THE EARTH’S SEA AND

ATMOSPHERETopic 1

KEY POINTSC1.1 The early atmosphere When the Earth was young, it was very hot and

there were lots of volcanoes. The gases that escaped from these formed the Earth’s atmosphere.

This early atmosphere contained a large amount of carbon dioxide, along with some ammonia, methane, nitrogen and water vapour. There was no oxygen.

As the Earth cooled, water vapour in the atmosphere condensed to form the oceans.

Theories about the sequence of events concerning the Earth’s atmosphere are not certain. Scientists have little evidence and they do not all agree.

KEY POINTSC1.2 A changing atmosphere About half of the atmospheric carbon dioxide in the

early atmosphere dissolved in the oceans. Early marine plants began to photosynthesise – they

removed carbon dioxide from the oceans and atmosphere and gave out oxygen. The fairly rapid expansion of plant life on land increased the rate of this. The amount of carbon dioxide in the atmosphere gradually decreased and the amount of oxygen increased.

Some of the dissolved carbon dioxide became part of the shells of marine organisms, such as coral. When these died they sank to the bottom of the ocean and over millions of years formed sedimentary carbonate rocks, such as limestone.

KEY POINTSC1.4 The atmosphere today Scientists monitor the composition of the

atmosphere in different places constantly. The main gas in the atmosphere today is nitrogen, which makes up 78% of the atmosphere. Some scientists think that this has built up from the many volcanic eruptions over the lifetime of the Earth.

The second most abundant gas in the atmosphere is oxygen, which makes up 21% of the atmosphere. This is there because of constant photosynthesis by the plants on the planet.

The third most abundant gas is argon, which makes up about 1%. It is a noble gas element and there are smaller amounts of other unreactive gases as well.

KEY POINTS - CONTINUED There is about 0.04% of carbon dioxide

in the atmosphere, but human activities such as burning fossil fuels, deforestation, cattle farming and rice growing are thought to be increasing this proportion.

MATERIALS FROM THE

EARTHTopic 2

KEY POINTSC1.5 Rocks and their formation Rocks deep inside the Earth can get hot enough to

melt – molten rock is called magma. This sometimes escapes to the Earth’s surface – when it erupts it is called lava.

Igneous rocks are formed when magma solidifies – this can happen underground or on the surface of the Earth. These rocks have a crystalline structure – if magma cools slowly, the crystals are bigger than if it cools quickly. Granite is an igneous rock.

Metamorphic rocks are formed when existing rocks are subjected to high temperature and high pressure. Marble is a metamorphic rock formed when underground limestone or chalk are subjected to the right conditions.

KEY POINTS Most sedimentary rocks are made from

particles of other rocks that have eroded and have been washed away in rivers and laid down to form layers called sediments. The layers pile up and compress the deeper layers of sediment over a very long time period – the particles get cemented together. Sandstone is a sedimentary rock.

The only types of rock that may contain fossils are sedimentary rocks. The sediments may contain dead plants and animals, so the hard parts of these can become trapped in the new sedimentary rock and an impression of their shape is left.

KEY POINTS Some sedimentary rocks are made from

the hard parts of marine organisms – this is how limestone and chalk are formed. These two rocks contain calcium carbonate – and so does marble because it is made from them.

Sedimentary rocks erode more easily than other types of rock because the particles are only cemented together – they are not made of crystals that interlock strongly.

KEY POINTS C1.6 Limestone and its uses Limestone is an important material and large

quantities of it are removed from the Earth in quarries. Explosives are used to break the limestone into pieces. These are cut or crushed and taken to the customers. Some is used in the form of regular blocks for constructing buildings. Much of it is used to make new materials.

Limestone (along with chalk and marble) contains calcium carbonate – when this is heated, it changes to make calcium oxide (lime) and carbon dioxide. This chemical reaction is called thermal decomposition.

KEY POINTS Limestone is needed for making cement,

concrete and glass. Cement is made by heating limestone with clay. Concrete is made by mixing sand, gravel, water and cement. Glass is made by heating calcium carbonate with sand and sodium carbonate.

Quarries are dusty, unsightly and noisy places. They often happen to be in attractive places and they might harm the tourist industry. They also take up land that could have been used for farming. Decisions have to be made about balancing the need for limestone and the effect that getting it can have on local people, the environment and the economy.

KEY POINTS C1.8 Chemical reactions All substances are made of atoms. An atom is

the smallest part of an element that can take part in chemical reactions.

A compound consists of atoms of two or more elements chemically joined together. The chemical formula of a compound shows the symbols of the elements it contains and the ratios in which their atoms are present.

In all chemical reactions, the atoms of the reactants rearrange to form new products. None of the atoms are destroyed in the reaction and no new ones are formed.

KEY POINTS Atoms are not made or destroyed in a chemical

reaction – they are only rearranged. So the total mass before and after any reaction stays the same.

Word equations show what happens in chemical reactions. In general:

reactants products For example, zinc carbonate decomposes on

heating – it makes zinc oxide and carbon dioxide:zinc carbonate zinc oxide + carbon dioxide

Reactions like this can also be represented by balanced equations:

ZnCO3(s) ZnO(s) + CO2(g)

KEY POINTS There are similarities between ‘families’ of

compounds. For example, other carbonate compounds decompose on heating, and have similar equations for the reaction – but some need higher temperatures than others. For example, calcium carbonate is more stable than copper carbonate – it needs stronger heating to break it down.

Another kind of chemical reaction is precipitation. This happens when soluble substances react to form an insoluble product, called the precipitate. For example, silver nitrate and potassium bromide are soluble – their solutions react to form insoluble silver bromide:

silver nitrate(aq) + potassium bromide(aq) potassium nitrate(aq) silver bromide(s)

KEY POINTS C1.9 Reactions of calcium compounds When water is added to calcium oxide a chemical

reaction happens that produces a lot of heat. The solution formed is calcium hydroxide – also called limewater.

When carbon dioxide is bubbled through limewater, it turns cloudy – a precipitate of insoluble white calcium carbonate is formed. This reaction can be used as a test for carbon dioxide – bubble the suspected gas through some limewater, and if the solution turns cloudy then the gas is carbon dioxide.

Acids are neutralised by alkalis – this reaction is called neutralisation. Calcium carbonate, calcium oxide and calcium hydroxide all neutralise acids.

KEY POINTS Some crops do not grow well if the soil is too

acidic, so farmers sometimes need to reduce the acidity of their soil. They spray powdered calcium carbonate, calcium oxide or calcium hydroxide over their fields to do this.

Nitrogen oxides and sulphur dioxide are formed when coal burns – and coal-fired power stations burn a lot of coal. These are acidic gases that can produce acid rain if they escape from the chimneys. Calcium carbonate is sprayed into the chimneys and it neutralises the acidic gases, so reducing the chance of acid rain forming.

ACIDSTopic 3

KEY POINTS C1.10 Indigestion Your stomach produces hydrochloric acid to kill

bacteria – the acid also helps enzymes to digest the food you eat. If too much acid is produced, you can get indigestion.

Indigestion remedies (antacids) contain substances that neutralise the extra acid. Substances used include calcium carbonate and magnesium hydroxide.

A base is a substance that neutralises an acid to produce a salt and water:

acid + base salt + water A base that is soluble in water is called an alkali.

KEY POINTS You can describe acids and alkalis using

the pH scale, which runs from 0 (highly acidic) to 14 (highly alkaline). A neutral solution (such as water) has a pH of 7.

Indicators such as litmus show acidity (red) and alkalinity (blue) by changing colour. Universal indicator has different colours across the full pH range.

KEY POINTS C1.12 Neutralisation Acids can be neutralised by metal oxides, metal

hydroxides or metal carbonates. The general equations for these reactions are:

acid + metal oxide salt + wateracid + metal hydroxide salt + wateracid + metal carbonate salt + water +

carbon dioxide

When hydrochloric acid is neutralised it makes chloride salts, sulfuric acid makes sulphate salts and nitric acid makes nitrate salts.

KEY POINTS Substances have to be carried from one

place to another and this can be hazardous. Hazard symbols are used to show the dangers of the substance inside a container – this enables people to know what precautions to take when using the substance, and what to do if there is an accident.

KEY POINTS C1.14 The importance of chlorine Chlorine is a yellow-green gas that is toxic, so

there are always safety concerns when it is being made, transported or used.

Electrolysis is the process of decomposing compounds by passing a direct current through an electrolyte.

The electrolysis of sea water produces chlorine because it contains dissolved sodium chloride.

You can test a gas to see if it is chlorine by holding a piece of damp blue litmus paper in it – if the gas is chlorine, the litmus paper will first turn red and then white.

KEY POINTS Chlorine compounds are used to

disinfect swimming pools and drinking water. Chlorine is also used to make bleach and some plastics, such as PVC.

KEY POINTS C1.15 Electrolysis of water The electrolysis of acidified water

produces oxygen and hydrogen. You can test for hydrogen by putting a

lighted splint near the mouth of a test tube containing the gas – there will be a squeaky pop as it reacts with oxygen in the air.

You can test for oxygen by putting a glowing splint into a test tube containing the gas – the splint will burst into flames again.

OBTAINING AND USING METALS

TOPIC 4

KEY POINTS C1.16 Ores A few metals are found on Earth as

uncombined elements – these are unreactive metals such as gold and platinum. Most metals are extracted from ores dug out of the Earth. An ore is a rock containing compounds from which a metal can be extracted at a profit.

Low reactivity metals are extracted by heating their ores with carbon. Iron is an example of a metal that can be extracted from its ore (for example, haematite) in this way.

KEY POINTS High reactivity metals are extracted from their

ores by electrolysis – this is expensive because of the amount of energy it takes. An example of a metal that has to be extracted from its ore (bauxite) in this way is aluminium.

The way in which a metal is extracted depends on its reactivity − the more reactive a metal is, the harder it is to extract. The metals can be organised into a reactivity series, which lists metals in order of reactivity. The most reactive metals are at the top and these are the hardest to extract from their ores – and among the most expensive. Metals above zinc in the reactivity series are extracted using electrolysis.

KEY POINTS C1.18 Oxidation and reduction Oxidation is the addition of oxygen to an

element or compound; reduction is the removal of oxygen from a compound.

Most ores are oxides of metals. To extract the metal, the metal oxide must lose oxygen – the metal oxide must be reduced. The reduction can be done by heating with carbon or by electrolysis, depending on the reactivity of the metal.

Most metals corrode – the corrosion of iron is called rusting. Corrosion nearly always involves the reaction of the metal with oxygen in the air – this is oxidation.

KEY POINTS Metals high in the reactivity series

corrode very easily; metals lower down don’t – which explains why gold can be found uncombined in some places.

Using a coating of some kind (for example oil or paint) can slow down the rate of corrosion of a metal.

KEY POINTS C1.19 Recycling metals Most metals can be recycled. Used metals are

taken to scrap yards where iron and steel are separated from non-magnetic metals using magnets. New products can be made by melting down the old metals are remoulding them.

Three of the main advantages of recycling are:– reserves of metal ores will last longer– it reduces the use of energy– there is less damage to the landscape from mining.

There are some disadvantages including the cost and the energy used in collecting, sorting and transporting metals to be recycled.

KEY POINTS C1.20 Properties of metals Metals are good conductors of heat and

electricity. They are strong, hard and can be hammered into shape (malleable). They can also be stretched into wires (ductile).

There are many uses for metals – few examples are:– aluminium is used to make aircraft because it

has a low density– copper is used to make electrical cables because

it is a good electrical conductor and is ductile– copper is also used to make water pipes because

it is malleable and does not react with water

KEY POINTS – gold is used to make jewellery because it is

attractive and unreactive– steel is used in construction because it is

strong and relatively cheap.

FUELSTOPIC 5

KEY POINTS C1.22 Crude oil Crude oil is a mixture of different hydrocarbon

molecules. This is a thick black mixture trapped in some sedimentary rocks. It is called a fossil fuel because it was formed from the remains of marine plants and animals.

Hydrocarbons are compounds that are made of hydrogen and carbon atoms only. The hydrocarbon molecules in crude oil have different numbers of carbon atoms and hydrogen atoms.

Crude oil will run out one day and it being made much more slowly than we are using it – this means that it is a non-renewable energy resource.

KEY POINTS C1.23 Crude oil fractions Crude oil is a mixture of different

compounds, which have different boiling points. The mixture is split up into fractions by fractional distillation. Each fraction is also a mixture of different compounds, but many fewer compounds.

The fractions containing hydrocarbons with the shortest molecules have lower boiling points, lower viscosity (they are runnier) and are easier to ignite (set alight) than fractions containing larger molecules.

KEY POINTS Each fraction has different uses. Gases

are used for heating and cooking. Petrol is used as a fuel for cars. Kerosene is used as aircraft fuel. Diesel oil is used as a fuel for some cars and trains. Fuel oil is used by ships and is burned in some power stations. Bitumen is used to make roads and to make some roofs waterproof.

KEY POINTS C1.24 Combustion Combustion is an oxidation reaction.

When fuels burn in air they combine with oxygen. The products of the reaction are carbon dioxide and water – energy is also released.

Complete combustion occurs when there is plenty of oxygen for the reaction.

Carbon dioxide can be detected using limewater – when carbon dioxide bubbles through it, the limewater turns milky.

KEY POINTS C1.25 Incomplete combustion Incomplete combustion happens when there is

not enough oxygen to allow a fuel to burn completely. Incomplete combustion produces water, some carbon dioxide, some carbon monoxide and carbon particles – the carbon particles are also known as soot.

Carbon monoxide is a toxic gas – it reduces the amount of oxygen that a person’s blood can carry. Some faulty boilers produce carbon monoxide. Deaths from carbon monoxide poisoning can be reduced by having boilers serviced regularly, and by using carbon monoxide alarms.

KEY POINTS C1.26 Acid rain Rainwater always has some carbon dioxide

dissolved in it, which makes it slightly acidic. Other gases produced in power stations and car engines can also dissolve in water in the air – these gases include sulfur dioxide. They make the rain more acidic – rain that is more acidic than normal (less than about pH 5.2) is called acid rain.

Acid rain makes lakes and rivers acidic, which harms fish and other life. It also damages trees, buildings made of limestone or marble, and makes metal corrode.

KEY POINTS The amount of acid rain falling in Europe

and North America has been reduced by removing sulfur from car fuels and by removing sulfur dioxide from power station waste gases.

KEY POINTS C1.27 Climate change Carbon dioxide, methane and water vapour in

the atmosphere help to keep the Earth warm by trapping heat energy – this is called the greenhouse effect.

Some human activities put more of these gases into the air. Burning fossil fuels gives out carbon dioxide; cows and rice fields give out methane.

There is evidence that the increasing proportion of carbon dioxide in the atmosphere is causing a slow increase in temperature which is leading to climate change.

KEY POINTS Scientists could reduce the amount of

carbon dioxide in the atmosphere by adding iron to the oceans – this is called iron seeding. It encourages plankton to grow, which use up carbon dioxide as they photosynthesise.

Carbon dioxide can be removed from the atmosphere and used to make hydrocarbons. These could be used as a replacement for fossil fuels.

KEY POINTS C1.28 Biofuels Biofuels are fuels made from plant or animal waste.

Any plant material that is burned is a biofuel. They are alternatives to fossil fuels – but the best thing about them is that they are renewable fuels.

Ethanol is a biofuel made using sugar cane or sugar beet. It can be mixed with petrol for use in car engines. Using ethanol helps to reduce the demand for petrol, and so conserves crude oil supplies.

Biodiesel is made from vegetable oils like rapeseed oil and used cooking oil from restaurants. Ordinary diesel engines can run on biodiesel or on a mixture of biodiesel and normal diesel oil.

KEY POINTS Using biofuels may also help to reduce the overall

amount of carbon dioxide that human activity puts into the atmosphere. When plants photosynthesise, they use carbon dioxide from the air. When a biofuel burns, it releases carbon dioxide back into the atmosphere. If the burning fuel emits the same amount of carbon dioxide as the plants absorbed, it is carbon neutral.

However, energy is needed to make fertilisers to help the plants to grow, to harvest the crops and to make the biofuel. At the moment this energy comes from fossil fuels, so burning biofuels can add carbon dioxide to the atmosphere overall.

Another disadvantage is that growing crops to make into biofuels reduces the amount of land that can be used to grow food.

KEY POINTS C1.29 Choosing fuels Different fuels have different properties. A good fuel

will burn easily producing a lot of heat energy, and it will not produce much pollution. It will also be easy to store and to transport.

Hydrogen produces only water when it burns in air – it also releases a lot of energy per kilogram. However, hydrogen is more difficult to store than petrol or diesel because it is a gas that must be compressed. It is also potentially more dangerous.

Hydrogen and oxygen can also be combined in a fuel cell to produce electricity. Cars and buses can be powered by fuel cells. Before cars with fuel cells can become widely used, hydrogen has to be easily and economically available.

KEY POINTS Petrol, kerosene and diesel are all non-

renewable fossil fuels. Methane is a non-renewable fossil fuel found in natural gas.

Coal is a non-renewable fossil fuel that produces a lot of ash when it burns.

KEY POINTS C1.31 Alkanes and alkenes The forces of attraction holding atoms together

in a molecule are called bonds. Alkanes are hydrocarbons that have only single bonds between carbon atoms. They are called saturated molecules – methane, ethane and propane are all alkanes.

Alkenes are hydrocarbons that have at least one double bond between carbon atoms. They are unsaturated molecules – ethene and propene are alkenes.

A molecule of methane has one carbon atom and four hydrogen atoms – its formula is CH4. The formula of ethane is C2H6; propane is C3H8.

KEY POINTS A molecule of ethene has two carbon

atoms and four hydrogen atoms – its formula is C2H4; propene is C3H6.

Bromine water is used to test for alkenes. It is an orange colour – if it is mixed with an unsaturated hydrocarbon, it reacts and the mixture becomes colourless. There is no reaction if bromine water is mixed with a saturated hydrocarbon.

KEY POINTS C1.32 Cracking Most crude oils contain more large

hydrocarbon molecules than small ones – and small ones (like those in the petrol fraction) are more useful than larger ones.

Cracking is used to split up long hydrocarbon molecules into shorter ones. Cracking is a thermal decomposition reaction that produces a mixture of molecule lengths, and also a mixture of saturated and unsaturated hydrocarbons – alkanes and alkenes.

KEY POINTS Liquid paraffin can be cracked in the

laboratory. Mineral wool is soaked in paraffin and put into a boiling tube and is heated. The tube is held horizontally so that the paraffin vapour can pass over pieces of heated porous pot. The gases produced are collected over water.

KEY POINTS C1.33 Polymerisation Lots of small unsaturated molecules, such as

ethene, can be joined together to form very long molecules called a polymer – this is polymerisation. The repeating units are called monomers. The polymer made from ethene is called poly(ethene).

Some polymers occur naturally – for example proteins and cellulose. Manufactured polymers are often called plastics.

Poly(ethene) is flexible, cheap and a good insulator. It is used for making plastic bags, cling film and insulation for electrical wires.

Poly(propene) is flexible and tough, and has a higher melting point. It is used to make buckets and bowls.

KEY POINTS Poly(chloroethene) is also called PVC – it

is tough and cheap. It is used to make window frames, gutters and pipes.

PTFE is also called Teflon® – it is tough, slippery and resistant to corrosion. It is used to make non-stick pans, containers for corrosive chemicals and stain-proof carpets.

KEY POINTS C1.34 Problems with polymers Manufactured polymers are not biodegradable –

this means that they do not rot. This is a useful property because it means that things made from polymers last a long time. However, it also means that they do not decompose when they are thrown away – they will last for thousands of years in landfill sites.

There is a big landfill problem – there is too much of it. There are ways of reducing the problem. For example, plastic waste can be incinerated and the energy used to generate electricity. However, some polymers produce toxic gases when they burn and these must be removed from the waste gases.

KEY POINTS Another development is that scientists

are developing biodegradable polymers that will rot relatively quickly when they are thrown away.

A better way of tackling the landfill problem is to reduce the amount of waste that needs to be dealt with by reusing items when we can. When we have finished with an object it can be recycled. Plastic objects that can be recycled have a symbol stamped on them to show the type of polymer they are made from.