C1 - Chemistry

AQA - C1 Chemistry

Friday 15th June 2012

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1.1 - Atoms, elements and compounds

100 different elements - which all substances are made from.

Periodic table is a list of the elements.

Each element is made of 1 type of atom.

Atoms are represented by chemical symbols - e.g. Na - Sodium.

Elements in the periodic table are arranged in columns, called groups. Elements in a group usually have similar properties.

Atoms have a tiny nucleus surrounded by electrons.

Elements react, their atoms join with atoms of other elements. Compounds are formed when two or more elements combine together.

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1.2 - Atomic structure

The nucleus at the centre of an atom contains two types of particle - protons and neutrons. Protons = Positve charge. Neutrons = No charge.

Electrons are tiny negatively charged particles that move around the nucleus. An atom has no overall charge because the number of protons is equal to the number of electrons and their charges are equal and opposite (proton +1 and electron -1).

Atoms of and element contain the same number of protons. This number is called the atomic number (or proton number). Elements are arranged in order of their atomic numbers on the periodic table. The atomic number is also the number of electrons in an atom of the element.

Mass number = total number of particles in the nucleus of an atom, so it is the number of protons plus the number of neutrons.

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1.3 - The arrangement of electrons in atoms

Each electron is an atom is in an energy level. Energy levels are represented as shells, with electrons in the lowest energy level closest to the nucleus.

Lowest energy level - first shell can hold two electrons and the second energy level can hold eight. Electrons occupy the lowest possible energy levels. E.g. The electronic structure of neon with 10 electrons is 2,8.

Elements in the same group of the periodic table have the same number of electrons in their highest energy level - e.g. Group 1 elements have one electron in their highest energy level.

Group 1 elements include lithium, sodium and potassium -> which react quickly with water and oxygen.

The atoms of the unreactive noble gases (in Group 0) all have very stable arrangements of electrons.

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1.4 - Forming bonds

Different elements combine they form compounds.

A metal reacts with a non - metal, ions are formed. Metal atoms lose one or more electrons to form positively charged ions. Non - metal atoms gain electrons to form negatively charged ions. The oppositely charged ions attract each other strongly and the compound has ionic bonds.

The chemical formula of an ionic compound tells us the simplest ratio if ions in the compound. For example, NaCl shows that sodium chloride is made from equal number of sodium ions and chloride ions.

Non - metals combine, their atoms share electrons to form covalent bonds and molecules are formed.

Chemical formula of a molecule tells us the number of atoms that have bonded together in the molecule. For example, H20 shows that a water molecule contains two hydrogen atoms and one oxygen atom. Covalent bonds can be shown as lines between the atoms that are bonded together.

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1.5 - Chemical equations

In chemical reactions the atoms in the reactants re-arrange themselves to form new substances, the products.

Atoms are neither created nor destroyed in a chemical reaction. The number and type of atoms remains the same before and after the reaction.

The mass of the products equals the mass of the reactants.

We can write chemical equations to represent reactions.

Word equations only give the names of the reactants and products. Symbol equations show the numbers of the numbers and types of atoms in the reactants and products.

When symbol equations are written they should always balanced.

This means that the numbers of each type of atom should be the same on both sides of a symbol equation.

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2.1 - Limestone and its uses

We quarry large amounts of limestone rock because it has many uses.

Blocks of limestone can be used for building. Limestone is used to make calcium oxide and cement.

We make concrete by mixing cement with sand, aggregate and water.

Limestone is mainly calcium carbonate - CaCO3

When heated strongly, calcium carbonate decomposes to make calcium oxide and carbon dioxide which is done on a large scale in lime kilns. The equation for this reaction is:

CaCO3     ---------->     CaO     +     CO2

Calcium carbonate     ---------->     Calcium oxide     +     Carbon dioxide

This type of reaction is called thermal decomposition

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2.2 - Reactions of carbonates

All metal carbonates react in similar ways when heated or when reacted with acids.

Metal carbonates decompose to the metal oxide and carbon dioxide when they are heated strongly enough.

A Bunsen burner flame cannot get hot enough to decompose sodium carbonate or potassium carbonate.

All carbonates react with acids to produce a salt, water and carbon dioxide gas. Limestone is damaged by acid rain because the calcium carbonate in the limestone reacts with acids in the rain.

Calcium hydroxide solution is called limewater. Limewater is used to test for carbon dioxide. The limewater turns cloudy because it reacts with carbon dioxide to produce the insoluble calcium carbonate.

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2.3 - The 'limestone reaction cycle'

When heated strongly the calcium carbonate in limestone decomposes to calcium oxide and carbon dioxide.

Water is added to calcium oxide and they react to produce calcium hydroxide.

Calcium hydroxide is an alkali. It can be used to neutralise acids. E.g. It is used by farmers to neutralise acidic soils and in industry to neutralise acidic gases.

Calcium hydroxide is not very soluble in water but dissolves slightly to make limewater.

Calcium hydroxide reacts with carbon dioxide to produce calcium carbonate - the main compound in limestone.

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2.4 - Cement and Concrete

To make cement, limestone is mixed with clay and heated strongly in a kiln. The product is ground up to make a fine powder.

Cement is mixed with sand and water to make mortar. Mortar is used to hold bricks and blocks together in buildings.

Concrete is made by adding aggregate to cement, sand and water. Small stones or crushed rock are used as aggregate. The mixture can be poured into moulds before it sets to form a hard solid.

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2.5 - Limestone issues

We depend on limestone to provide building materials. Cement and concrete are needed in most buildings.

Quarrying limestone can have negative impacts on the environment and on people living near to the quarries.

Cement works are often close to limestone quarries. Making cement involves heating limestone with clay in large kilns. This uses a large area of land and a lot of energy.

Advantages for an area in which limestone is to be quarried: More employment opportunities for local people, more customers and trade for local businesses and improved roads.

Disadvantages for an area in which limestone is to be quarried: Dust and noise, more traffic and loss of habitats for wildlife.

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3.1 - Extracting metals

Rock that contains enough of a metal or a metal compound to make it worth extracting the metal is called an ore.

Mining ores often involves digging up large amounts of rock. The ore may need to be concentrated before the metal is extracted. These processes can produce large amounts of waste and may have major impacts on the environment.

A few unreactive metals, low in the reactivity series, such as gold are found in the Earth as the metal. Gold can be separated from rocks by physical methods. Most metals are found as compounds - so the metals have to be extracted by chemical reactions.

Metals can be extracted from compounds by displacement using a more reactive element. Metals which are less reactive than carbon can be extracted from their oxides by heating with carbon. A reduction reaction  takes place as carbon removes the oxygen from the oxide to produce the metal. This method is used commercially if possible.

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3.2 - Iron and Steels

Many of the ores used to produce iron contain iron (III). Iron (III) oxide is reduced at high temperatures in a blast furnace using carbon. The iron produced contains about 96%  iron. The impurities make it hard and brittle and so it has only a few uses as cast iron. Removing all of the carbon and other impurities makes pure iron, but this is too soft for many uses.

Most iron is used to make steels. Steels are alloys of iron because they are mixtures of iron with carbon and other elements. Alloys can be made so that they have properties for specific uses.

The amounts of carbon and other elements are carefully adjusted when making steels. Low carbon steels are easily shaped and high-carbon steels are hard.

Some steels, such as stainless steels, contain larger quantities of other metals. They resist corrosion.

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3.3 - Aluminium and titanium

Aluminium has a low denstiy although it is quite high in the reactivity series. It is resistant to corrosion.

Aluminium is more reactive than carbon and so its oxide cannot be reduced using carbon. It has to be extracted by electrolysis of molten aluminium oxide. The process require high temperature and a lot of electrcity. This makes aluminium expensive to extract.

Pure aluminium is not very strong but aluminium alloys are stronger and harder. They have many uses.

Titanium is resistant to corrosion and very strong. It has low densitry compared with other strong metals.

Titanium oxide can be reduced by carbon, But the metal reacts with carbon making it brittle.

Titanium is extracted from its ore by a process that involves several stages and large amounts of energy. The high costs of the process make titanium expensive.

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3.4 - Extracting copper

Copper can be extracted from copper-rich ores by smelting. This means heating the ore strongly in a furnace.

Smelting produces impure copper, which can be purified by electrolysis.

Smelting and purifying copper ore require huge amounts of heating and electricity.

Copper-rich ores are a limited resource. Scientists are developing new ways of extracting copper from low-grade ores. These methods can have less environmental impact than smelting.

Phytomining uses plants to absorb copper compounds from the ground. The plants are burned and produce ash from which copper can be extracted.

Bioleaching uses bacteria to produce solutions containing copper compounds.

Solutions of copper compounds can be reacted with a metal that is more reactive than copper, such as scrap iron, to displace the copper.

Copper can also be extracted from solutions of copper compounds by electrolysis.

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3.5 - Useful metals

Elements from the central block of the periodic table are known as the transition metals.

They are all metals and have similar properties.

They are good conductors of heat and electricity.

Many of them are strong, but can be bent or hammered into shape. These properties make them useful as materials for buildings, vehicles, containers, pipes and wires.

Copper is a very good conductor of heat and does not react with water, it can be bent but it is hard enough to keep its shape. These properties make it useful for making pipes and tanks in water and heating systems. It is a very good conductor of electricity as well and so it is used for electrical wiring.

Most of the metals we use are not pure elements.

Pure iron, copper, gold and aluminium are soft and easily bent. They are often mixed with other metals to make alloys that are harder so that they keep their shape.

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Iron is made into steels.

Gold used for jewellery is usually an alloy.

Most of the aluminium used for buildings and aircraft is alloyed.

Copper alloys include bronze and brass.

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3.6 - Metallic issues

Mining for metal ores involves digging up and processing large amounts of rock. This can produce large amounts of waste material and effect large areas of the environment.

Recycling metals saves the energy needed to extract the metal. Recycling saves resources because less ore needs to be mined. Also, less fossil fuel is needed to provide the energy to extract the metal from its ore.

The benefits of using metals in construction should be carefully considered against the drawbacks.

Some benefits of using metals in construction: They are strong, they can be bent into shape, they can be made into flexible wires and they are good electrical conductors.

Some drawbacks of using metals in construction: Obtaining metals from ores causes pollution and uses up limited resources, metals are more expensive than other materials such as concrete and iron and steel can rust.

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4.1 - Fuels from crude oil

Crude oil contains many different compounds that boil at different temperatures. These burn under different conditions and so crude oil needs to be separated to make useful fuels.

We can separate a mixture of liquids by distillation. Simple distillation of crude oil can produce liquids that boil within different temperature ranges. These liquids are called fractions.

Most of the compounds in crude oil are hydrocarbons. This means that their molecules contain only hydrogen and carbon. Many of these hydrocarbons are alkanes, with the general formula CnH2n+2. Alkanes contain as many hydrogen atoms as possible in each molecule and so we call them saturated hydrocarbons.

We can represent molecules in different ways. A molecular formula shows the number of each type of atom in each molecule, e.g. C2H6 represents a molecule of the atoms are bonded together.

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4.2 - Fractional distillation

Crude oil is separated into fractions at refineries using fractional distillation. This can be done because the boiling point of a hydrocarbon depends on the size of its molecule. The larger the molecule, the higher the boiling point of the hydrocarbon.

The crude oil is vapourised and fed into a fractionating column. This is a tall tower that is hot at the bottom and gets cooler going up the column.

Inside the column there are many trays which holes to allow gases through. The vapours move up the column getting cooler as they go up. The hydrocarbons condense to liquids when they reach the level that is at their boiling point. Different liquids collect on the trays at different levels and there are outlets to collect the fractions.

Hydrocarbons with the smallest molecules have the lowest boiling points and are collected at the top of the column. The fractions collected at the bottom of the column contain hydrocarbons with the highest boiling points.

Fractions with low boiling ranges have low viscosity so they are runny liquids, very flammable so they ignite easily. They burn with clean flames, producing little smoke. This makes them very useful as fuels.

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4.3 - Burning fuels

When pure hydrocarbons burn completely they are oxidised to carbon dioxide and water. The fuels we use are not always burned completely. They may also contain other substances.

In a limited supply of incomplete combustion may produce carbon monoxide. Carbon may also be produced and some of the hydrocarbons may not burn. This produces solid particles that contain soot (carbon) and unburnt hydrocarbons called particulates.

Most fossil fuels contain sulfur compounds. When the fuel burns these sulfur compounds produce sulfur dioxide. Sulfur dioxide causes acid rain.

At high temperatures produced when fuels burn, oxygen and nitrogen in the air may combine to form nitrogen oxides. Nitrogen oxides also cause acid rain.

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4.4 - Cleaner fuels

We  burn large amounts of fuels and this releases substances that spread throughout the atmosphere and effect the environment.

Burning any fuel that contains carbon produces carbon dioxide. Carbon dioxide is a greenhouse gas that many scientists believe is the cause of global warming. Incomplete combustion of these fuels produces the poisonous gas carbon monoxide. It can also produce tiny solid particulates that reflect sunlight and so cause global dimming.

Burning fuels also produces sulfur dioxide and nitrogen oxides. These gases dissolve in water droplets and react and react with oxygen in the air to produce acid rain.

We can remove harmful substances from waste gases before they are released into the atmosphere. Sulfur dioxide is removed from the waste gases from power stations. Exhaust systems of cars are fitted with catalytic converters to remove carbon monoxide and nitrogen oxides. Filters can remove particulates.

Sulfur can be removed from fuels before they are supplied to users so that less sulfur dioxide is produced when the fuel is burned.

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4.5 - Alternative fuels

Biofuels are made from plant or animal products and are renewable. Biodiesel can be made from vegetable oils extracted from plants.

There are advantage to using biodiesel. For example, it makes little contribution to carbon dioxide levels because the carbon dioxide given off when it burns was taken from the atmosphere by plants as they grew.

There are also disadvantages, for example the plants that are grown for biodiesel use large areas of farmland.

Ethanol made from sugar cane or sugar beat is a biofuel. It is a liquid and so can be stored and distributed like other liquid fuels. It can be mixed with petrol.

HOW SCIENCE WORKS:

Using hydrogen as a fuel has the advantage that it produces only water when it is burned.

It is a gas so it takes up a large volume. That makes it difficult to store in the quantities needed for combustion in engines.

It can be produced from water by electrolysis but this requires large amounts of energy.

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5.1 - Cracking hydrocarbons

Large hydrocarbon molecules can be broken down into smaller molecules by a process called cracking.

Cracking can be done in two ways:

1. By heating a mixture of hydrocarbon vapours and steam to a very high temperature

2. By passing hydrocarbon vapours over a hot catalyst

During cracking thermal decomposition reactions produce a mixture of smaller molecules. Some of the smaller molecules are alkanes, which are saturated hydrocarbons with the general formula CnH2n+2. These alkanes with smaller molecules are more useful as fuels.

Some of the other smaller molecules formed are hydrocarbons with the general formula CnH2n. These are called alkenes. Alkenes are unsaturated hydrocarbons because they contain fewer hydrogen atoms than alkanes with the same number of carbon atoms.

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              800oC + catalyst

C10H22  -------------------->  C5H12 + C3H6 + C2H4

Decane ---------------------> pentane + propene + ethene

An example of a cracking reaction.

Alkenes have a double bond between two carbon atoms and this make them more reactive than alkanes. Alkenes react with bromine water turning it from orange to colourless.

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5.2 - Making polymers from alkenes

Plastics are made of very large molecules called polymers. Polymers are made from many small molecules joined together. The small molecules used to make polymers are called monomers. The reaction to make a polymer is called polymerisation.

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Loss of ethene (C2H4) molecules join together to form poly(ethene), commonly called polythene. In the polymerisation reaction the double bond in each ethene molecule becomes a single bond and thousands of ethene molecules join together in long chains.

n  C = C     ---------->     ---- C -- C ----

  where n is a large number

Other alkenes can polymerise in a similar way. E.g. propene (C3H6) can form poly(propene).

Many of the plastics we use as bags, bottles, containers and toys are made from alkenes.

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5.3 - New and useful polymers

Materials scientists can desingn new polymers to make materials with special properties for particular uses. Many of these materials are used for packaging clothing and medical applications.

New polymer materials for dental fillings have been developed to replace fillings that contain mercury. Light-sensitive polymers are used in sticking plasters to cover wounds so the plasters can be easily removed. Hydrogeis are polymers that can trap water and have many used including dressingsfor wounds.

Shape-memory polymers change back to their original shape when temperature or other conditions are changed. An example of this type of smart polymer is a material used for stiching wounds that changes shape when heated to body temperature.

The fibres used to make fabrics can be coated with polymers to make them waterproof and breathable.

The plastic used to make many drinks bottles can be recycled to make polyester fibres for clothing as well as filling pillows and duvets.

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5.4 - Plastic waste

Many polymers are no biodegradable. This means that plastic waste is not broken down when left in the environment. Unless disposed of properly, plastic rubbish gets everywhere. It is unsightly and can harm wildlife. Even when put into landfill sites it takes up valuable space.

We are using more plastics that are biodegradable. Microorganisms can break down biodegradable plastics. These plastics break down when in contact with soil. 

Plastics made from non-biodegradable polymers can have cornstarch mixed into the plastic. Microorganisms break down the cornstarch so the plastic breaks down into very small pieces that can be mixed with soil or compost.

Biodegradable plastics can be made from plant material. One example is a polymer made from cornstarch that is used as biodegradable food packaging.

Some plastics can be recycled but there are many different types of plastic and sorting is difficult.

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5.5 - Ethanol

Ethanol has the formula C2H6O. It is often written C2H5OH. This shows the OH group in the molecule and that means it is an alcohol.

Ethanol can be produced by the fermentation of sugar from plants using yeast.

Enzymes in the yeast cause the sugar to convert to ethanol and carbon dioxide. This method is used to make alcoholic drinks.

Ethanol can also be made by the hydration of ethene.

Ethene is reacted with steam at a high temperature in the presence of a catalyst. The ethene is obtained from crude oil by cracking.

Ethanol produced by fermentation uses a renewable resource, sugar from plants.

Fermentation is done at room temperature. However, fermentation can only produce dilute aqueous solution of ethanol. The ethanol must be separated from the solution by fractional distillation to give pure ethanol.

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Ethanol produced from ethene uses a non-renewable resource, crude oil.

The reaction can be run continuously and produces pure ethanol, but requires a high temperature.

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6.1 - Extracting vegetable oil

Some seeds, nuts and fruits are rich in vegetable oils. The oils can be extracted by crushing and pressing the plant material, followed by removing water and other impurities. Some oils are extracted by distilling the plant material mixed with water. This produces a mixture of oil and water from which the oil can be seperated.

When eaten, vegetable oils provide us with a lot of energy and important nutrients. Vegetable oils release a lot of energy when they burn in air and so can be used as fuels. They are used to make biofuels such as biodiseal.

The molecules in vegetable oils have hydrocarbon chains. Those with carbon-carbon double bonds (C=C) re unsaturated. If there are several double bonds in each molecule, they are called polyunsaturated. Unsaturated oils react with bromine water, turning it orange to colourless.

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6.2 - Cooking with vegetable oils

The boiling points of vegetable oils = higher than water. So food is cooked at higher temperatures in oil. This means it cooks faster, It also changes the flavour, colour and texture of the food. Some of the oil is absorbed and so the energy content of the food increases.

Unsaturated oils can be reacted with hydrogen so that some or all of the carbon-carbon double bonds become single bonds. This reaction is called hydrogenation and is done at about 60oC using a nickel catalyst. The hydrogenated oils have higher melting points because they are more saturated. This reaction is also called hardening because the hydrogenated oils are solids at room temperature. This means they can be used as spreads and to make pastries and cakes that require solid fats.

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6.3 - Everyday emulsions

Oil and water do not mix and usually separate from each other, forming two layers. I f we shake, stir or beat the liquids together tiny droplets form that can be slow to separate, This type of mixture is called an emulsion.

Emulsions are opaque and thicker than the oil and water they are made from. This improves their texture, appearance and their ability to coat and stick to solids. Milk, cream, salad dressings and ice cream are examples of emulsions. Some water-based paints and many cosmetic creams are also emulsions.

Emulsifiers are substances that help stop the oil and water from seperating into layers. Most emulsions contain emulsifiers to keep the emulsion stable.

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6.4 - Food issues

There are benefits and drawbacks to using vegetable oils and emulsifiers in foods.

Vegetable oils are high in energy and contain important nutrients, They contain unsaturated fats that are believed to be better for your health than saturated fats.

Animal fats and hydrogenated vegetable oils contain saturated fats and are used in many foods. Saturated fats have been linked to heart disease.

Emulsifiers stop oil and water separating into layers. This makes foods smoother, creamier and more palatable. However, because they taste better and it is less obvious that they are high in fat, you may be tempted to eat more.

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7.1 - Structure of the Earth

The Earth is almost spherical. Diameter of about 12 800km. At the surface is a thin, solid crust. Crust = a very thin layer that varies in thickness between about 5 km and 70km.

The mantle is under the crust. About 3000km thick. It goes almost halfway to the centre of the Earth. The mantle is almost entirely solid but parts of it can flow very slowly.

The core is about about half the diameter of the Earth. It has a high proportion of the magnetic metals iron and nickel. It has a liquid outer part and a solid inner part.

The atmosphere surrounds the Earth. Most of the air within 10km of the surface and most of the atmosphere is within 100km of the surface.

All raw materials and other resources that we depend on come from the crust, the oceans and the atmosphere. The resources available to us are limited.

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7.2 - The restless Earth

Scientists now believe the Earth's crust and upper part of the mantle is cracked into massive pieces called tectonic plates. Tectonic plates move a few centimetres each year because of convection currents in the mantle beneath them. The convection currents are caused by energy released by the decay of radioactive elements heating up the mantle.

Where the plates meet, huge forces build up. Eventually the rocks give way, changing shape or moving suddenly causing earthquakes, volcanoes or mountains to form. Scientists till do not know enough about what is happening inside the Earth to predict exactly when and where earthquakes or volcanic eruptions will happen.

Alfred Wegener put forward the idea of continental drift in 1915. Other scientists at that time did not accept his ideas, mainly because he could not explain why the continents moved. They believed that the Earth was shrinking as it cooled . In the 1960's scientists found new evidence and the theory of plate tectonics was developed.

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7.3 - The Earth's atmosphere in the past

Scientists think that the Earth was formed about 4.5 billion years ago. In the first billion years the surface was covered with volcanoes that released carbon dioxide, water vapour and nitrogen.

As the Earth cooled most of the vapour condensed to form the oceans. So the early atmosphere was mainly carbon dioxide with some water vapour. Some scientists believe there was also nitrogen and possibly some methane and ammonia.

In the next two billion years bacteria, algae and plants evolved. Algae and plants used carbon dioxide for photosynthesis and this released oxygen. As the number of plants increased the amount of carbon dioxide in the atmosphere decreased and the amount of oxygen increased.

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7.4 - Life on Earth

The plants that produced the oxygen in the atmosphere probably evolved from simple organisms like plankton and algae in the ancient oceans. But we do not know how the molecules of the simplest things were formed. Many scientists have suggested theories of how life began but no one knows for sure because we have insufficient evidence.

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7.5 - Gases in the atmosphere

Plants took up much of the carbon dioxide in the Earth's early atmosphere. Animals ate the plants and much of the carbon ended up in the plant and animal remains as sedimentary rocks and fossil fuels. Limestone was formed from the shells and skeletons of marine organisms. Fossil fuels contain carbon and hydrogen from plants and animals.

Carbon dioxide dissolves in the oceans and some probably formed insoluble carbonate compounds that were deposited on the seabed and became sedimentary rocks.

By 200 million years ago the proportions of gases in the atmosphere has stabilised and were much the same as today. The atmosphere is now almost four-fifths nitrogen and just over one-fifth oxygen. Other gases, including carbon dioxide, water vapour and noble gases, make up about 1% of the atmosphere.

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7.6 - Carbon dioxide in the atmosphere

For about 200 million years the amount of carbon dioxide in the atmosphere has remained about the same.

This is because various natural processes that move carbon dioxide into and out of the atmosphere had achieved a balance.

These processes involve carbon compounds in plants, animals, the oceans and rocks. The organic carbon cycle shows the some of these processes.

Carbon dioxide dissolves in water, particularly the oceans, and reactions of inorganic carbonate compounds are also important in maintaining a balance.

In the recent past the amount of carbon dioxide that human activity has released into the atmosphere has increased dramatically. This has been mainly caused by the large increase in the amount of fossil fuels that we burn.

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Comments

Eleanor

very useful!:)x

Kimberley

Thanks

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