Topic 10: Chemistry of the environment

This topic looks at the chemistry of the water we drink and the air we breathe. You will learn how to test for water and judge its purity, how water supplies are treated, why fertilisers can damage rivers, and how pollutant gases and greenhouse gases affect our environment.

Chemical test for water and test for purity

Two simple chemical tests show that a liquid contains water. Anhydrous copper(II) sulfate turns from white to blue when water is added, and anhydrous cobalt(II) chloride turns from blue to pink. Both tests only prove that water is present, not that the sample is pure water. Purity is judged from physical properties instead: pure water boils at exactly 100 degrees Celsius and freezes at exactly 0 degrees Celsius at normal atmospheric pressure. Dissolved impurities raise the boiling point and lower the freezing point, so a sample that boils above 100 degrees Celsius or freezes below 0 degrees Celsius is not pure. Distilled water is used in chemical experiments because tap water contains dissolved salts that could interfere with reactions.

Treatment of the water supply

Natural water from rivers and reservoirs contains suspended solids, dissolved substances and harmful microbes, so it must be treated before it is safe to drink. The water is first passed through filter beds of sand and gravel to remove insoluble solids and particles. It is then treated to kill bacteria and other microorganisms; this sterilisation is usually done by adding chlorine, though ozone or ultraviolet light can also be used. Although treated water is clean and safe, it is not chemically pure because it still contains dissolved salts. Distillation would remove these dissolved salts, but it is far too expensive and energy hungry to use for an entire public water supply.

Fertilisers, NPK and eutrophication

Plants need certain elements to grow well, the three most important being nitrogen, phosphorus and potassium. Fertilisers supply these elements: nitrogen for healthy leaves and stems, phosphorus for strong roots, and potassium for flowers and fruit. NPK fertilisers are mixtures that contain compounds of all three elements together. Ammonium salts and nitrates are common nitrogen-containing fertilisers because they are soluble and easily taken up by roots. However, excess fertiliser can be washed off fields into rivers and lakes, where it causes eutrophication. The extra nutrients make algae grow rapidly, forming a bloom that blocks light. Plants below die and are broken down by aerobic bacteria, which multiply and use up the dissolved oxygen, so fish and other aquatic life suffocate.

Composition of clean air

Clean, dry air is a mixture of gases in roughly fixed proportions. About 78 percent is nitrogen and about 21 percent is oxygen, which together make up almost the whole atmosphere. The remaining 1 percent is mostly argon, a noble gas, together with a small but important amount of carbon dioxide, around 0.04 percent. Clean air also contains traces of other noble gases and varying amounts of water vapour. Knowing these proportions helps you recognise pollutants, which are gases added to the air that are not normally part of this clean mixture.

Air pollutants: sources and effects

Several pollutants are released into the air, mainly by burning fuels. Carbon monoxide (CO) forms during the incomplete combustion of carbon-containing fuels when there is not enough oxygen; it is a toxic gas that combines with haemoglobin in the blood and reduces the amount of oxygen carried around the body. Sulfur dioxide (SO2) is produced when fuels containing sulfur impurities are burned; it dissolves in rain to form acid rain, which damages buildings, harms plants and acidifies lakes. Oxides of nitrogen (NO and NO2), often written together as NOx, form when nitrogen and oxygen in the air react in the high temperatures inside engines; they also cause acid rain and contribute to respiratory problems. Particulates are tiny solid particles such as carbon (soot) released by incomplete combustion; breathing them in can cause respiratory illness, and they may also be linked to increased cloud formation.

Catalytic converters

Catalytic converters are fitted to the exhaust systems of motor vehicles to reduce harmful emissions. They contain catalysts, usually platinum and other precious metals, spread over a honeycomb to give a large surface area. As exhaust gases pass through, the catalysts speed up reactions that convert the most harmful pollutants into less harmful ones. Carbon monoxide and oxides of nitrogen react together so that carbon monoxide is changed into carbon dioxide and the oxides of nitrogen are changed into harmless nitrogen gas. In this way a catalytic converter removes two pollutants in a single reaction, although the carbon dioxide it produces is still a greenhouse gas.

Carbon dioxide, methane and climate change

Carbon dioxide (CO2) and methane (CH4) are greenhouse gases. They allow energy from the Sun to reach the Earth's surface but absorb the heat energy radiated back, trapping it in the atmosphere and keeping the planet warm. This natural greenhouse effect is essential for life, but human activity has increased the amounts of these gases. Carbon dioxide is added by burning fossil fuels and is removed by photosynthesis, so cutting down forests reduces how much can be absorbed. Methane is released from cattle and other livestock and from decomposing waste in landfill sites and rice fields. The rising concentration of greenhouse gases is increasing the greenhouse effect, which contributes to climate change and global warming.