Contents: 9 sections
Cambridge IGCSE Co-ordinated Sciences 0654 and Combined Science 0653 · Core and Extended
Syllabus points
- Describe the chemical tests for water, and the difference between water that is safe to drink and water that is pure.
- Describe the treatment of the domestic water supply.
- State the composition of clean dry air.
- Name the common air pollutants, their sources and their effects.
- Describe how a catalytic converter reduces pollution from a car engine.
- Describe the manufacture of sulfuric acid by the Contact process, and the use of fertilisers.
Testing for water
There are two tests, and they are constantly confused with one another, so learn them as a pair and learn which colour comes first.
| Test substance | Colour when dry (anhydrous) | Colour with water added |
|---|---|---|
| Cobalt(II) chloride | Blue | Pink |
| Copper(II) sulfate | White | Blue |
Anhydrous means the water of crystallisation has been driven out, usually by heating. Adding water re-forms the hydrated salt, which is why the colour change works as a test.
The trap is the direction. Copper(II) sulfate is familiar as the blue chemical, so "blue to white" feels right, but that is what you see when you heat the hydrated crystals and drive the water off. The test runs the other way: white to blue. The same reversal is offered for cobalt chloride, as pink to blue.
Both tests show only that water is present. To show that a liquid is pure water, you measure its physical constants: pure water boils at exactly 100 °C and freezes at exactly 0 °C at normal pressure. Any dissolved substance raises the boiling point and lowers the freezing point.
Safe to drink is not the same as pure. Drinking water contains dissolved salts and added chlorine, so it is a mixture and not chemically pure, and it is perfectly safe. This distinction is the whole of one common question.
Treating the water supply
- Filtration through beds of sand and gravel, which removes insoluble solids. Anything dissolved passes straight through with the water, so filtration cannot remove dissolved impurities.
- Chlorination, adding chlorine to kill bacteria and other microorganisms.
Chlorine makes the water safe, not pure. It removes none of the dissolved salts and in fact adds a substance to the water, so any statement saying chlorine is added to purify the water fails on that word alone.
Clean dry air
| Gas | Approximate proportion |
|---|---|
| Nitrogen | 78% |
| Oxygen | 21% |
| Argon and other noble gases | About 1% |
| Carbon dioxide | About 0.04% |
Air is a mixture, not a compound, which is why the proportions vary slightly and why its components can be separated by physical means such as fractional distillation of liquid air.
Air pollutants
| Pollutant | Where it comes from | What it does |
|---|---|---|
| Carbon monoxide | Incomplete combustion of fuels, where oxygen is limited | Toxic. Binds to haemoglobin so the blood carries less oxygen |
| Sulfur dioxide | Burning fossil fuels that contain sulfur impurities | Acid rain |
| Oxides of nitrogen | Nitrogen and oxygen from the air combining in the heat of an engine cylinder | Acid rain, and photochemical smog |
| Carbon dioxide | Complete combustion of fuels, and deforestation | Enhanced greenhouse effect and climate change |
| Methane | Cattle, rice fields and decomposing waste | Enhanced greenhouse effect |
| Particulates | Incomplete combustion, especially of diesel | Breathing problems, and they darken buildings |
Three sources are examined more than the rest.
Oxides of nitrogen come from the air, not from the fuel. Air is roughly four fifths nitrogen and one fifth oxygen, and the temperature inside an engine cylinder is high enough for these two normally unreactive gases to combine directly. Petrol and diesel are hydrocarbons and contain no nitrogen at all, so "nitrogen from the air reacts with the fuel" is a near miss that fails on chemistry.
Sulfur dioxide comes from sulfur impurities in the fuel, which is why removing sulfur at the refinery is the way to deal with it, not a catalytic converter.
Carbon monoxide comes from incomplete combustion, when there is not enough oxygen for every carbon atom to reach carbon dioxide.
Acid rain kills trees and fish, acidifies lakes and soils, and erodes limestone buildings and statues. The enhanced greenhouse effect is caused by an increase in greenhouse gases; the greenhouse effect itself is natural and is what keeps the planet habitable.
Catalytic converters
A catalytic converter turns exhaust pollutants into gases already present in clean air. It handles two of them:
Carbon monoxide is oxidised to carbon dioxide:
2CO + O₂ → 2CO₂
Nitrogen monoxide is reduced to nitrogen, sometimes using the carbon monoxide as the reducing agent so that both pollutants go at once:
2NO → N₂ + O₂
2NO + 2CO → N₂ + 2CO₂
Three points decide the questions on this.
Which gases increase and which decrease. The pollutants going in are carbon monoxide and nitrogen monoxide, and both are consumed. The gases leaving in greater quantity are carbon dioxide and nitrogen. Reading the question carefully matters here, because it is easy to spot the two harmful gases and answer with those without noticing that the question asked which gases increase.
Where the nitrogen oxide is made. N₂ + O₂ → 2NO is a real reaction, but it happens in the engine cylinder, not in the converter. The converter is the device that undoes it. An option containing that equation has the converter manufacturing a pollutant instead of removing one.
Sulfur dioxide is not removed by a converter. The equation 2SO₂ + O₂ → 2SO₃ is the key step of the Contact process in a chemical works, not something a car does.
A converter also has to be hot to work, which is why most of a short journey's pollution is emitted in the first few minutes before it warms up.
The Contact process
Sulfuric acid is made in three steps:
- Burn sulfur in air: S + O₂ → SO₂
- Oxidise the sulfur dioxide: 2SO₂ + O₂ ⇌ 2SO₃, over a vanadium(V) oxide catalyst at about 450 °C and about 2 atmospheres pressure.
- Convert the sulfur trioxide to sulfuric acid, in practice by dissolving it in concentrated sulfuric acid to give oleum and then adding water.
Step 2 is the reversible one, and that is what makes the conditions a compromise. A lower temperature would give a higher yield, because the forward reaction is exothermic, but the reaction would be far too slow, so 450 °C is chosen as a balance between yield and rate.
Sulfuric acid is used to make fertilisers, detergents, paints and car batteries.
Fertilisers
Fertilisers replace the elements crops remove from the soil. The three that matter are nitrogen, phosphorus and potassium, so an NPK fertiliser contains all three. Ammonium nitrate, ammonium sulfate and ammonium phosphate are common examples.
- Nitrogen is needed to make amino acids and proteins, so it drives leaf and stem growth.
- Phosphorus is needed for root growth.
- Potassium is needed for flowering and fruiting, and for enzyme function.
The environmental cost is eutrophication. Excess fertiliser leaches into rivers and lakes, algae grow rapidly, the bloom dies, decomposers multiply and use up the dissolved oxygen as they respire, and fish suffocate.
Common mistakes
- Saying anhydrous copper(II) sulfate turns from blue to white when water is added. That is the test running backwards.
- Confusing the cobalt chloride and copper sulfate tests, or mixing one colour from each.
- Saying a positive water test proves the liquid is pure water. It proves only that water is present.
- Saying chlorine purifies the water supply. It makes it safe.
- Saying filtration removes dissolved impurities.
- Saying oxides of nitrogen come from nitrogen in the fuel.
- Saying sulfur dioxide comes from the air rather than from sulfur in the fuel.
- Saying a catalytic converter removes sulfur dioxide.
- Saying a catalytic converter converts carbon dioxide into carbon monoxide. It is the other way round.
- Including N₂ + O₂ → 2NO among the converter reactions.
- Naming step 1 or step 3 of the Contact process as the reversible step.
- Saying the greenhouse effect itself is the pollution problem, rather than its enhancement.