Manufactured products need raw materials taken from natural resources. This topic covers how useful materials are obtained, how water is made safe to drink, and how the benefit of a process is weighed against its environmental cost.
The Earth supplies everything we use, but not at the same rate we use it. A finite resource is one that forms far more slowly than it is consumed, so the supply effectively runs down: crude oil, natural gas, coal and metal ores all took millions of years to form. A renewable resource is replaced at a similar rate to its use, so it can last indefinitely provided it is managed so that use does not outrun replacement. Timber, cotton and wool are all regrown within a human lifetime.
"Renewable" describes the rate at which a resource is replaced. It does not mean that a resource is artificial, harmless or unlimited: crude oil is entirely natural and is still finite.
Sustainable development means meeting the needs of people today without stopping future generations meeting theirs. Chemistry contributes to it by making processes use less energy, by finding ways to use lower-grade ores, and by recovering materials that would otherwise be thrown away.
Potable water is water that is safe to drink. It is not the same as pure water: potable water still contains dissolved salts and minerals, and a chemist would call it a mixture. Pure water, in the chemical sense, contains only H2O.
In the UK most drinking water comes from fresh surface or ground water. Real treatment has several stages; the two studied at GCSE are:
Where fresh water is scarce, drinking water can be produced from sea water by desalination, either by distillation or by reverse osmosis through a membrane. Both remove the dissolved salts, and both need a great deal of energy, so desalination is generally used only where fresh water is scarce or other sources are less practical.
Sewage and agricultural waste water carry organic matter and harmful microbes, and industrial waste water may also carry toxic metal compounds. Treatment happens in stages:
Traditional extraction, by reduction with carbon or by electrolysis, needs ore rich enough to be economic to process. As the rich deposits are used up, chemists have turned to biological methods that work on low-grade ores containing only small amounts of metal.
Phytomining grows plants on soil or low-grade ore containing metal compounds. The plants take up metal ions through their roots; they are then harvested and burned, and the ash contains the metal compound in a much higher concentration than the original soil.
Bioleaching uses bacteria instead of plants. They bring about chemical reactions involving minerals in the ore, producing a solution called a leachate that contains dissolved metal compounds.
Both methods give material containing a more concentrated metal compound, from which the metal still has to be recovered chemically. For copper this can be done by displacement using scrap iron, or by electrolysis. The advantage is access to ore that would otherwise be waste; the disadvantage is that both methods are slow.
Reducing, reusing and recycling all reduce demand for raw materials, but they are different processes. Reusing means using the object again as it is, such as refilling a glass bottle. Recycling means breaking the object down and making something new from the material, such as crushing that bottle into cullet, melting it and reshaping it.
Recycling metals is particularly worthwhile because it avoids some of the most energy-intensive and environmentally damaging stages of obtaining metal from new ore. Recycled metal is melted and cast; where a specific alloy is needed the composition may have to be adjusted.
Glass bottles that are not reused are crushed, melted and reshaped. Some products are difficult to recycle because their materials are hard to separate, which is a design problem as much as a chemical one.
A life cycle assessment (LCA) works out the total environmental impact of a product across its whole life, not just its manufacture. It considers four stages in order: extracting and processing the raw materials, manufacturing and packaging, using the product over its lifetime, and finally disposing of it, including the transport at every stage.
Some of what an LCA measures is straightforward: the mass of raw material, the energy used, the volume of water. Other parts are not. Putting a number on the effect of a pollutant means making a judgement about how much harm it causes, and different assessors reach different figures. That makes an LCA partly subjective, so an LCA published by a company selling the product should be read carefully.
An alloy is a mixture containing a metal, not a compound, because the components are not chemically bonded in fixed proportions. Many metals are used as alloys, because alloying can improve hardness, strength, corrosion resistance or other useful properties.
In a pure metal, similarly sized positive metal ions form regular layers that slide over one another when a force is applied, so pure metals bend easily. In an alloy, ions of different sizes distort the regular arrangement, making it harder for the layers to slide, so the alloy is harder.
Common alloys:
Corrosion is the destruction of a metal by reaction with substances around it. For iron the process is called rusting, and it needs both oxygen and water. Iron will not rust in dry air, and it will not rust in water that has been boiled to remove dissolved oxygen. Salt does not cause rusting but speeds it up.
Two common ways of preventing it are barrier methods and sacrificial protection. A barrier keeps oxygen and water off the surface: painting, greasing, or coating with plastic. Aluminium protects itself this way, forming a thin oxide layer that stops further attack, so aluminium appears less reactive than it really is.
Sacrificial protection works differently. A more reactive metal is attached to the iron, and because it is more reactive it corrodes instead. Blocks of magnesium bolted to a ship's hull do this job. Galvanising, which is coating iron with zinc, does both at once: the zinc is a barrier, and if it is scratched it still corrodes in preference to the iron.
Ceramics are made by shaping and then firing. Clay ceramics such as porcelain and brick are made from wet clay that hardens permanently when heated; glass is made mainly from sand, and soda-lime glass is produced by melting sand with sodium carbonate and limestone. Ceramics are hard, rigid and brittle, and many common ceramics are good electrical and thermal insulators.
Polymers are long-chain molecules, and their properties depend on which monomer was used and the conditions of manufacture. The same monomer can give different materials: low density poly(ethene) is flexible and made at high pressure, while high density poly(ethene) is made with a catalyst at lower pressure and is more rigid. Thermosoftening polymers soften when heated and can be remoulded, because the separate chains slide apart. Thermosetting polymers do not soften readily, because cross-links hold the chains together; at high enough temperatures they decompose instead.
A composite is made of fibres or fragments of one material held in a matrix of another, so it combines properties neither has alone. Concrete is gravel in cement, fibreglass is glass fibres in resin, and wood is natural cellulose fibres in an organic matrix.
The Haber process makes ammonia, the starting point for nitrogen fertilisers. The raw materials are nitrogen from the air and hydrogen, usually obtained from natural gas.
The reaction is reversible: N2 + 3H2 ⇌ 2NH3. The forward reaction is exothermic, so a low temperature would give a higher yield at equilibrium, but it would be far too slow to be useful. The conditions used are a compromise between yield and rate:
The ammonia formed is cooled until it liquefies and is removed, and the unreacted nitrogen and hydrogen are recycled through the reactor.
Nitrogen, phosphorus and potassium are three of the major mineral nutrients plants need, and the three supplied by NPK fertilisers. An NPK fertiliser is a formulation containing compounds of all three in measured proportions.
Ammonia from the Haber process is the source of the nitrogen. Reacting it with nitric acid gives ammonium nitrate, which is unusually rich in nitrogen because both parts of the compound supply it.
Potassium chloride and potassium sulfate are obtained from mined mineral deposits and processed for use. Phosphate rock is also mined, but it is not soluble enough for plants to take up, so it is treated with acid first: nitric acid gives phosphoric acid and calcium nitrate, sulfuric acid gives single superphosphate, and phosphoric acid gives triple superphosphate.
In industry this happens continuously on a large scale with integrated stages; in a school laboratory the same salts are made in small batches by titration and crystallisation.
Back to Using Resources practice · View all Using Resources questions
