C1A AQA CHEMISTRY
Early ideas about atoms
The word atom comes from atomos, an ancient Greek word meaning
indivisible. The Greek philosopher Democritus (460-370 BCE)
maintained that all matter could be divided and sub-divided into
smaller and smaller units, and eventually there would be a tiny
particle that could not be divided any further - an atom. This was
remarkable because there was no way ancient Greeks could support
this theory by observation or experiment.
John Dalton
John Dalton (1766-1844)
Understanding of atoms didn’t progress much beyond Democritus’
theory until the English chemist John Dalton (1766-1844) started to
look at it in the 1800s. Dalton did experiments, worked out some
atomic weights, and invented symbols for atoms and molecules. His
most important conclusions are summarised below.
Dalton's theories about atoms took a long time to be accepted by
scientists. Some of his ideas about gases were incorrect, and it was
difficult for many years to do the experiments needed to support his
theories, because atoms are too small to see.
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Atoms and elements
Although the word 'atom' comes from the Greek for indivisible, we now
know that atoms are not the smallest particles of matter. Instead, they
have a small central nucleus surrounded by even smaller particles
called electrons.
The structure of the atom
All substances are made from atoms. And, as Dalton suggested, any
given element is made of atoms of just one particular sort. The atoms
of any element are different from the atoms of any other element. So
iron contains a different sort of atoms from those of sulphur, and the
atoms in carbon are different from those of oxygen.
Chemical symbols
the atoms of each element are represented by chemical symbols.
These usually consist of one or two different letters, but sometimes
three letters are used for newly-discovered elements. The first letter in
a chemical symbol is always an UPPERCASE letter, and the other
letters are always lowercase. So, the symbol for magnesium is Mg and
not mg, MG or mG.
Every element has its own chemical symbol. For example, iron is Fe,
sulphur is S, sodium is Na and oxygen is O.
The periodic table
There are more than 100 different elements. The periodic table is a
chart showing all the elements arranged in a particular way. The
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vertical columns in the periodic table are called groups. Each group
contains elements that have similar properties.
The periodic table
The periodic table has eight main groups. For example, group 1
contains very reactive metals such as sodium - Na - while group 7
contains very reactive non-metals such as chlorine - Cl.
Note that you will never find a compound in the periodic table,
because these consist of two or more different elements joined
together by chemical bonds.
Reactions and compounds
New substances are formed by chemical reactions. When elements
react together to form compounds their atoms join to other atoms
using chemical bonds. For example, iron and sulphur (often spelt
'sulphur') react together to form a compound called iron sulphide
(often spelt 'sulphide'), and sodium and oxygen react together to form
sodium oxide.
Chemical bonds involve electrons from the reacting atoms. Bonds can
form when:
•
•
electrons are transferred from one atom to another, so that one
atom gives electrons and the other takes electrons, or
Electrons are shared between two atoms.
Chemical formulae
The chemical formula of a compound shows how many of each type of
atom join together to make the units that make the compound up. For
example, in iron sulphide every iron atom is joined to one sulphur
atom, so we show its formula as FeS. In sodium oxide, there are two
sodium atoms for every oxygen atom, so we show its formula as
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,Na2O. Notice that the 2 is written as a subscript, so Na2O would be
wrong.
The diagram below shows that one carbon atom and two oxygen
atoms combine to make up the units of carbon dioxide - its chemical
formula should therefore be written as CO2.
Carbon dioxide units contain one carbon atom and two oxygen atoms
Sometimes you see more complex formulae such as Na2SO4 and
Fe(OH)3:
•
•
A unit of Na2SO4 contains two sodium atoms, one sulphur atom
and four oxygen atoms joined together.
A unit of Fe(OH)3 contains one iron atom, three oxygen atoms
and three hydrogen atoms (the brackets show that the 3 applies
to O and H).
Equations
When elements are joined to cause a chemical reaction, no atoms are
made or lost during the process - but at the end of it they are joined
differently from the way they were at the start. This means that the
mass of the substances at the start - the reactants - is the same as
the mass of the substances at the end - the products.
Copper and oxygen reaction - getting a balanced equation
We use balanced equations to show what happens to the different
atoms in reactions. For example, copper and oxygen react together to
make copper oxide.
Take a look at the word equation for the reaction, here:
copper + oxygen
copper oxide
You can see that copper and oxygen are the reactants, and copper
oxide is the product.
If we just replace the words shown above by the correct chemical
formulae, we will get an unbalanced equation, as shown here:
Cu + O2
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CuO
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Notice that we have unequal numbers of each type of atom on the lefthand side compared with the right-hand side. To make things equal,
we need to adjust the number of units of some of the substances until
we get equal numbers of each type of atom on both sides of the arrow.
Here is the balanced symbol equation:
2Cu + O2
2CuO
You can see that now we have two copper atoms and two oxygen
atoms on each side. This matches what happens in the reaction.
Two atoms of copper react with two atoms of oxygen to form two units of
copper oxide
Fuels from crude oil
Crude oil is a mixture of compounds called hydrocarbons. Many
useful materials can be produced from crude oil. It can be separated
into different fractions using fractional distillation, and some of these
can be used as fuels. Unfortunately, there are environmental
consequences when fossil fuels such as crude oil and its products are
used.
Hydrocarbons and alkanes
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons. This means
that they only contain hydrogen and carbon atoms, joined together by
chemical bonds. There are different types of hydrocarbon, but most of
the ones in crude oil are alkanes.
Alkanes
The alkanes are a family of hydrocarbons that share the same general
formula. This is:
CnH2n+2
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The general formula means that the number of hydrogen atoms in an
alkane is double the number of carbon atoms, plus two. For example,
methane is CH4 and ethane is C2H6. Alkane molecules can be
represented by displayed formulae in which each atom is shown as its
symbol (C or H) and the chemical bonds between them by a straight
line.
Notice that the molecular models on the right show that the bonds are
not really at 90°
Alkanes are saturated hydrocarbons. This means that their carbon
atoms are joined to each other by single bonds. This makes them
relatively unreactive, apart from their reaction with oxygen in the air,
which we call burning or combustion.
Boiling point and state at room temperature
Hydrocarbons have different boiling points, and can be either solid,
liquid or gas at room temperature:
•
•
•
Small hydrocarbons with only a few carbon atoms have low
boiling points and are gases.
Hydrocarbons with between five and 12 carbon atoms are
usually liquids.
Large hydrocarbons with many carbon atoms have high boiling
points and are solids.
Distillation
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Distillation is a process that can be used to separate a pure liquid
from a mixture of liquids. It works when the liquids have different
boiling points. Distillation is commonly used to separate ethanol (the
alcohol in alcoholic drinks) from water.
The mixture is heated in a flask. Ethanol has a lower boiling point
than water so it evaporates first. The ethanol vapour is then cooled
and condensed inside the condenser to form a pure liquid. The
thermometer shows the boiling point of the pure ethanol liquid. When
all the ethanol has evaporated from the solution, the temperature
rises and the water evaporates.
This is the sequence of events in distillation:
heating
evaporating
cooling
condensing
Fractional distillation
Fractional distillation differs from distillation only in that it separates
a mixture into a number of different parts, called fractions. A tall
column is fitted above the mixture, with several condensers coming off
at different heights. The column is hot at the bottom and cool at the
top. Substances with high boiling points condense at the bottom and
substances with low boiling points condense at the top. Like
distillation, fractional distillation works because the different
substances in the mixture have different boiling points.
Fractional distillation of crude oil
Because they have different boiling points, the substances in crude oil
can be separated using fractional distillation. The crude oil is
evaporated and its vapours allowed to condense at different
temperatures in the fractionating column. Each fraction contains
hydrocarbon molecules with a similar number of carbon atoms.
Oil fractions
The diagram below summarises the main fractions from crude oil and
their uses, and the trends in properties. Note that the gases condense
at the top of the column, the liquids in the middle and the solids stay
at the bottom.
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The main fractions include refinery gases, gasoline (petrol), naphtha,
kerosene, diesel oil, fuel oil, and a residue that contains bitumen.
These fractions are mainly used as fuels, although they do have other
uses too.
Hydrocarbons with small molecules make better fuels than
hydrocarbons with large molecules because they are volatile, flow
easily and are easily ignited.
Combustion of fuels
Complete combustion
Fuels burn when they react with oxygen in the air. The hydrogen in
hydrocarbons is oxidised to water (remember that water, H2O, is an
oxide of hydrogen). If there is plenty of air, we get complete
combustion and the carbon in hydrocarbons is oxidised to carbon
dioxide:
hydrocarbon + oxygen
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water + carbon dioxide
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Incomplete combustion
if there is insufficient air for complete combustion, we get incomplete
combustion instead. The hydrogen is still oxidised to water, but
instead of carbon dioxide we get carbon monoxide. Particles of carbon,
seen as soot or smoke, are also released.
Sulphur
Most hydrocarbon fuels naturally contain some sulphur compounds.
When the fuel burns, the sulphur it contains is oxidised to sulphur
dioxide.
Summary
the combustion of a fuel may release several gases into the
atmosphere, including:
•
•
•
•
•
water vapour
carbon dioxide
carbon monoxide
particles
sulphur dioxide
These products may be harmful to the environment.
Clouds of smoke and other combustion products are emitted from
chimneys
Sulphur dioxide
Sulphur dioxide is produced when fuels that contain sulphur
compounds burn. It is a gas with a sharp, choking smell. When
sulphur dioxide dissolves in water droplets in clouds, it makes the
rain more acidic than normal. This is called acid rain.
Effects of acid rain
Acid rain reacts with metals and rocks such as limestone. Buildings
and statues are damaged as a result. Acid rain damages the waxy
layer on the leaves of trees and makes it more difficult for trees to
absorb the minerals they need for healthy growth. They may die as a
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result. Acid rain also makes rivers and lakes too acidic for some
aquatic life to survive.
Reducing acid rain
Sulphur dioxide can be removed from waste gases after combustion of
the fuel. This happens in power stations. The sulphur dioxide is
treated with powdered limestone to form calcium sulphate. This can
be used to make plasterboard for lining interior walls, so turning a
harmful product into a useful one.
Sulphur can be removed from fuels at the oil refinery. This makes the
fuel more expensive to produce, but it prevents sulphur dioxide being
produced. You may have noticed ‘low sulphur’ petrol and diesel on
sale at filling stations.
Global warming
Carbon dioxide from burning fuels causes global warming, a process
capable of changing the world’s climate significantly.
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As you can see from the graphs, the amount of carbon dioxide in the
atmosphere has increased steadily over the past 150 years, and so
has the average global temperature.
Carbon dioxide is a greenhouse gas. It absorbs heat energy and
prevents it escaping from the Earth’s surface into space. The greater
the amount of carbon dioxide in the atmosphere, the more heat energy
is absorbed and the hotter the Earth becomes.
Effects of global warming
A rise of just a few degrees in world temperatures will have a dramatic
impact on the climate:
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Global weather patterns will change, causing drought in some
places and flooding in others.
Melting of polar ice caps will raise sea levels, causing increased
coastal erosion and flooding of low-lying land – including land
where major cities lie.
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Global dimming
Tiny particles that are released when fuels are burned cause global
dimming. Like global warming, this process may change rainfall
patterns around the world.
The amount of sunlight reaching the Earth’s surface has decreased by
about 2 per cent every ten years, because more sunlight is being
reflected back into space. The particles from burning fuels reflect
sunlight, and they also cause more water droplets to form in the
clouds. This makes the clouds better at reflecting sunlight back into
space.
It is likely that global dimming has hidden some of the effects of global
warming, by stopping some of the Sun’s energy reaching the Earth’s
surface in the first place. Governments around the world are
introducing controls on pollution. There is the possibility that as the
air becomes less polluted by smoke and soot, global dimming will
decrease, causing the effects of global warming to become more
obvious.
Limestone
Limestone is mainly calcium carbonate, CaCO3. When it is heated, it
breaks down to form calcium oxide and carbon dioxide. Calcium oxide
reacts with water to produce calcium hydroxide.
Limestone and its products have many uses, including being used to
make mortar, cement, concrete and glass
Thermal decomposition
Metal carbonates such as calcium carbonate break down when heated
strongly. This is called thermal decomposition. Here are the equations
for the thermal decomposition of calcium carbonate:
calcium carbonate
CaCO3
heat
heat
calcium oxide + carbon dioxide
CaO + CO2
Other metal carbonates decompose in the same way. Here are the
equations for the thermal decomposition of copper carbonate:
copper carbonate
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heat
copper oxide + carbon dioxide
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CuCO3
heat
CuO + CO2
Notice that in both examples the products are a metal oxide and
carbon dioxide. The carbon dioxide gas can be detected using
limewater. Limewater turns cloudy white when carbon dioxide is
bubbled through it.
Metals high up in the reactivity series - such as calcium - have
carbonates that need a lot of energy to decompose them. Metals low
down in the reactivity series - such as copper - have carbonates that
are easily decomposed. This is why copper carbonate is often used at
school to show these reactions. It is easily decomposed, and its colour
change, from green copper carbonate to black copper oxide, is easy to
see
Copper carbonate decomposes to form copper oxide and carbon
dioxide when heated
Quicklime and slaked lime
For your exam, you need to know how quicklime and slaked lime are
obtained from limestone.
Making quicklime
If limestone is heated strongly, it breaks down to form calcium oxide
and carbon dioxide. Calcium oxide is also called quicklime. It is yellow
when hot, but white when cold.
Here are the equations for this reaction:
calcium carbonate heat calcium oxide + carbon dioxide
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CaCO3
heat
CaO + CO2
This is a thermal decomposition reaction.
Quicklime (calcium oxide)
Making slaked lime
Calcium oxide reacts with water to form calcium hydroxide, also called
slaked lime.
Here are the equations for this reaction:
calcium oxide + water
CaO + H2O
calcium hydroxide
Ca(OH)2
A lot of heat is produced in the reaction, which may even cause the
water to boil.
Slaked lime (calcium hydroxide)
Summary
Using common names instead of chemical names, this is what
happens:
limestone
heat
quicklime + carbon dioxide
quicklime + water
slaked lime
Uses of limestone
Limestone, quicklime and slaked lime are all used to neutralise excess
acidity - which may be caused by acid rain - in lakes and in soils.
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Limestone is used as a building material, and to purify iron in blast
furnaces. It's also used in the manufacture of glass, and of cement
(one of the components of concrete).
The flow chart below summarises the main uses of limestone and its
products.
Glass
Glass is made by melting sand and then cooling it. Flat sheets of glass
for windows are made by floating molten glass on a layer of molten tin.
Glass manufacturers add sodium carbonate to sand during the
manufacturing process, to reduce the melting temperature of the sand
and so save energy. The sodium carbonate decomposes in the heat to
form sodium oxide and carbon dioxide, but this makes the glass
soluble in water. Calcium carbonate (limestone) is therefore also
added, to stop the glass dissolving in water. The calcium carbonate
decomposes in the heat to form calcium oxide and carbon dioxide.
About 90 per cent of glass is soda-lime glass, or bottle glass.
Environmental, social and economic considerations
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The limestone industry
You need to be able to evaluate some of the effects of the limestone
industry. Here are the main ones:
Benefits
Disadvantages
Limestone is a valuable natural
resource, used to make things
such as glass and concrete.
Limestone quarries are visible
from long distances and may
permanently disfigure the local
environment.
Limestone quarrying provides
employment opportunities that
support the local economy in towns
around the quarry.
Quarrying is a heavy industry
that creates noise and heavy
traffic, which damages people's
quality of life.
Advantages and disadvantages of various building materials
Limestone, cement and mortar slowly react with carbon dioxide
dissolved in rainwater, and wear away. This damages walls made from
limestone, and it leaves gaps between bricks in buildings. These gaps
must be filled in or “pointed”. Pollution from burning fossil fuels
makes the rain more acidic than it should be, and this acid rain
makes these problems worse.
Concrete is easily formed into different shapes before it sets hard. It is
strong when squashed, but weak when bent or stretched. However,
concrete can be made much stronger by reinforcing it with steel. Some
people think that concrete buildings and bridges are unattractive.
Glass is usually brittle and easily shattered, but toughened glass can
be used for windows. While glass is transparent and so lets light into
a building, buildings with lots of glass can be too hot in the summer.
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Metals
Metals are very useful. Ores are naturally occurring rocks that contain
metal or metal compounds in sufficient amounts to make it
worthwhile extracting them. For example, iron ore is used to make
iron and steel. Copper is easily extracted, but ores rich in copper are
becoming more difficult to find. Aluminium and titanium are metals
with useful properties, but they are expensive to extract. Most
everyday metals are mixtures called alloys.
Methods of extracting metals
The Earth's crust contains metals and metal compounds such as gold,
iron oxide and aluminium oxide, but when found in the Earth these
are often mixed with other substances. To be useful, the metals have
to be extracted from whatever they are mixed with. A metal ore is a
rock containing a metal, or a metal compound, in a high enough
concentration to make it economic to extract the metal.
The method used to extract metals from the ore in which they are
found depends on their reactivity. For example, reactive metals such
as aluminium are extracted by electrolysis, while a less-reactive metal
such as iron may be extracted by reduction with carbon or carbon
monoxide.
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Thus the method of extraction of a metal from its ore depends on the
metal's position in the reactivity series:
potassium
sodium
calcium
extract by electrolysis
magnesium
aluminium
carbon
zinc
iron
tin
lead
extract by reaction with carbon or carbon monoxide
hydrogen
copper
silver
gold
platinum
extracted by various chemical reactions
Reactivity and extraction method
Note that gold, because it is so unreactive, is found as the native
metal and not as a compound, so it does not need to be chemically
separated. However, chemical reactions may be needed to remove
other elements that might contaminate the metal. Making iron
In the blast furnace
Iron is extracted from iron ore in a huge container called a blast
furnace. Iron ores such as haematite contain iron oxide. The oxygen
must be removed from the iron oxide to leave the iron behind.
Reactions in which oxygen is removed are called reduction reactions.
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Blast furnace in a modern steel works
Carbon is more reactive than iron, so it can push out or displace the
iron from iron oxide. Here are the equations for the reaction:
iron oxide + carbon
2Fe2O3 + 3C
iron + carbon dioxide
4Fe + 3CO2
In this reaction, the iron oxide is reduced to iron, and the carbon is
oxidised to carbon dioxide.
In the blast furnace, it is so hot that carbon monoxide can be used to
reduce the iron oxide in place of carbon:
iron oxide + carbon monoxide
Fe2O3 + 3CO
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iron + carbon dioxide
2Fe + 3CO2
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Raw materials for the reaction
The table shows the raw materials for extracting iron and their
function in the process.
Raw material Contains
Function
iron ore
(haematite)
iron oxide
a compound that contains iron
coke
carbon
burns in air to produce heat, and reacts to
form carbon monoxide (needed to reduce
the iron oxide)
limestone
calcium
carbonate
helps to remove acidic impurities from the
iron by reacting with them to form molten
slag
air
oxygen
allows the coke to burn, and so produces
heat and carbon monoxide
Steel
Iron
Pure iron is soft and easily shaped. This is because its atoms are
arranged in a regular way that lets layers of atoms slide over each
other. Pure iron is too soft for many uses.
Layers of atoms slide over each other when metals are bent or
stretched
Iron from the blast furnace is an alloy of about 96 per cent iron with
carbon and some other impurities. It is hard, but too brittle for most
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uses. So, most iron from the blast furnace is converted into steel by
removing some of the carbon.
Steel
Carbon is removed by blowing oxygen into the molten metal. It reacts
with the carbon producing carbon monoxide and carbon dioxide.
These escape from the molten metal. Enough oxygen is used to
achieve steel with the desired carbon content. Other metals are often
added, such as vanadium and chromium.
There are many different types of steel, depending on the other
elements mixed with the iron. The table summarises the properties of
some different steels.
Type of steel
Iron alloyed with:
Properties
Typical use
low carbon
steel
about 0.25 per cent
carbon
easily shaped
car body
panels
high carbon
steel
up to 2.5 per cent
carbon
hard
cutting tools
resistant to
corrosion
cutlery and
sinks
stainless steel chromium and nickel
Alloys
The properties of a metal are changed by including other elements,
such as carbon. A mixture of two or more elements, where at least one
element is a metal, is called an alloy. Alloys contain atoms of different
sizes, which distort the regular arrangements of atoms. This makes it
more difficult for the layers to slide over each other, so alloys are
harder than the pure metal.
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It is more difficult for layers of atoms to slide over each other in alloys
Copper, gold and aluminium are too soft for many uses. They are
mixed with other metals to make them harder for everyday use. For
example:
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Brass, used in electrical fittings, is 70 per cent copper and 30
per cent zinc.
18 carat gold, used in jewellery, is 75 per cent gold and 25 per
cent copper and other metals.
Duralumin, used in aircraft manufacture, is 96 per cent
aluminium and 4 per cent copper and other metals
The transition metals
You need to know where to find the transition metals in the periodic
table. The transition metals are found in the large block between
Groups 2 and 3 in the periodic table. Most metals are placed here,
including iron, titanium, copper and nickel.
The transition metals (in blue)
Common properties
The transition metals have these properties in common:
•
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•
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They are metals.
They form coloured compounds.
They are good conductors of heat and electricity.
They can be hammered or bent into shape easily.
They are less reactive than alkali metals such as sodium, they
have higher melting points (but mercury is a liquid at room
temperature) and they are hard and tough.
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•
They have high densities.
Smart alloys can return to their original shape after being bent. They
are useful for spectacle frames and dental braces.
Copper
Copper is a transition metal. It is soft, easily bent and it is a good
conductor of electricity. This makes copper useful for electrical wiring.
Copper does not react with water, which makes it useful for plumbing.
Copper is purified by electrolysis. Electricity is passed through
solutions containing copper compounds, such as copper sulphate
(sometimes spelt sulphate). Pure copper forms on the negative
electrode.
Problems
we are running out of ores rich in copper. Research is being carried
out to find new ways to extract copper from the remaining ores,
without harming the environment too much. This research is very
important, as traditional mining produces huge open-cast mines, and
the remaining ores are low-grade, which means that they contain
relatively little copper and produce a lot of waste rock.
Aluminium and titanium
Aluminium and titanium are two metals with a low density. This
means that they are lightweight for their size. They also have a very
thin layer of their oxides on the surface, which stops air and water
getting to the metal, so aluminium and titanium resist corrosion.
These properties make the two metals very useful.
A cross section of aluminium that shows the outer layer of oxide
Aluminium is used for aircraft, trains, overhead power cables,
saucepans and cooking foil. Titanium is used for fighter aircraft,
artificial hip joints and pipes in nuclear power stations.
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Extraction
Unlike iron, aluminium and titanium cannot be extracted from their
oxides by reduction with carbon:
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Aluminium is more reactive than carbon, so the reaction does
not work.
Titanium forms titanium carbide with carbon, which makes the
metal brittle.
Aluminium extraction is expensive because the process needs a lot of
electrical energy. Titanium extraction is expensive because the
process involves several stages and a lot of energy. This especially
limits the uses of titanium.
Recycling
Aluminium is extensively recycled because less energy is needed to
produce recycled aluminium than to extract aluminium from its ore.
Recycling preserves limited resources and requires less energy, so it
causes less damage to the environment.
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