Key Stage 4 science · Chemistry – Earth and atmospheric science

GCSE air pollution and drinking water: a guide for parents

In Years 10 and 11, GCSE chemistry students study the common pollutants that burning fuel releases into the air, namely sulfur dioxide, oxides of nitrogen and particulates, and where each one comes from. They also look at the water resources on Earth and how water is made safe to drink. The topic joins up ideas from earlier years, such as combustion, mixtures and separation, with real questions about health, cars, industry and supply.

Air pollutants and potable water, explained

Most air pollution in this topic starts with burning. When a fuel burns in plenty of air, the products are mainly carbon dioxide and water, but real fuels and real engines are not perfect. Sulfur dioxide forms when fuels that contain sulfur impurities, such as coal and some oils, are burned, because the sulfur reacts with oxygen. It dissolves in water in clouds to make acid rain, which damages stone buildings, trees and life in lakes, and it irritates the lungs. Schools now use the spelling “sulfur”; older books, and the national curriculum wording, spell it “sulphur”.

Oxides of nitrogen have a different origin. They are not made from the fuel at all. Inside a hot engine, the nitrogen and oxygen that were both in the air react together, and this happens only at very high temperatures. Oxides of nitrogen also contribute to acid rain and to breathing problems, particularly for people with asthma. Particulates are tiny solid or liquid specks, such as soot, that come from incomplete combustion, especially in diesel engines. They can be small enough to reach deep into the lungs, and they also reduce the sunlight reaching the ground, an effect called global dimming. Many schools also teach carbon monoxide alongside these, a poisonous gas from incomplete combustion, though it is not named in the statutory wording.

Students are expected to link each pollutant to its source and its effect, and to know that these are not the same as the gases behind climate change. Carbon dioxide is treated separately, because it is not harmful to breathe at normal levels; its main effect is on the climate, which is covered in a neighbouring topic. Ways of reducing these pollutants include removing sulfur from fuels before they are burned, removing sulfur dioxide from power station waste gases, fitting catalytic converters to car exhausts to turn oxides of nitrogen back into nitrogen, and using cleaner transport.

The second half of the topic is about water. Most of the water on Earth is salt water in the oceans. Of the fresh water, a large share is frozen in ice caps and glaciers or lies deep underground, so only a small fraction is easily available in rivers, lakes, reservoirs and shallow aquifers. Potable water means water that is safe to drink, and it is not the same as pure water: tap water contains dissolved minerals and is perfectly healthy. In the UK, rainwater collects in reservoirs, rivers and underground rocks called aquifers. Water from a suitable source is treated by passing it through filter beds to remove solid particles, then sterilising it with chlorine, ozone or ultraviolet light to kill microbes. Where fresh water is scarce, sea water can be desalinated by distillation or by reverse osmosis, but both need a lot of energy. Used water goes through sewage treatment before it returns to rivers.

Words your child will hear

Potable water: Water that is safe to drink. It can still contain dissolved substances.
Pure water: Water containing nothing else at all. It boils at exactly 100 °C at normal pressure.
Particulates: Tiny solid or liquid particles, such as soot, carried in the air.
Acid rain: Rain made more acidic by dissolved sulfur dioxide and oxides of nitrogen.
Incomplete combustion: Burning with too little oxygen, which can make soot (carbon) and carbon monoxide.
Desalination: Removing dissolved salts from sea water to make fresh water.
Reverse osmosis: Forcing water through a very fine membrane that holds back the dissolved salts.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about common atmospheric pollutants: sulphur dioxide, oxides of nitrogen, particulates and their sources
Students should be taught about the Earth's water resources and obtaining potable water

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Explain how water from a reservoir in a rainy area is made potable, and how this differs from making drinking water from sea water in a hot, dry country.

Start with the source. Rain collects in the reservoir, so the water is fresh. It still holds dissolved minerals, some floating debris and microbes, so it is not yet safe to drink.
First stage: screening and filtration. Water passes through a screen to remove large objects, then through beds of sand and gravel. These trap the small solid particles, such as silt, which the screens miss.
Second stage: sterilisation. Chlorine, ozone or ultraviolet light is used to kill bacteria and other microbes. Dissolved minerals are left in, which is fine, because potable does not mean pure.
Now the sea water case. Filtering cannot remove dissolved salt, so a different method is needed. One is distillation: boil the sea water, collect and cool the steam, and the salt stays behind. The other is reverse osmosis: high pressure forces water through a membrane that holds back the salt.
Compare the two routes. The reservoir route needs little energy but depends on rainfall. Desalination works anywhere near the sea, but heating or pressurising large volumes of water uses a great deal of energy and is expensive, so it is used mainly where fresh water is scarce.

Reservoir water is potable after filtration to remove solids and sterilisation to kill microbes. Sea water must also have its dissolved salt removed, by distillation or reverse osmosis, which costs far more energy.

Common mistakes and how to help

Thinking potable water and pure water mean the same thing

In everyday speech, “clean” and “pure” are used loosely, and adverts often describe bottled water as pure. In chemistry, pure means a single substance only.

What helps: Ask what is dissolved in tap water (calcium, magnesium and other minerals, usually harmless) and how you could prove it is not pure. Boiling point and the white scale left in a kettle both give clues.

Believing oxides of nitrogen come from the fuel

Students learn that sulfur dioxide comes from sulfur in the fuel, so they assume the same pattern holds for every pollutant.

What helps: Point out that air is about four-fifths nitrogen and one-fifth oxygen. In a very hot engine, those two gases react with each other, so even a fuel with no nitrogen at all can lead to oxides of nitrogen.

Mixing up the effects of different gases, for example saying carbon dioxide causes acid rain

Climate change, acid rain and air quality are all “environmental problems”, and they blur together in news stories.

What helps: Make a small table with the gas, its source and its main effect. Sulfur dioxide and oxides of nitrogen go with acid rain and breathing problems; particulates with lung damage and dimming; carbon dioxide with warming.

Thinking particulates are only visible smoke

Children picture black smoke from a chimney or exhaust, so they assume clean-looking air has no particulates.

What helps: Explain that the most worrying particles are far too small to see individually. Ask what the dark, gritty film is that builds up on a window ledge near a busy road over a few weeks.

How to help at home

Where does our tap water come from? Look at the annual water quality information that most water companies publish, or simply ask: is your supply from a reservoir, a river or underground? Your child can then sketch the journey from rain to tap, naming each treatment step.
Filter a muddy sample, then talk about why not to drink it. Stir some garden soil into a jug of water and pour it through a coffee filter, or through sand and gravel in a plastic bottle that an adult has cut open. The water turns clearer, but it is not safe to drink, which makes the point that filtering is only one stage.
Collect particulates on a card. Fix two clean white cards outside with sticky tape, one on the side of your home or fence that faces the street and one somewhere sheltered from traffic, such as a back garden. Keep away from the kerb when putting them up. After a week, compare the grey marks. It is a rough test, but it gets children thinking about how to make results comparable.
Talk about journeys. Ask your child what a car engine releases and why a catalytic converter helps. Discuss short journeys, idling outside school and what could reduce pollution, such as walking, cycling or sharing a lift.
Estimate water use. Time a shower or fill a bucket from the tap for ten seconds to work out the flow in litres per minute. Then compare with how much fresh water the family might use in a day, and ask why saving water matters even in a rainy country.

Builds on

Key Stage 3: The Earth, the rock cycle and resources
Key Stage 3: The carbon cycle, atmosphere and climate