Using the Earth's resources and obtaining potable water

Humans use the Earth's resources to provide warmth, shelter, food and transport. Natural resources, supplemented by agriculture, provide food, timber, clothing and fuels. Finite resources from the Earth, oceans and atmosphere are processed to provide energy and materials.

Chemistry plays an important role in improving agricultural and industrial processes to provide new products, and in sustainable development — development that meets the needs of current generations without compromising the ability of future generations to meet their own needs.

You should be able to state examples of natural products that are supplemented or replaced by agricultural and synthetic products, and distinguish between finite and renewable resources given appropriate information.

The distinction to make: a finite resource is one being used up faster than it is replaced, so supplies will eventually run out — crude oil, metal ores, and the fossil fuels. A renewable resource is one that is replaced at the same rate or faster than it is used, so it can be maintained — timber from replanted forests, and crops.

You should also be able to extract and interpret information about resources from charts, graphs and tables, and use orders of magnitude to evaluate the significance of data. When a table gives reserves and annual use, dividing one by the other gives the number of years remaining — and it is the order of magnitude of that answer, tens of years or thousands, that decides whether the figure matters.

Water of appropriate quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes. Water that is safe to drink is called potable water.

Potable water is not pure water in the chemical sense, because it contains dissolved substances. You should be able to distinguish between potable water and pure water: pure water is H₂O and nothing else, while potable water is safe to drink but still a mixture — and the dissolved salts it contains are harmless at these levels.

The methods used to produce potable water depend on available supplies of water and local conditions.

In the United Kingdom, rain provides water with low levels of dissolved substances — fresh water — that collects in the ground and in lakes and rivers. Most potable water is produced by:

• choosing an appropriate source of fresh water • passing the water through filter beds • sterilising.

Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.

You should be able to give reasons for the steps used: an appropriate source has low levels of dissolved substances to start with, so less treatment is needed; the filter beds remove insoluble solids such as grit and leaves; and sterilising kills microbes that would otherwise cause disease.

If supplies of fresh water are limited, desalination of salty water or sea water may be required. Desalination can be done by distillation, or by processes that use membranes such as reverse osmosis. These processes require large amounts of energy.

You should be able to describe the differences in treatment of ground water and salty water. The difference is what has to be removed. Ground water already has low levels of dissolved salts, so it only needs filtering and sterilising — both cheap. Salty water has high levels of dissolved salts, which filtering cannot remove, so the water must be separated from them by distillation or reverse osmosis — which is why desalination is used only where fresh water is scarce.

Urban lifestyles and industrial processes produce large amounts of waste water that require treatment before being released into the environment. Sewage and agricultural waste water require removal of organic matter and harmful microbes. Industrial waste water may require removal of organic matter and harmful chemicals.

Sewage treatment includes:

• screening and grit removal • sedimentation to produce sewage sludge and effluent • anaerobic digestion of sewage sludge • aerobic biological treatment of effluent.

Following the four steps through: screening removes large objects and grit that would damage the equipment. Sedimentation lets the mixture stand so that heavy solids sink as sewage sludge, while the liquid effluent floats above. The sludge is then digested anaerobically — by bacteria without oxygen — which produces methane that can be burned for energy, and a fertiliser. The effluent is treated aerobically, with air pumped through so that bacteria with oxygen break down the remaining organic matter.

You should be able to comment on the relative ease of obtaining potable water from waste, ground and salt water.

The Earth's resources of metal ores are limited.

Copper ores are becoming scarce, and new ways of extracting copper from low-grade ores include phytomining and bioleaching. These methods avoid traditional mining methods of digging, moving and disposing of large amounts of rock.

Phytomining uses plants to absorb metal compounds. The plants are harvested and then burned to produce ash that contains metal compounds.

Bioleaching uses bacteria to produce leachate solutions that contain metal compounds.

The metal compounds can be processed to obtain the metal. For example, copper can be obtained from solutions of copper compounds by displacement using scrap iron, or by electrolysis.

You should be able to evaluate alternative biological methods of metal extraction, given appropriate information.

The advantages are that these methods work on low-grade ores that would not be worth mining conventionally, they avoid digging, moving and disposing of large amounts of rock, and they use less energy. The disadvantages are that both are slow — phytomining takes a growing season or more — and the metal compounds still have to be processed afterwards, which itself costs energy.