Transcript Key Points - Science Cloud

Topic 1
C1.1 The early atmosphere
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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.
C1.2 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.
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C1.4 The atmosphere today
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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.
 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.
Topic 2
C1.5 Rocks and their formation
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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.
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.
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 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.
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.
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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.
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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.
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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)
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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)
C1.9 Reactions of calcium compounds
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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.
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.
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Topic 3
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.
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 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.
C1.12 Neutralisation
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Acids can be neutralised by metal oxides, metal
hydroxides or metal carbonates. The general
equations for these reactions are:
acid + metal oxide  salt + water
acid + metal hydroxide  salt + water
acid + metal carbonate  salt + water +
carbon dioxide
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When hydrochloric acid is neutralised it makes
chloride salts, sulfuric acid makes sulphate salts
and nitric acid makes nitrate salts.
 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.
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.
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 Chlorine
compounds are used to disinfect
swimming pools and drinking water. Chlorine
is also used to make bleach and some
plastics, such as PVC.
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.
TOPIC 4
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.
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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.
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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.
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 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.
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:
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– reserves of metal ores will last longer
– it reduces the use of energy
– there is less damage to the landscape from mining.
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There are some disadvantages including the cost
and the energy used in collecting, sorting and
transporting metals to be recycled.
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
– gold is used to make jewellery because it is
attractive and unreactive
– steel is used in construction because it is strong
and relatively cheap.
TOPIC 5
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.
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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.
 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.
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.
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.
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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.
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 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.
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.
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 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.
C1.28 Biofuels
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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.
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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.
C1.29 Choosing fuels
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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.
 Petrol,
kerosene and diesel are all nonrenewable fossil fuels. Methane is a nonrenewable fossil fuel found in natural gas.
 Coal is a non-renewable fossil fuel that
produces a lot of ash when it burns.
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.
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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.
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.
 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.
C1.33 Polymerisation
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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.
 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.
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.
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 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.