Topic 10: Using resources

Humans rely on the Earth, the oceans and the atmosphere to provide everything from drinking water to metals and fertilisers. Chemistry helps us judge how to use these resources wisely, treat the water we drink, and develop materials that suit a particular job. This topic looks at how we balance our needs against the limits of the planet.

Finite resources and sustainable development

Natural resources, with help from farming, supply us with food, fuels, timber, clothing and many other materials. Some are renewable, meaning they can be replaced at the same rate they are used, such as wood from managed forests. Many others are finite: fossil fuels and most metal ores formed over millions of years and cannot be replaced once used. The chemical industry processes raw materials to make products of more value, but this uses energy and creates waste. Sustainable development means meeting the needs of people today without spoiling the ability of future generations to meet their own needs. Chemists support this by finding methods that use less energy, fewer raw materials and produce less pollution.

Potable water and desalination

Potable water is water that is safe to drink. It is not pure water in the chemical sense, because it still contains dissolved substances, but the levels of dissolved salts and microbes are low enough to be harmless. In the UK, rain provides fresh water that collects in lakes, rivers and underground rock called aquifers. To make this safe, the water is passed through filter beds to remove solids and then sterilised to kill microbes, using chlorine, ozone or ultraviolet light. Where fresh water is scarce, salty sea water can be treated by desalination. This is done either by distillation, which boils the water and condenses the vapour, or by reverse osmosis using membranes. Both methods need large amounts of energy, which makes them expensive.

Treating waste water

Sewage and farm waste must be treated before the water is returned to the environment, otherwise it would pollute rivers and harm living things. Sewage treatment first involves screening to remove large objects and grit. The water is then allowed to settle in tanks, producing a liquid called effluent and a solid called sludge. Air is bubbled through the effluent so that aerobic bacteria can break down the remaining organic matter and any harmful microbes. The sludge is treated separately by anaerobic bacteria, which digest it and produce biogas that can be used as a fuel. Treating waste water uses less energy than desalinating sea water, but more than treating fresh ground water.

Life cycle assessments

A life cycle assessment, or LCA, is carried out to work out the total environmental impact of a product over its whole life. It considers four main stages: extracting and processing raw materials, manufacturing and packaging, using and operating the product, and finally its disposal. At each stage the use of energy, water, natural resources and the production of waste and pollutants are considered. Some parts, such as the mass of material used or the energy needed, can be measured fairly accurately. Other parts, such as the effect of pollutants, are harder to give a number to and require judgement, so they can be misused to support a chosen conclusion in advertising.

Reduce, reuse and recycle

Using less of a finite resource is the most effective way to make it last longer, so reducing the amount we use is the first priority. Reusing items, such as refilling glass bottles, saves the energy and materials needed to make new ones. Recycling involves processing used materials so they can be made into new products. Metals can be melted down and recast, often using far less energy than extracting the metal from its ore. Glass bottles can be crushed, melted and reshaped. Recycling reduces the use of limited raw materials, cuts energy use and reduces the waste sent to landfill, which all support sustainable development.

Alternative methods of metal extraction (Higher)

Copper-rich ores are running out, so chemists have developed ways to extract copper from low-grade ores that were once ignored. Phytomining uses plants grown on soil that contains copper. The plants absorb copper compounds and are then burned, leaving ash rich in copper compounds. Bioleaching uses bacteria to produce solutions called leachates that contain copper compounds. In both methods the copper is obtained from solution either by displacement using scrap iron or by electrolysis. These methods avoid the digging, moving and disposing of large amounts of rock, so they cause less damage to the environment, although they are often slow.

The Haber process and its conditions (Higher)

The Haber process makes ammonia, NH3, which is used to manufacture fertilisers. Nitrogen, N2, is obtained from the air and hydrogen, H2, is obtained from natural gas. The reaction is reversible: N2 + 3H2 forms 2NH3. The gases are passed over an iron catalyst at a temperature of about 450 degrees Celsius and a pressure of around 200 atmospheres. These conditions are a compromise. A lower temperature would give more ammonia because the forward reaction is exothermic, but the reaction would be too slow, so a moderate temperature is chosen. A high pressure pushes the equilibrium towards ammonia and speeds the reaction, but very high pressures are costly and dangerous to build for. The ammonia is cooled so it liquefies and is removed, while unreacted N2 and H2 are recycled.

NPK fertilisers (Higher)

Plants need three main elements to grow well: nitrogen, phosphorus and potassium. NPK fertilisers are formulations that contain compounds of all three in the right proportions. Ammonia from the Haber process is used to make ammonium salts and nitric acid, providing the nitrogen. Potassium chloride, potassium sulfate and phosphate rock are mined directly, but phosphate rock cannot be used straight away. It is treated with nitric acid or with sulfuric acid to make soluble compounds such as phosphoric acid, calcium phosphate and ammonium phosphate, which plants can absorb. Industrial production uses large reactors and trained workers, unlike preparing a single salt in the school laboratory.

Corrosion and its prevention

Corrosion is the destruction of materials by chemical reactions with substances in the environment. The rusting of iron is a common example and needs both air and water to take place. One way to prevent corrosion is to keep out air or water using a barrier such as paint, oil or grease, or a coating of another metal. Galvanising coats iron with zinc, and this also gives sacrificial protection: the zinc is more reactive than iron, so it corrodes instead of the iron even if the surface is scratched. Aluminium does not corrode away because it forms a thin oxide layer that sticks tightly to the surface and protects the metal underneath.

Alloys, ceramics, polymers and composites

Materials are chosen to match the job they must do. Most metals in everyday use are alloys, mixtures of a metal with other elements that are harder and more useful than the pure metal. Bronze, brass, steels and gold alloys are examples. Ceramics such as glass and clay pottery are hard, brittle and resist heat. Soda-lime glass is made by heating sand, sodium carbonate and limestone, while borosilicate glass melts at higher temperatures. Polymers are long-chain molecules whose properties depend on the monomers used and the conditions of manufacture. Composites are made of two materials, fibres or fragments held in a binder called the matrix, giving combined properties, as in fibreglass and reinforced concrete.