Using resources, for the exam.

Part 1What the Earth gives us

Humans use the Earth's resources to keep warm, to build, to move around, and to stay healthy. These resources come from the Earth, the oceans and the atmosphere. Some are natural; many are processed or improved by chemistry.

A finite (non-renewable) resource is one that is being used up faster than it forms — once it's gone, it's gone on a human timescale. Fossil fuels (coal, oil, gas), metal ores and many minerals are finite. A renewable resource is one that re-forms at a similar rate to the rate we use it, such as timber from replanted forests.

Natural products — like rubber, cotton, wood and natural fertilisers — were once our only option. Chemistry now provides synthetic alternatives and improvements: synthetic rubber, man-made fabrics, fertilisers made in industry, and a huge range of materials. The job of the chemist is to make these processes more efficient and to use less of the finite resources.

Part 2Sustainable development & the chemist's job

Sustainable development meets the needs of people today without damaging the ability of future generations to meet their own needs. In chemistry that means using less energy, using fewer finite resources, and producing less waste.

Examples you should be able to give: extracting copper from low-grade ores using phytomining and bioleaching (instead of digging up scarce high-grade ore), recycling metals and glass rather than making them from raw materials, and finding ways to use a smaller quantity of a finite raw material per product.

Part 1Potable is not pure

Potable water is water that is safe to drink. It is not the same as pure water: potable water still contains small amounts of dissolved salts and other substances — it just has low enough levels of dissolved solids and harmful microbes to be safe.

Good-quality potable water should have a pH between 6.5 and 8.5, low levels of dissolved salts, and no harmful microbes. In the UK most potable water comes from fresh water — rain water that collects in reservoirs, lakes and rivers, and in aquifers (rocks that trap groundwater).

So treating fresh water takes two steps: pass it through filter beds to remove solid bits, then sterilise it to kill microbes. UK water companies sterilise using chlorine, ozone or ultraviolet light.

Part 2When fresh water is scarce — desalination

Where there isn't much fresh water, countries treat sea water instead. Removing the dissolved salts is called desalination. It can be done by distillation (boil the sea water and condense the pure water vapour) or by reverse osmosis (push water through a membrane that traps the salt ions).

The big drawback is energy: both methods need a lot of energy (distillation has to boil the water; reverse osmosis needs high pressure). That makes desalination expensive, so it's mainly used where fresh water is genuinely short.

Part 1Sewage treatment, step by step

Homes, farms and factories produce huge volumes of waste water that must be cleaned before it's returned to rivers and the sea. Sewage contains organic matter and harmful microbes; agricultural and industrial waste can also contain harmful chemicals.

Sewage treatment works in a set order:

Screening & grit removal takes out large solids and grit. Sedimentation lets the waste settle into a liquid (effluent) on top and a solid (sludge) at the bottom. The effluent is treated by aerobic digestion (air is bubbled through so bacteria break down organic matter and harmful microbes). The sludge is treated by anaerobic digestion (bacteria break it down without air, which also produces methane gas as a fuel and a fertiliser as a by-product).

Part 2Choosing where to get your water

It's worth comparing the three sources by how much treatment — and energy — each needs. Ground water from aquifers usually only needs sterilising. Waste water needs the full sewage treatment above. Sea water needs desalination, which uses the most energy of all.

The general rule: the more contaminated the starting water, the more energy and processing it takes to make it potable. That's why treating sewage to return it to rivers is far cheaper than desalinating sea water — but still uses much more energy than simply sterilising clean ground water.

Part 1The four stages of an LCA

A life cycle assessment (LCA) works out the environmental impact of a product across its whole life — "from cradle to grave". You assess four stages:

1. Extracting and processing the raw materials — this uses energy and resources, and can pollute land, water and air. 2. Manufacturing and packaging the product. 3. Using the product over its lifetime (including energy used and any waste). 4. Disposal at the end of life — landfill, transport to the dump, and whether it can be recycled.

At every stage you consider how much energy and raw material is used, and how much waste and pollution (including carbon dioxide and other gases) is produced.

Part 2Reduce, reuse, recycle

We can cut the use of finite resources, energy, waste and pollution by handling materials more carefully. The order of preference is reduce, then reuse, then recycle:

Reduce — use less material in the first place (e.g. lighter packaging). This saves the most resources. Reuse — use the same item again without reprocessing it (e.g. refilling a glass bottle). Recycle — process a used material into a new product. For metals, this means melting and re-forming them; the amount of separating needed depends on the metal and what it's wanted for. Glass bottles can be crushed, melted and re-shaped into new glass products.

Recycling metals saves the finite ore and uses much less energy than extracting the metal from its ore in the first place — a key reason it's worth doing.

Part 1Rusting needs water and oxygen

Corrosion is the destruction of a metal by reactions with substances in its surroundings. The corrosion of iron and steel is called rusting. Rust is hydrated iron(III) oxide.

The key fact: iron rusts only when both water AND oxygen are present. Remove either one and the iron will not rust. (Salt water and acids speed rusting up, but they aren't needed for it to happen.) You can prove this with three test tubes — only the nail with access to both air and water rusts.

Part 2Two ways to stop rust

There are two big strategies, and you should be able to explain both.

1. Barrier methods keep out the water and oxygen. Painting, coating with oil or grease, and covering with plastic all work — but only while the coating is unbroken. If a painted surface is scratched, rusting starts at the scratch.

2. Sacrificial protection uses a more reactive metal in contact with the iron. The more reactive metal corrodes instead of the iron — it is "sacrificed". Galvanising (coating iron with zinc) does both jobs at once: the zinc is a barrier and, because zinc is more reactive than iron, it protects the iron sacrificially even if the layer is scratched.