil fuels containing sulfur compounds |
Complete combustion has enough oxygen and forms carbon dioxide. Incomplete combustion has insufficient oxygen and can form carbon monoxide and solid particulates.
For a source question, name both the pollutant and the precise process or material that produces it—for example, sulfur dioxide from burning a sulfur-containing fossil fuel.
Nitrogen oxides form from nitrogen and oxygen in the hot engine; they are not described as coming from nitrogen compounds in petrol.
Match air pollutants to their adverse effects
| Pollutant | Required adverse effect |
|---|---|
| carbon dioxide | increased global warming, leading to climate change |
| methane | increased global warming, leading to climate change |
| carbon monoxide | toxic gas |
| particulates | increased risk of respiratory problems and cancer |
| oxides of nitrogen | acid rain, photochemical smog and respiratory problems |
| sulfur dioxide | acid rain |
Keep each cause-and-effect pair exact. If a question asks for one effect, state a listed effect for that named pollutant rather than giving a general statement such as 'damages the environment'.
Carbon monoxide harms people because it is toxic. Carbon dioxide and methane are linked here to global warming and climate change, not acute poisoning.
Sulfur dioxide causes acid rain; oxides of nitrogen have the wider listed set of acid rain, photochemical smog and respiratory effects.
Explain how greenhouse gases cause global warming
Carbon dioxide and methane are greenhouse gases because they reduce the loss of thermal energy from Earth to space.
| Stage | Thermal-energy change |
|---|---|
| 1 | Energy reaches Earth; the surface absorbs some and reflects some. |
| 2 | The warmed surface emits thermal energy towards the atmosphere and space. |
| 3 | Carbon dioxide and methane absorb some of this emitted thermal energy. |
| 4 | The gases emit thermal energy in different directions, including back towards Earth. |
| 5 | Less thermal energy escapes to space, so the average global temperature rises. |
A complete explanation links absorption and emission by greenhouse gases to reduced thermal-energy loss, then links that reduced loss to global warming.
Do not say greenhouse gases create energy or simply make more sunlight enter. Their required role is to absorb and re-emit thermal energy, reducing its loss to space.
Explain strategies that reduce climate change and acid rain
| Environmental issue | Strategy | Why it helps |
|---|---|---|
| climate change | plant trees | photosynthesis removes carbon dioxide from air |
| climate change | reduce livestock farming | lowers methane emissions from animal digestion |
| climate change | decrease fossil-fuel use | lowers carbon dioxide emissions from combustion |
| climate change | increase hydrogen and renewable energy such as wind and solar | replaces some fossil-fuel combustion |
| acid rain | use catalytic converters in vehicles | lowers emissions of oxides of nitrogen |
| acid rain | use low-sulfur fuels | lowers sulfur dioxide formation |
| acid rain | flue-gas desulfurisation using calcium oxide | neutralises/removes acidic sulfur dioxide |
Explain a strategy by naming the pollutant it reduces and the source or removal process it changes.
The two issues need different pollutant links: carbon dioxide and methane for climate change; oxides of nitrogen and sulfur dioxide for acid rain.
Calcium oxide is used in flue-gas desulfurisation to remove sulfur dioxide; it is not added to the fuel to prevent combustion.
Describe the reaction of photosynthesis
Photosynthesis is the reaction in which carbon dioxide and water produce glucose and oxygen, in the presence of chlorophyll and using energy from light.
| Role | Required item |
|---|---|
| reactants | carbon dioxide and water |
| products | glucose and oxygen |
| condition | chlorophyll present |
| energy source | light |
A complete description includes both reactants, both products, chlorophyll and light energy. It also explains why photosynthesis removes carbon dioxide from the atmosphere.
Chlorophyll and light are required for the reaction but are not written as reactants or products. Respiration runs in the opposite overall direction and must not be substituted.
Write the word equation for photosynthesis
carbon dioxide + water → glucose + oxygen
Write the reactants on the left and the products on the right. The arrow means 'produces'; it does not mean the reaction is being written backwards as respiration.
Chlorophyll and light energy may be written above or near the arrow as conditions, but they are not joined to either side with a plus sign.
A list such as 'carbon dioxide, water, glucose, oxygen' is not a word equation: the plus signs and correctly directed arrow are essential.
Explain nitrogen oxides in car engines and catalytic converters
At the high temperatures in a car engine, nitrogen and oxygen from the air gain enough energy to react and form oxides of nitrogen such as nitrogen monoxide, NO.
| Substance entering converter | Change | Less harmful product |
|---|---|---|
| nitrogen monoxide, NO | reduced / loses oxygen | nitrogen, N₂ |
| carbon monoxide, CO | oxidised / gains oxygen | carbon dioxide, CO₂ |
A representative catalytic-converter equation is: 2CO + 2NO → 2CO₂ + N₂.
The two pollutants react together on the catalyst surface: carbon monoxide removes oxygen from nitrogen monoxide, so both are converted into less harmful products.
Nitrogen oxides form in the hot engine, not in the catalytic converter. The converter removes them after combustion; it does not stop nitrogen and oxygen entering the engine.
Write the symbol equation for photosynthesis
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The coefficients balance 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side.
Read the equation as six carbon dioxide molecules reacting with six water molecules to form one glucose molecule and six oxygen molecules.
Do not change the subscripts inside CO₂, H₂O, C₆H₁₂O₆ or O₂ when balancing. Change only the coefficients placed in front of whole formulas.