Earth and atmosphere, Year 9

feeding; respiration

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

为什么Photosynthesis 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

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

为什么Decomposers 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

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

为什么Fossil 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

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

为什么The 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

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

为什么Greenhouse 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

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

为什么The 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

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

为什么Coal, 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

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

为什么Growing 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

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

为什么Non-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

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

为什么In 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

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

为什么The 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

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

为什么Renewable 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

提示Haematite is one for iron, and bauxite is one for aluminium.

为什么Most 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

提示Think where gold sits in the reactivity series.

为什么Gold 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

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

为什么Carbon 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

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

为什么The 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

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

为什么The 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

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

为什么Using 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

提示It is measured in joules.

为什么Recycling 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

提示One of the two needs a furnace.

为什么Melting 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.