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

Sustainable development

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

The whyChemists 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

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

The whyA 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

HintThink of what cloth was made from before the chemical industry.

The whyOther 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

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

The why(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

HintCompare the rate of removal with the rate of regrowth.

The whyA 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

HintThe first word comes from the Latin for 'to drink'.

The whyTo 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

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

The whyTo 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

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

The whyThe 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

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

The whyFresh 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

HintThe solid is whatever extra mass the dish has gained.

The why52.61 − 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

HintA pure substance changes state at one fixed temperature.

The whyDissolved 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

HintPicture what a coarse sieve would catch.

The whyRags, 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

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

The whyThe 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

HintIt takes place in closed tanks with the air kept out.

The whyBacteria 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

HintThe microbes doing this job respire the way we do.

The whyThis 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

HintCompare how much has to be taken out of each one.

The whyGround 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.