Earth and atmosphere, Year 11: the Earth's resources and clean water

Sustainable development

提示The first word describes something that can be kept going for a long time.

为什么Chemists contribute by finding ways to use less of the limited raw materials, less energy and less water, to make less waste, and to replace finite resources with renewable ones.

Crude oil and metal ores

提示Pick the ones that cannot be regrown or re-bred within a human lifetime.

为什么A finite resource is used up faster than it is replaced, so it will eventually run out: crude oil took millions of years to form, and ores are not being replaced at all. Timber and wool are renewable because trees and sheep can be grown again.

Natural fibres such as cotton and wool

提示Think of what cloth was made from before the chemical industry.

为什么Other examples are synthetic rubber in place of the natural rubber from trees, and manufactured fertilisers alongside manure. Synthetic products can be made in large amounts with chosen properties, but many depend on a finite raw material.

200

提示Divide the amount in the ground by the amount removed each year; the powers of ten subtract.

为什么(4 × 10⁸) ÷ (2 × 10⁶) = (4 ÷ 2) × 10² = 200 years. The deposit is two orders of magnitude larger than the amount used each year. The figures are invented for this question.

Trees must be replanted at least as fast as they are cut down

提示Compare the rate of removal with the rate of regrowth.

为什么A resource is renewable only while it is replaced as quickly as it is used. A forest that is felled faster than it regrows shrinks like any finite store, so the label depends on how the resource is used.

Potable water

提示The first word comes from the Latin for 'to drink'.

为什么To be safe, drinking water must have low enough levels of dissolved salts and of microbes. It does not have to contain nothing but H2O.

It contains dissolved substances

提示Think about what the label on a bottle of mineral water lists.

为什么To a chemist, pure means a single substance with nothing else in it. Tap water contains dissolved salts, and usually a little chlorine, at levels that are safe to drink.

To kill harmful microbes

提示Some things in the supply are too small to be caught by sand and gravel, and they can cause disease.

为什么The three steps are choosing a suitable source of fresh water, passing it through filter beds and sterilising it. Chlorine, ozone or ultraviolet light can be used for the last step.

energy

提示It is what is needed to boil sea water, or to force it through a membrane at high pressure.

为什么Fresh water from rain needs only filtering and sterilising, because it is already low in dissolved salts. Salty water has to have the salts taken out, so desalination is used mainly where fresh water is scarce.

0.25

提示The solid is whatever extra mass the dish has gained.

为什么52.61 − 52.36 = 0.25 g of solid in 50 cm³ of water. Pure water would leave nothing behind, so the mass of the residue measures how much was dissolved in the sample.

Check that it boils at exactly 100 °C

提示A pure substance changes state at one fixed temperature.

为什么Dissolved salts raise the boiling point, so impure water boils above 100 °C. Two supporting checks are that a sample leaves no solid when it is evaporated and that its pH is 7.

Large solid objects and grit

提示Picture what a coarse sieve would catch.

为什么Rags, paper, plastics and grit would block or damage the pipes and pumps further on, so they are taken out first. The stages that follow deal with the organic matter and the harmful microbes.

sludge; effluent

提示The first sinks to the bottom of the tank; the second is what flows off the top.

为什么The sewage is left to stand in large tanks so that the solids settle out. The two parts are then treated separately, by different kinds of bacteria.

Anaerobic

提示It takes place in closed tanks with the air kept out.

为什么Bacteria break the sludge down without oxygen. The liquid part of the sewage is treated the other way, by bacteria that need oxygen.

The bacteria that break down the organic matter need oxygen

提示The microbes doing this job respire the way we do.

为什么This is aerobic biological treatment. The bacteria feed on the remaining organic matter and reduce the harmful microbes, so that the water is clean enough to be released into rivers or the sea.

Ground water needs only filtering and sterilising; waste water needs many more treatment steps

提示Compare how much has to be taken out of each one.

为什么Ground water is fresh and fairly clean. Waste water contains organic matter, harmful microbes and sometimes harmful chemicals, so it needs every stage of sewage treatment first. Salt water needs desalination, which costs a great deal of energy.