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C 1b Revision Guide
The Earth has a layered structure, including the core, mantle and crust. The crust and upper
mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can
cause earthquakes and volcanoes where they meet.
The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it
has been much as it is today.
The structure of the Earth
The Earth is almost a sphere. These are its main layers, starting with the outermost layer:
1. The crust (relatively thin and rocky).
2. The mantle (has the properties of a solid, but can flow - very slowly).
3. The core (made from nickel and iron).
.
Note that the radius of the core is just over half the radius of the Earth. The core itself consists
of a solid inner core and a liquid outer core
Plate tectonics
The Earth's crust and upper part of the mantle are broken into large pieces called tectonic
plates. These are constantly moving at a few centimetres each year. Although this doesn't
sound like very much, over millions of years the movement allows whole continents to shift
thousands of kilometres apart. This process is called continental drift.
The plates move because of convection currents in the Earth's mantle. These are driven by
the heat produced by the decay of radioactive elements and heat left over from the formation of
the Earth.
Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against
each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the
boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.
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It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in
places known for having earthquakes. You may wish to view this BBC News item from 2006
about the 100th anniversary of San Francisco’s great earthquake.
Alfred Wegener
Alfred Wegener (1880 - 1930)
The theory of plate tectonics and continental drift were proposed at the beginning of the
last century by a German scientist, Alfred Wegener. Before his time it was believed that
the planet's features, such as mountains, were caused by the crust shrinking as the
Earth cooled after it was formed.
It took more than 50 years for Wegener’s theory to be accepted. This was because it was
difficult to work out what the mechanism was that could make whole continents move,
and it was not until the 1960s that enough evidence was discovered
So what was the evidence for Wegener's theory?
1. Plate tectonics explained why earthquakes and volcanoes were concentrated in
specific places - around the boundaries of moving plates.
2. The match in shape between the east coast of South America and the west coast of
Africa suggests both were once part of a single continent. There are similar patterns of
rocks and similar fossils on both sides of the Atlantic - including the fossil remains of
land animals that would have been unable to swim across an ocean.
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Composition of the Earth's atmosphere
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie
chart shows the proportions of the main gases in the atmosphere.
It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to
respire, and fuels to burn - is the next most abundant gas. These two gases are both
elements and account for about 99% of the gases in the atmosphere. The remaining
gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in
much smaller proportions.
Oxygen in the air
The percentage of oxygen in the air can be measured by passing a known volume of air over
hot copper, and measuring the decrease in volume as the oxygen reacts with it. Here are the
equations for this reaction:
copper + oxygen
2Cu + O2
copper oxide
2CuO
The oxygen in this air will also react with the hot copper, causing a small error in the final
volume recorded. It is also important to let the apparatus cool down at the end of the
experiment; otherwise the final reading will be too high
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Evolution of the atmosphere
The early atmosphere
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere
was probably formed from the gases given out by volcanoes. It is believed that there was
intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were
smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most
of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon
dioxide, are similar to the early atmosphere of the Earth.
The Earth and its atmosphere provide all the things we need
Changes in the atmosphere
So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of
oxygen go up?
The proportion of oxygen went up because of photosynthesis by plants.
The proportion of carbon dioxide went down because:
•
•
•
It was locked up in sedimentary rocks, such as limestone, and in fossil fuels.
It was absorbed by plants for photosynthesis.
It dissolved in the oceans.
The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be
removed. This means that the level of carbon dioxide in the atmosphere is increasing. Check
that you understand the effects of this by going to Combustion of fuels
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Vegetable oils are obtained from plants. They are important ingredients in many foods, and can
be hardened through a chemical process to make, for example, margarine. They can also be
used as fuels, for example as biodiesel.
Food additives are chemicals added by food manufacturers to certain foods, including
vegetable oils, to improve their shelf-life, taste and appearance.
Vegetable oils
Vegetable oils are natural oils found in seeds, nuts and some fruit.
Extracting vegetable oils
The plant materials are crushed and pressed to squeeze the oil out. Olive oil is obtained this
way. Sometimes the oil is more difficult to extract and has to be dissolved in a solvent. Once
the oil is dissolved, the solvent is removed by distillation, and impurities such as water are also
removed, to leave pure vegetable oil. Sunflower oil is obtained in this way.
Structure of vegetable oils
Molecules of vegetable oils consist of glycerol and fatty acids. Glycerol has three carbon
atoms and fatty acids have long chains of carbon atoms.
In the diagram below you can see how three long chains of carbon atoms are attached to a
glycerol molecule, with its three carbon atoms. Together they combine to make one molecule of
vegetable oil.
The structure of a vegetable oil molecule
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Fatty acids
The long fatty acid chains stop vegetable oils dissolving in water.
The fatty acids in some vegetable oils are saturated, and only have single bonds between their
carbon atoms. Saturated oils tend to be solid at room temperature, and are sometimes
called vegetable fats instead of oils. Lard is an example of saturated oil.
The fatty acids in some vegetable oils are unsaturated, and have double bonds between some
of their carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are
useful for frying food. They can be divided into two categories:
•
•
Monounsaturated fats have one double bond in each fatty acid
Polyunsaturated fats have many double bonds.
Hardening vegetable oils
Unsaturated vegetable oils tend to be liquid at room temperature, but they can also be
‘hardened’, through a chemical process called hydrogenation, to make them solid at room
temperature.
Testing for unsaturation
The carbon-carbon double bonds in unsaturated oils can be detected using the elements
bromine or iodine. These elements react with the double bonds in the oils, and the more double
bonds there are, the more bromine or iodine is used up.
You can check for unsaturated fats using a simple test with bromine water. The test is similar to
one used to differentiate alkenes from alkanes.
Bromine water is a dilute solution of bromine, which is normally orange-brown in colour. It
becomes colourless when shaken with an alkene, or with unsaturated fats. When shaken with
alkanes or saturated fats, its colour remains the same.
Hydrogenation
During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at
about 60°C. A nickel catalyst is used to speed up the reaction. The double bonds are
converted to single bonds by the hydrogenation. In this way unsaturated fats can be made into
saturated fats.
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Saturated vegetable oils are solid at room temperature, and have a higher melting point than
unsaturated oils. This makes them suitable for making margarine or for commercial use in the
making of cakes and pastry.
Vegetable oils in food, fuel and emulsions; trans fats
Vegetable oils are important nutrients and provide a lot of energy. You must be careful not to
eat excessive amounts to avoid becoming overweight.
Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste
cooking oil and rapeseed oil. Such fuels are carbon neutral, which means that they release
only as much carbon dioxide when they burn as was used to make the original oil by
photosynthesis. This helps to reduce global warming. However, some people are concerned
about whether it is ethical to use food crops in this way, instead of using them to feed hungry
people.
Emulsions
Vegetable oils do not dissolve in water. If a mixture of oil and water is shaken, then left to
stand, eventually a layer of oil will form on the surface of the water.
If an emulsifier is added to the oil and water, a mixture called an emulsion forms. Emulsions are
more viscous than oil or water on their own, and contain tiny droplets of one of the liquids
spread through the other liquid. The table below gives some examples of emulsions.
Type of
emulsion
Oil droplets in water
Examples
Egg yolk, milk, ice cream, salad cream, Margarine, butter, skin cream,
mayonnaise
moisturising lotion
Water droplets in oil
Trans fats
Partially hydrogenated vegetable oils may contain trans fats. These are thought to cause
health problems such as heart disease in humans, and food manufacturers are being
encouraged to reduce the amount of them in our food Food additives
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As we have seen, processed foods, including vegetable oils, may have chemicals added to
them. These chemical additives have different jobs, including extending a food product’s shelf
life and improving its taste and appearance. You can find additives listed on the ingredients
label of such foods, and many of these additives have E numbers to identify them. The table
below describes some of them.
Type of
additive
Example
colouring
Tartrazine (E102) orange colouring for soft drinks, sweets and sauces
emulsifier
Lecithin (E322)
allows oil and water mix to make margarine, ice cream
and salad cream
preservative
Benzoic acid
(E210)
used in many foods to stop harmful micro-organisms
growing
sweetener
Aspartame
(E951)
low-calorie drinks and food
Typical use
Additives with an E number have been licensed by the European Union. Some are natural and
some are artificial, but they have all been tested for safety and passed for use.
The UK Food Standards Agency (FSA) has strict limits on the amount of colourings allowed in
food. Some additives can lead people to have allergic reactions to them, and colourings are
banned from use in baby foods.
Food scares
A red dye called 'Sudan I' was quite recently banned for use in food, because it was thought to
be a health risk. Some of the dye had been used in chilli powder before the ban came into
force, and was later added in this form to some foods by mistake. Hundreds of food items with
this ingredient had to be taken off supermarket shelves and destroyed as a result.
Polymers and ethanol from oil
In order for it to be useful to us, crude oil is broken down in oil refineries into its component
parts, known as fractions, and these can then be used for many different purposes.
Fractions that are produced by the distillation of crude oil can go through a process called
cracking. This chemical reaction produces smaller hydrocarbons, including alkanes and
alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can be used to make
polymers. Ethene can be used to make ethanol.
Cracking
Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They
do not flow easily and are difficult to ignite. Crude oil often contains too many large
hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is
where cracking comes in.
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Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful
hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and
passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller
hydrocarbon molecules.
Cracking is an example of a thermal decomposition reaction.
Some of the smaller molecules formed by cracking are used as fuels, and some of them are
used to make polymers in plastics manufacture.
Alkenes
The products of cracking include alkenes (for example ethene and propene). The alkenes are a
family of hydrocarbons that share the same general formula. This is:
CnH2n
The general formula means that the number of hydrogen atoms in an alkene is double the
number of carbon atoms. For example, ethene is C2H4 and propene is C3H6. Alkene 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.
Note the double bond between two of the carbon atoms
Alkenes are unsaturated hydrocarbons. They contain a double bond, which is shown as two
lines between two of the carbon atoms. The presence of this double bond allows alkenes to
react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used
as fuels. But they are more useful than that. They can be used to make ethanol (alcohol) and
polymers (plastics) - two crucial products in today's world
Ethanol
Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful
as a fuel. For use in cars and other vehicles, it is usually mixed with petrol.
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Ethanol from ethene and steam
Ethanol can be manufactured by reacting ethene (from cracking crude oil fractions) with
steam. A catalyst of phosphoric acid is used to ensure a fast reaction. Here are the equations
for the reaction:
Notice that ethanol is the only product. The process is continuous – as long as ethene
and steam are fed into one end of the reaction vessel, ethanol will be produced. These
features make it an efficient process, but there is a problem. Ethene is made from crude
oil, which is a non-renewable resource. It cannot be replaced once it is used up and it
will run out one day. Ethanol from non-renewable or renewable resources
Ethanol can be made by reacting ethene with steam, but it can also be made by a process
called fermentation.
Fermentation
Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The
enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this
process happen:
Unlike ethene, sugar from plant material is a renewable resource.
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Non-renewable v renewable
The table summarises some of the advantages and disadvantages of making ethanol using
non-renewable or renewable resources.
ethene and steam
sugar from plant material
Type of raw materials non-renewable
renewable
Type of process
continuous (runs all the time)
batch (stop-start)
Labour
few workers needed
a lot of workers needed
Rate of reaction
fast
slow
Conditions needed
high temperature and pressure warm, normal pressure
Purity of product
pure
impure – needs treatment
Energy needed
a lot
a little
Which method of making ethanol is the best? In fact, more than 90 per cent of the world’s
ethanol is made by fermentation.
Polymers
Alkenes can be used to make polymers. Polymers are very large molecules made when many
smaller molecules join together, end-to-end. The smaller molecules are called monomers. In
general:
lots of monomer molecules
a polymer molecule
Alkenes can act as monomers because they have a double bond:
•
•
Ethene can polymerise to form poly(ethene), which is also called polythene.
Propene can polymerise to form poly (propene), which is also called polypropylene.
Different polymers have different properties, so they have different uses.
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The table below gives some examples.
Polyethene
plastic bags and bottles
Polypropene
crates and ropes
Polychloroethene
water pipes and insulation on electricity cables
Smart materials
Polymers have properties that depend on the chemicals they are made from, and the
conditions in which they are made. Modern polymers have many uses, including:
•
•
•
•
•
waterproof coatings
fillings for teeth
dressings for cuts
hydrogels for making soft contact lenses and disposable nappy liners
shape memory polymers for shrink-wrap packaging
Branches
Polymer molecules can have branches coming off them, which change the properties of the
polymer. The table compares two types of poly(ethene).
Polymer
LDPE
HDPE
low-density poly(ethene) high-density poly(ethene)
Branches on polymer molecules many
few
Relative strength
weak
strong
Maximum useable temperature
85°C
120°C
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Plasticisers
Plasticisers are substances that let the polymer molecules slide over each other more easily.
This makes the polymer softer and more flexible. For example, poly (chloroethene) or PVC is a
hard polymer. Unplasticised PVC, usually called uPVC, is used to make pipes and window
frames. PVC with plasticisers is soft and flexible. It is used for floor coverings, raincoats and car
dashboards.
Slime
Poly (ethenol) is a polymer that dissolves in water to make slime. The viscosity of the
slime can be changed to make it thick or runny by varying the amount of water.
Problems with polymers
One of the useful properties of polymers is that they are unreactive, so they are suitable for
storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of
polymers.
Biodegradable
Most polymers, including poly (ethene) and poly (propene) are not biodegradable. This means
that micro-organisms cannot break them down, so they may last for many years in rubbish
dumps. However, it is possible to include chemicals that cause the polymer to break down
more quickly. Carrier bags and refuse bags made from such degradable polymers are already
available.
Incineration
Polymers can be burnt or incinerated. They release a lot of heat energy when they burn and
this can be used to heat homes or to generate electricity.
There are problems with incineration. Carbon dioxide is produced, which adds to global
warming. Toxic gases are produced unless the polymers are incinerated at high temperatures.
Recycling
Many polymers can be recycled. This reduces the disposal problems and the amount of crude
oil used. But the different polymers must be separated from each other first, and this can be
difficult and expensive to do.
Polymers have recycling symbols
like this one for PVC to show what they are
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