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5.2 Metals
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Metals are conductors of electricity, malleable (easy to bend into
shape) and hard. These properties can be explained by looking at
how metal atoms are bonded.
Some of the outer electrons of the metal atoms
move into the spaces between the atoms,
forming a ‘sea of electrons’ as shown in the
diagram. Since they have lost negative electrons,
they have become ions – atoms carrying a
positive charge. There is a strong attraction
between the positive metal ions and the
negative electrons.
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a Explain why the ions in a metal are positive.
This strong attraction pulls the metal ions close to each other and
holds them in place. A lot of energy is needed to pull the metal
ions apart, making the metal hard and giving it a high melting
point.
b Suggest why some metals are harder than others.
The sea of electrons between the ions is mobile. When a potential
difference is applied to the metal, e.g. by connecting it to an
electrical battery, electrons can flow easily between the ions to
carry the current. Metals are therefore good conductors of
electricity.
c Suggest why a non-metal such as sulphur does not conduct
electricity.
Metals are easy to shape. When hit with a hammer a metal dents,
i.e. it changes shape rather than breaking.
The diagram shows how the metal ions move when a force is
applied. Layers of metal ions move over each other. A new shape
is formed and the metal does not break.
This enables metals such as steel to be pressed into complex
shapes, such as those needed for the parts of a car body.
d A non-metal such as sulphur breaks if hit with a hammer.
Suggest why.
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A mixture of metals is called an alloy.
Bronze is a mixture of copper and tin.
Brass is a mixture of copper and zinc.
Solder is a mixture of lead and tin.
atom of second metal
The diagram shows the arrangement of
metal atoms in a pure metal and an
alloy.
The layers of atoms in the pure metal
can slide over each other quite easily.
In the alloy the different-sized atoms of
the second metal prevent the layers of
atoms from sliding so easily. The alloy
is harder than the pure metal.
Pure metal.
atom of first metal
Alloy.
A mixture of two metals to make an alloy is often harder and has
a higher melting point than either pure metal. This means that
alloys can be used for jobs that neither metal would be good for
on its own.
Duralumin is an alloy of aluminium, magnesium and copper. This
alloy still has a low density but is much stronger than aluminium
alone. Duralumin is used in aircraft construction.
e In the Bronze Age weapons were made of the alloy bronze.
Suggest why a bronze axe would be better than a copper
one.
Steel is a mixture of iron with a small proportion of the non-metal
carbon, though steel is still called an alloy. The presence of up to
4% of carbon in mild steel makes the alloy much harder but less
brittle than iron.
f Why would a hammer made of steel be better than one
made of iron?
TASKS
1 Use your knowledge of the structure of metals, including the arrangement of particles, to
explain the following.
a Wires in home electricity cables are made of copper, not polythene.
b Steel, not iron, is used to make car bodies.
c Solder, not lead, is used to join copper water pipes.
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Thermoplastic and thermosetting polymers
Polymers are made by joining large numbers of small molecules
together to form long chains. The small molecules are called
monomers. Each polymer molecule is a long chain of atoms, usually
carbon and hydrogen. The chains are held together by weak forces
between them. Most polymers are tough, but because the forces
between the chains are easily broken, polymers melt easily and are
flexible.
In these diagrams the lines represent the long polymer molecules.
In the upper diagram the long chains are shown tangled up with
each other. This gives the polymer a much higher melting point
than the monomer it is made from, but it still melts at a moderate
temperature.
Straight-chain.
The longer the polymer chains, the more tangled they get and the
higher the melting point of the polymer.
a Explain why longer polymer chains give the polymer a
higher melting point.
The lower diagram shows polymer chains that have side chains.
These make the polymer chains even more tangled. Polymers
with branched chain molecules have higher melting points than
those with straight chain molecules.
Branched-chain.
b What other differences would you expect between polymers
with side chains and those without them?
Although the chain length and amount of side branching affect
the melting point of these polymers, they all melt at a fairly low
temperature. They are called thermoplastic polymers or
thermosoftening polymers.
With some polymers it is possible to join one chain to another
chemically. This is called cross-linking. The polymer chains in
the upper diagram are not cross-linked, but those in the lower
one are. Strong bonds have been formed between the polymer
chains, similar to the bonds in the chains.
Thermosoftening.
c What type of bonds are present between the polymer chains?
Thermoplastic polymers that are not cross-linked melt at fairly
Thermosetting.
low temperatures. This means that the polymer can be
manufactured and then re-melted into a mould to make the shape
of the article. Examples are poly(ethene) and poly(vinyl chloride).
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Polymers
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Cross-linked polymers are very difficult to melt. Most decompose
before they reach their melting point. These polymers are made in
the shape of the final article, because they cannot be re-melted
into a new shape. They are called thermosetting polymers, and
are good for making articles that need to get hot without melting.
An example is melamine, the polymer used to make kitchen work
surfaces.
d What other articles would be better made from
thermosetting rather than thermoplastic polymers?
The cross-linking between polymer chains forms a rigid network
in the polymer, as shown in this diagram.
This makes thermosetting polymers harder and tougher than
thermosoftening polymers.
The cross-links are formed by chemical reactions between side
groups on the polymer chains. An example is shown in the
diagram below.
COOH
H2N
polymer chains
COHN
H 2O
cross-linked
polymer chains
e Why does the polymer poly(ethene) not form cross-links
between the polymer chains?
TASKS
1 The table gives information about some
polymers.
Use information from the table and your
knowledge of polymers to suggest which
of these five polymers is best for the
following uses, and why its properties
are most suited to this job.
a car seat fabric
b cling film
c kitchen work surface
d washing-up bowl
e window frames.
Polymer
Type of chain
Plasticiser
low-density polythene
unbranched
yes
high-density polythene
branched
no
melamine
cross-linked
no
uPVC
unbranched
no
PVC
unbranched
yes
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Compounds with high melting points have giant structures in
which each particle is held rigidly in place by bonds to the
particles around it. There are two types of giant structures: ionic
and molecular.
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Aluminium oxide has a giant ionic structure, shown in this
diagram. The grey spheres represent aluminium ions, Al3+, and
the red spheres represent oxygen ions, O2. This type of structure
is known as a lattice.
The ions are held together by electrostatic attraction. This forms
very strong bonds, and gives the compound a very high melting
point.
a Use the charges on aluminium and oxygen ions to work out
the formula of aluminium oxide.
Silicon dioxide has a giant molecular structure, shown in this
diagram. The grey circles represent silicon atoms and the red
circles represent oxygen atoms.
There are no ions in this structure. The bonds between the silicon
and oxygen atoms are covalent.
For this lattice to melt, all of these very strong covalent bonds
need to be broken. This would require a lot of energy. Silicon
dioxide therefore has a very high melting point and is very hard.
Sand is largely composed of silicon dioxide.
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b Sandpaper (tough paper coated with sand) is used to smooth
wood. Suggest why sand is a good material for this use.
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When silicon dioxide and other oxides with similar lattice
structures are melted and then cooled rapidly, the atoms do not
return to their original positions.
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A new arrangement of atoms is formed, as shown in the diagram.
This is the structure of glass.
Ceramic articles are made from fired clay. Clay is a natural
material dug out of the ground, and is a mixture of many
compounds. The clay is carefully shaped before being fired in an
oven. At the high temperature of the oven, some of the substances
in clay form glass. The ceramic material produced consists of
many tiny crystals of silicate minerals held together by glass.
c Suggest why ceramic materials are very hard.
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5.5 Composites
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Plasticisers
PVC, poly(vinyl chloride), is a widely used thermoplastic. It is a
hard and tough polymer, which melts at a fairly low temperature.
It is used to make CDs and many other articles.
PVC can also be used to make cling film, which is soft and
flexible.
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You may wonder how materials with such different properties can
be made from the same substance. To make cling film a
plasticiser is added to the PVC. This makes it much softer and
more flexible. Various plasticisers can be used; most belong to a
group of organic compounds called esters.
Composites are stronger
For some uses, composites are better than metals, polymers or
ceramics. Examples are glass-reinforced plastic used for sports car
bodies, and steel-reinforced concrete used as a building material.
The use of two different substances in a composite makes the
combined material much stronger than either substance would be
on its own. Composites are therefore better for uses where more
strength is needed than could be provided by metals, polymers or
ceramics alone.
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5.3 Polymers
1 There are two forms of polythene, called high-density
polythene and low-density polythene. High-density polythene
is harder and is used to make articles such as milk crates. Lowdensity polythene is used for plastic bags, etc. Both types are
made of very long hydrocarbon molecules that become
entangled in each other.
a A piece of each type of polythene is collected. The two pieces
have the same size. What do the names high-density and
low-density polythene suggest about the masses of each
piece of polythene?
b Suggest how the very long polythene molecules are
differently arranged in high-density and low-density
polythene.
c Would you expect any difference in melting point between
the two types of polythene? Explain your answer.
5.5 Composites
1 PVC is a hard and rigid polymer used to make articles such as
CDs. When a plasticiser is added to PVC, it can be used as a
substitute for leather in the manufacture of articles such as
briefcases and handbags.
a What change to the properties of PVC is made by the
plasticiser?
b Which type of PVC would you use to manufacture the frames
for double-glazed windows? Explain your answer.
c A plasticiser commonly used in the manufacture of cling film
is soluble in oils and fats. It is recommended that this type of
cling film is not used to wrap cheese. Suggest why.
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Materials for making things: questions
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1 Solder is an alloy containing lead and tin. It is used to make electrical connections
between components in solid-state electrical circuits, e.g. in computers, mobile
phones and radios. The solder is melted using a soldering iron and then allowed to
solidify as it cools, making a firm and conductive joint between the components.
Pure lead and pure tin have melting points that are too high. Using them in the
molten state could damage heat-sensitive components. A mixture of the two
metals has a lower melting point than either pure metal.
The table shows melting points of pure lead and tin, and mixtures containing
different proportions of the two metals.
Percentage of lead
Percentage of tin
Melting point in °C
100
0
327
95
5
307
80
20
255
45
55
193
40
60
186
30
70
189
20
80
200
10
90
213
0
100
232
a On a piece of graph paper plot melting point (on the y-axis) against percentage
of lead.
[2]
b Draw two intersecting curves which meet at the minimum melting point of
183°C.
[4]
c What is the best composition of solder to use on a circuit board? Explain
why.
[2]
d It is possible to obtain lead-free solder. One example is an alloy of 96.5% tin
and 3.5% silver with a melting point of 221°C. Suggest one advantage and one
disadvantage of using this solder.
[2]
2 The polymer poly(ethene), commonly called polythene, melts easily at a fairly
low temperature.
a How does this property make polythene useful for the manufacture of many
cheap household articles?
[1]
b When polythene is heated it gradually softens, then becomes semi-liquid and
finally melts to form a very viscous liquid. Polythene does not have a specific
melting point.
Use ideas about the shape of polythene molecules and their arrangement in a
sample of polythene to answer the following questions.
i What is happening to the molecules of polythene as the material softens
when heated?
[1]
ii Why does polythene not have an exact melting point?
[2]
iii Why is the liquid formed when polythene melts very thick?
[2]
c Some polymers, such as ABS, do not melt when heated. Use ideas about
the arrangement of molecules in these polymers to explain why they do
not melt.
[3]
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Materials for making things: questions
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3 This article appeared in The Sunday Times newspaper. Use information from the
article and your knowledge of polymers to answer the questions.
Green plastic bags boost eco shopping
By Mark Ludlow
A BRITISH firm is about to produce the
world’s first environmentally friendly
plastic bag at competitive prices in a
move that promises to revolutionise
supermarket packaging.
Enpol, a plastic that is 100%
biodegradable and does not use
hazardous chemicals in its production,
can also be used for bin liners, compost
bags and food packaging. It is believed
to be the first eco-friendly plastic
produced at a price comparable with
polythene and other existing packaging
materials.
Vyvyan Howard, a leading toxicologist
at Liverpool University, said existing
plastic products often took decades to
break down and contaminated the
environment in the process. One
recent study found traces of DEHP, a
chemical compound used to make
plastic more flexible, in 80% of foods.
The new plastic breaks down in two
weeks to seven months.
Howard said Enpol was a breakthrough because it was a green
product that could compete in price
with conventionally made plastics. “Up
until now people have had to choose
between cheap non-biodegradable or
expensive biodegradable plastic. Now
there is a choice and it’s the same
price.”
Polyval, the London-based company
behind the new product, says Enpol will
revolutionise plastic packaging. While
the government considers a plan to tax
consumers 10p per plastic shopping
bag, Polyval believes it has produced a
competitive green alternative.
Paul Taylor, its chairman, said the
company had invested £7m to perfect
Enpol’s production. “The end result is an
environmentally friendly product which
is 212 times stronger than polythene,” he
said. The strength to weight ratio of the
plastic also means lower transport and
warehouse costs for plastic producers.
Polyval has set up a production plant in
the West Country, with its 100-strong
staff expected to grow to 350 once
production begins next year. The
company plans to float on the London
stock exchange and wants to sell its
technology to plastic manufacturers
across Europe.
Its first commercial sales are likely to
be in America in a multi-million-dollar
deal to supply roadside compost bags
for the recycling of garden waste.
© NI Syndication, London (Sunday October 13, 2002)
a Give two reasons why Enpol is more ‘environmentally friendly’ than
polythene.
[2]
b Explain the meaning of the term ‘biodegradable’.
[2]
c What proof is given in the passage that existing plastic packaging causes
contamination of food?
[1]
d Enpol is not the first biodegradable plastic to be made. Why is it likely to be
more successful than the others?
[1]
e Suggest why the government is considering a plan to tax shoppers for using
plastic supermarket bags.
[2]
f What other, non-environmental advantage does Enpol have over polythene? [2]
g Why might Enpol be good news for people living in the West of England? [1]
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Materials for making things: questions
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4 Read this passage about catalytic converters, then answer the questions.
The catalytic converter, an invention that has sharply reduced smog from
cars, has now become a significant and growing cause of global warming,
according to the Environmental Protection Agency.
Hailed as a miracle by Detroit automakers even today, catalytic converters
have been reducing smog for 20 years. The converters are made from a
porous ceramic material coated with a thin layer of catalyst. They convert the
poisonous exhaust gas carbon monoxide into the less harmful carbon
dioxide. They also break down compounds of nitrogen and oxygen from car
exhaust that can combine with hydrocarbons, also in car exhaust, to form
smog.
But researchers have suspected for years that the converters sometimes
rearrange the nitrogen-oxygen compounds to form nitrous oxide, known as
laughing gas. And nitrous oxide is a potent greenhouse gas, more than 300
times more potent than carbon dioxide, the most common of the gases that is
warming the atmosphere, according to experts.
This spring, the United States Environmental Protection Agency (EPA)
published a study estimating that nitrous oxide now comprises about 7.2% of
the gases that cause global warming. Cars and trucks, most fitted with
catalytic converters, produce nearly half of that nitrous oxide, the study said.
(Other sources of nitrous oxide include everything from nitrogen-based
fertiliser to manure from farm animals.)
The EPA study also showed that nitrous oxide is one of a few gases for which
emissions are increasing rapidly. Collectively known as greenhouse gases,
they trap heat in the Earth’s atmosphere.
The increase in nitrous oxide, the study notes, stems from the growth in the
number of miles travelled by cars that have catalytic converters. And the
problem has worsened as improvements in catalytic converters, changes that
have eliminated more of the nitrogen-oxygen compounds that cause smog,
have conversely produced more nitrous oxide.
a Porous ceramic materials have a large surface area in contact with the air
around them. Suggest why catalytic converters use porous ceramic material
coated with catalyst.
[3]
b What advantage is gained by the use of catalytic converters?
[4]
c Explain why the use of catalytic converters is harming the environment.
[2]
d Why have improvements in the design of catalytic converters caused more
harm to the environment?
[2]
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