Producing potable water by sedimentation, filtration and sterilisation, desalination by distillation and reverse osmosis, treating waste water, and the sustainable extraction of metals by phytomining and bioleaching.

A six-mark Level of Response question. Reward describing the fresh-water treatment: an appropriate fresh-water source (such as a river or reservoir) is chosen; the water passes through filtration to remove insoluble solids (sedimentation lets larger particles settle out, then it is filtered, for example through beds of sand and gravel, to remove smaller solids); then it is sterilised to kill microorganisms (bacteria), commonly by adding chlorine, or by using ozone or ultraviolet light. Then explain the sea-water point: sea water contains a high concentration of dissolved salts (it is salty), and filtration and sterilisation do not remove dissolved salts, so to make sea water potable the salt must be removed by distillation (the water is boiled and the steam condensed, leaving the salt behind) or by reverse osmosis. This is not normally used for fresh water because fresh water has only low levels of dissolved salts, so it does not need the salt removing, and distillation needs a lot of energy and is expensive. Markers reward the fresh-water steps (choosing a source, filtration/sedimentation, sterilisation), and the explanation that sea water needs distillation (or reverse osmosis) to remove the dissolved salt because filtering does not remove dissolved salts, while fresh water is low in salts and distillation is too energy-expensive to use routinely. A common error is to say filtration removes the salt from sea water.

A C6.2 application question. Reward describing phytomining: plants are grown on land (or low-grade ore) that contains low concentrations of copper compounds; the plants absorb the copper compounds through their roots as they grow and concentrate them in their tissues; the plants are then harvested and burned, and the ash contains a relatively high concentration of copper compounds, from which the copper can be extracted (for example by reacting with acid to make a solution, then by displacement with scrap iron or by electrolysis). An advantage of phytomining (and bioleaching) over traditional mining is that they allow copper to be obtained from low-grade ores that would otherwise be uneconomic, conserving the limited supply of high-grade ore; they also cause less damage to the landscape and produce less rock waste than digging large mines, and use less energy. Markers credit the phytomining steps (plants absorb copper compounds, are harvested and burned, copper extracted from the ash) and a valid advantage (uses low-grade ores, conserves high-grade ore, less environmental damage). A common error is to confuse phytomining (plants) with bioleaching (bacteria).