Earth and atmosphere, Year 9

feeding; respiration

HintFirst ask how anything in grass gets inside a rabbit, then what every living cell does to release energy.

The whyPhotosynthesis builds the carbon into the plant, eating moves it along the food chain, and the animal then releases it again as carbon dioxide. The same atom may go round this loop countless times.

Decomposers, such as bacteria and fungi

HintThey are the living things that cause rotting; many are too small to see.

The whyDecomposers feed on dead material and respire as they do so, which releases the carbon as carbon dioxide. Without them, carbon would stay locked in dead plants and animals.

Burn the coal

HintThink what a power station does with the fuel it is supplied with.

The whyFossil fuels are a store of carbon taken out of the air by living things long ago. Combustion joins that carbon to oxygen and releases it as carbon dioxide in moments, far faster than any natural process puts it back underground.

It stays the same

HintAtoms can be rearranged and moved about; think whether any are made or destroyed.

The whyThe cycle only moves carbon between stores: the air, living things, the oceans, rocks and fossil fuels. What human activity changes is where the carbon is, with more of it now in the air.

Colder

HintThink what a blanket does, and what happens when it is taken away.

The whyGreenhouse gases that are naturally in the air keep the planet's average surface temperature at about 15 °C. Without them it would be around −18 °C, too cold for most life. The natural greenhouse effect is a good thing.

The enhanced greenhouse effect

HintThree words: the name of the natural process with one more word in front of it.

The whyThe natural effect has kept the Earth warm for millions of years. The enhanced effect is the part added since people began burning fossil fuels on a large scale, and it is what is changing the climate.

Burning fossil fuels

HintThink how most power stations, cars and boilers get their energy.

The whyCoal, oil and gas are mostly carbon and hydrogen. Burning them joins the carbon to oxygen from the air, and the carbon dioxide goes up the chimney or out of the exhaust. Clearing forests adds more.

Fewer trees are left to take carbon dioxide in

HintThink which process in green leaves uses this gas as a raw material.

The whyGrowing trees remove carbon dioxide from the air by photosynthesis and store the carbon as wood. Cut them down and that removal stops; if the wood is burned or left to rot, its stored carbon is released as well.

A non-renewable resource

HintCoal, oil and natural gas are the best-known examples.

The whyNon-renewable does not mean it can never form again. It means it forms so slowly that, for people, what is in the ground now is all there will ever be.

plants; plankton

HintOne grew in swampy forests on land; the other drifted in the sea in vast numbers.

The whyIn both cases the remains were buried under layers of mud before they could rot away, then squeezed and heated over millions of years. That is why these fuels are found underground and under the sea bed.

fossil

HintThe same word describes the preserved remains of ancient life found in rocks.

The whyThe energy in these fuels first arrived as sunlight, captured by living things long ago and stored in their remains. Burning them releases in moments energy that took a very long time to collect.

Trees regrow in years; coal takes millions of years to form

HintCompare how long you would wait for a new supply of each.

The whyRenewable means the supply can be replaced about as fast as it is used. A felled tree can be replaced by planting another, but nobody can wait millions of years for new coal.

An ore

HintHaematite is one for iron, and bauxite is one for aluminium.

The whyMost metals are found joined to other elements, often oxygen, inside rock. Whether a rock counts as an ore depends on cost: it must hold enough metal to pay for digging it up and getting the metal out.

It is very unreactive

HintThink where gold sits in the reactivity series.

The whyGold does not react with oxygen or water, so over millions of years it has stayed as the pure metal. Iron reacts with both, so in nature it is always found combined with other elements in its ores.

Zinc

HintCarbon can only take oxygen away from a metal that holds on to it less strongly than carbon does.

The whyCarbon is more reactive than zinc, so it can pull the oxygen away from zinc oxide and leave the metal. It cannot do the same to aluminium oxide, so aluminium has to be extracted using electricity, which costs far more.

iron; carbon dioxide

HintOne product is the metal you set out to get; the other is the gas made when the element doing the extracting joins with oxygen.

The whyThe carbon takes the oxygen from the iron oxide and leaves as a gas, so the metal is left behind on its own. The furnace is hot enough for the metal to run out as a liquid.

The metal's ore

HintIt is the rock that fresh supplies have to be extracted from.

The whyThe Earth holds a limited amount of each ore, and the richest deposits are mined first. Every tonne of metal that is recycled is a tonne that does not have to be dug up, so the supply lasts longer and less land is quarried.

It has burned away into waste gases

HintAsk what is left of a fuel after it has done its job.

The whyUsing a metal does not change what it is, so it can be melted and shaped again. Using a fuel means burning it, which turns it into carbon dioxide and water vapour that drift off into the air.

energy

HintIt is measured in joules.

The whyRecycling is a means of saving resources, so each case has to be weighed up. For aluminium the saving is huge; for some mixed or dirty plastics, cleaning and sorting can cost more than is gained.

Washing and refilling it

HintOne of the two needs a furnace.

The whyMelting glass takes a very hot furnace, so recycling it still uses a lot of energy. Reusing the bottle as it is skips that step altogether, which is why "reduce, reuse, recycle" puts recycling last.