AQA Science A Chemistry notes

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ChemistrySummary

AQA GCSE Science

New Unit 2

Chemistry Summary Notes

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Steve Bishop

Science A

Notes

AQA GCSE SCIENCE Chemistry Unit 2 summary notes

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Contents

C1.1 THE FUNDAMENTAL IDEAS IN CHEMISTRY ..................................................... 3

C1.1.1 ATOMS ........................................................................................................................................... 4

C1.1.2 THE PERIODIC TABLE ................................................................................................................ 6

C1.1.3 CHEMICAL REACTIONS ............................................................................................................. 7

C1.2 LIMESTONE AND BUILDING MATERIALS .......................................................... 8

C1.2.1 CALCIUM CARBONATE .............................................................................................................. 9

C1.3 METALS AND THEIR USES .....................................................................................11

C1.3.1 EXTRACTING METALS ............................................................................................................ 12

C1.3.2 ALLOYS ....................................................................................................................................... 15

C1.3.3 PROPERTIES AND USES OF METALS ................................................................................ 16

C1.4 CRUDE OIL AND FUELS ...........................................................................................17

C1.4.1 CRUDE OIL ................................................................................................................................. 18

C1.4.2 HYDROCARBONS..................................................................................................................... 19

C1.4.3 HYDROCARBONS FUELS ....................................................................................................... 20

C1.5 OTHER USEFUL SUBSTANCES FROM CRUDE OIL .....................................21

C1.5.1 OBTAINING USEFUL SUBSTANCES FROM CRUDE OIL ................................................. 22

C1.5.2 POLYMERS ................................................................................................................................ 23

C1.5.2 POLYMERS ................................................................................................................................ 23

C1.5.3 ETHANOL .................................................................................................................................... 24

C1.6 PLANT OILS AND THEIR USES .............................................................................25

C1.6.1 VEGETABLE OILS ..................................................................................................................... 26

C1.6.2 EMULSIONS ............................................................................................................................... 27

C1.6.3 SATURATED AND UNSATURATED OILS ............................................................................ 28

C1.7 CHANGES IN THE EARTH AND ITS ATMOSPHERE .....................................29

C1.7.1 THE EARTH’S CRUST ............................................................................................................. 30

C1.7.2 THE EARTH’S ATMOSPHERE ............................................................................................... 31


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C1.1 The fundamental ideas in chemistry Atoms and elements are the building blocks of chemistry. Atoms contain protons, neutrons and electrons. When elements react they produce compounds. Key words/ concepts Atom Element Compound Periodic table Nucleus Proton Neutron Atomic number Mass number Energy level Electron shell Noble gases Unreactive Stable Molecules Ions Covalent bonds Reaction Reactants Products

C1.1.1 Atoms a) All substances are madeatom is called an element.elements. Elements are shown in the

The groups contain elementsThe numbers above the columnsLi, Na, K, Rb, Cs, Fr are all b) Atoms of each elementatom of oxygen, and Na representsThe periodic table above shows c) Atoms have a small centraland around which there are

d) The relative electrical charges

Name ofProton Neutron Electron


made of atoms. A substance that is made of onlyelement. There are about 100 different naturally occurring

the periodic table.

elements with similar properties. columns show the group number. all in group I and have similar chemical properties.

element are represented by a chemical symbol, eg Orepresents an atom of sodium. shows the names and the symbols of the elements.

central nucleus, which is made up of protons andare electrons.

charges are:

of particle Charge +1

0 Electron –1


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only one sort of occurring

properties.

O represents an

elements.

and neutrons

e) In an atom, the numbernucleus. Atoms have no overall f) All atoms of a particular different elements have different g) The number of protons the protons and neutrons

Lithium has a mass numberIt will have 3 protons. Mass number – atomic number7 – 3 – 4 It will have 4 neutrons. For the atom to have no chargeSo, there will be 3 electrons.

h) Electrons occupy particular Each electron in an atom iselectrons in an atom occupyshells).

This element has 3 protons,3, so there must be 3 electrons It will have a mass number The first two electrons fill theshell. It has one electron in the ouIt has two shells (or energy


number of electrons is equal to the number of protonsoverall electrical charge.

element have the same number of protons. different numbers of protons.

in an atom of an element is its atomic number. in an atom is its mass number.

number of 7, its atomic number is 3.

number = the number of neutrons

charge the number of electrons (-) = number ofelectrons.

particular energy levels.

is at a particular energy level (in a particular occupy the lowest available energy levels (innermost

protons, 4 neutrons in its nucleus. The nucleus has

electrons to make the atom neutral.

number of 7 and an atomic number of 3.

the first shell and so the third electron will be

outer shell so it will be in group 1 in the Periodicenergy levels) so it is in period 2 of the Periodic Table.


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protons in the

Atoms of

umber. The sum of

of protons (+)

shell). The (innermost available

has a charge of +

be in the third

Periodic Table Table.

C1.1.2 The periodic a) Elements in the same groupelectrons in their highest energysimilar chemical properties.

TheChemistnineteenthundiscoveredpropertiespatterns

Columns in the table are knownRows are known as periodsGroups go down, periods b) The elements in Group are unreactive because theirhave the maximum number The noble gases are: Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) Radon (Rn) The electron structure It has two shells (so it is in(though we call this group

Period 2


periodic table

group in the periodic table have the same numberenergy level (outer electrons) and this gives them

properties.

The periodic table was first developed by a RussiChemist Dmitri Mendeleev (1834-1907) at thenineteenth century. At the time many elementsundiscovered and Mendelev was able to predictproperties of the yet to be discovered elementspatterns in his Periodic Table.

known as groups periods

across.

0 of the periodic table are called the noble gases.their atoms have stable arrangements of electrons

number of electrons possible in their outer shell.

structure of neon is 2, 8:

in period 2) and 8 electrons in its outer shell sogroup 0!).


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number of them

Russian the end of the

elements were predict the

elements from the

gases. They electrons. They

so group 8

Group 0

C1.1.3 Chemical a) When elements react, theirinvolves giving, taking or sharing

Compounds formed from metalsIn ions the atoms transfer Compounds formed from nonIn molecules the atoms areIn covalent bonds the atoms b) Chemical reactions can

carbon or by symbol equations.

The chemicals before the products. c) No atoms are lost or madeequals the mass of the reactants. On the left there is: one atom of carbon (C) and This must balance with the CO2 is one atom of carbon There is the same numberthe equation is balanced.


Chemical reactions

their atoms join with other atoms to form compounds.sharing electrons to form ions or molecules.

metals and non-metals consist of ions. transfer (give or take) electrons.

non-metals consist of molecules. are held together by covalent bonds.

atoms share electrons.

can be represented by word equations:

carbon + oxygen → carbon dioxide

C + O2 → CO2

reactants → products

arrow are the reactants. Those after the arrow

made during a chemical reaction so the mass ofreactants. Atoms are never destroyed or created.

and two atoms in a molecule of oxygen (O2).

the right:

carbon combined with two of oxygen

is the same number of carbon and oxygen atoms on the left as onbalanced.


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compounds. This

arrow are the

of the products created.

on the right, so


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C1.2 Limestone and building materials Rocks provide essential building materials. Limestone is a naturally occurring resource that provides a starting point for the manufacture of cement and concrete. In this section you should be able to:

• know that limestone is needed for buildings and that the positive benefits of using this material should be considered against the negative aspects of quarrying.

• consider and evaluate the environmental, social and economic effects of exploiting limestone and producing building materials from it.

• evaluate the developments in using limestone, cement and concrete as building materials, and their advantages and disadvantages over other materials.

Key words Limestone Decompose Thermal decomposition Salt Cement Concrete Mortar Chemical compounds Calcium carbonate CaCO3 Calcium oxide CaO Carbon dioxide CO2 Calcium hydroxide Ca(OH)2 Limewater Carbonates


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C1.2.1 Calcium carbonate a) Limestone, mainly composed of the compound calcium carbonate (CaCO3), is quarried and can be used as a building material. b) Calcium carbonate can be decomposed by heating (thermal decomposition) to make calcium oxide and carbon dioxide. Calcium oxide is often called quicklime. The word equation is:

calcium carbonate → calcium oxide + carbon dioxide As a symbol equation:

CaCO3 → CaO + CO2 c) The carbonates of magnesium, copper, zinc, calcium and sodium decompose on heating in a similar way. d) Calcium oxide reacts with water to produce calcium hydroxide, which is an alkali that can be used in the neutralisation of acids. It reacts with water to form slaked lime, calcium hydroxide. This reaction gives out lots of heat (exothermic)

calcium oxide + water → calcium hydroxide

CaO + H2O → Ca(OH)2 The two by the bracket means that there are two OH molecules joined with the Ca atom:

Ca

OH OH Slaked lime is an alkali so it can be used on acidic soils to increase the pH levels.

Recall: In a chemical reaction atoms are never destroyed or created, but new substances are formed. We can show this by using symbols (formulae) in the equations:

calcium carbonate → calcium oxide + carbon dioxide

CaCO3 → CaO + CO2 This is a balanced equation; the number of calcium, oxygen and carbon atoms on the left hand side balances the number on the right hand side. The number of atoms of each element is the same on both sides


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e) A solution of calcium hydroxide in water (limewater) reacts with carbon dioxide to produce calcium carbonate. Limewater is used as a test for carbon dioxide. As carbon dioxide is bubbled through it, it will turn ‘milky’ as the insoluble calcium carbonate if formed.

The equation is:

CO2 + Ca(OH)2 → CaCO3 + H2O

f) Carbonates react with acids to produce carbon dioxide, a salt and water. For example:

Calcium carbonate + hydrochloric acid → Calcium chloride + carbon dioxide + water

CaCO3 + HCl → CaCl + CO2 + H2O Salt Carbon dioxide water

This is why limestone (calcium carbonate) is damaged by acid rain. g) Limestone has a number of uses. It is heated with clay in a kiln to make cement. Cement is mixed with sand to make mortar. Mortar is used to bind bricks together. Cement is mixed with sand and aggregate (loose small chippings) to make concrete. Concrete is harder than mortar, it is very difficult to compress (squash). It is weak when it is pulled apart (tension). Limestone can also be used to make glass. Sand and sodium carbonate are also needed to make glass. Making new quarries is always controversial. The advantages are:

• jobs; • improved roads; • more money in the area,

leading to better facilities.

The disadvantages are: • quarry can be an eyesore; • dust from the quarry; • noise from blasting; • heavy lorry traffic.

Some of these can be resolved by:

• restricted working hours; • use of rail to transport the minerals; • restoration of the landscape when mining has stopped. • have fewer but bigger quarries.


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C1.3 Metals and their uses Metals are very useful in our everyday lives. Ores are naturally occurring rocks that provide an economic starting point for the manufacture of metals. Iron ore is used to make iron and steel. Copper can be easily extracted but copper-rich ores are becoming scarce so new methods of extracting copper are being developed. Aluminium and titanium are useful metals but are expensive to produce. Metals can be mixed together to make alloys. You should be able to:

• consider and evaluate the social, economic and environmental impacts of exploiting metal ores, of using metals and of recycling metals

• know that metal ores are obtained by mining and that this may involve digging up and processing large amounts of rock.

• evaluate the benefits, drawbacks and risks of using metals as structural materials.

Key words Ores Alloys Extraction Carbon Oxides Electrolysis Furnace Smelting Depleted Phytomining Bioleaching Electrode Corrosion Conductor

Metals Elements Iron Copper Aluminium Titanium Gold Transition metals Alloys Steel Compounds Iron oxide


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C1.3.1 Extracting metals a) Ores contain enough metal to make it economical to extract the metal. The economics of extraction may change over time. Two common ores are bauxite (aluminium ore, Al2O3) and haematite (iron ore Fe2O3)

Bauxite Three different types of haematite

b) Ores are mined and may be concentrated before the metal is extracted and purified. c) Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal. d) Metals that are less reactive than carbon can be extracted from their oxides by reduction with carbon. In simple terms reduction is the removal of oxygen:

Al2O3 reduction Al

Fe2O3 reduction Fe

Iron oxide (haematite) canas carbon is more reactive

e) Metals that are more reactiveelectrolysis of molten compounds.extraction of these metals

f) Copper can be extracted(smelting). The copper canis limited.

Copper is extracted electricity. Copper-rich ores aremajor environmental


can be reduced with carbon in the blast furnacereactive than iron. So carbon is able to reduce iron oxide.

reactive than carbon, such as aluminium, are extractedcompounds. The use of large amounts of energy

metals makes them expensive.

extracted from copper-rich ores by heating the ores incan be purified by electrolysis. The supply of copper

from its ores by chemical processes that involve

are being depleted and traditional mining and extractionenvironmental impacts.


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furnace to make iron oxide.

extracted by energy in the

in a furnace copper-rich ores

involve heat or

extraction have

g) New ways of extractingthe environmental impact Copper can also be extracted • phytomining uses plantsare burned to produce ashbe extracted from the ash. • bioleaching uses bacteriasulfur and copper. This separatesIt has the advantage that itthe traditional method. It has h) Copper can be obtaineddisplacement using scrap During electrolysis positive

i) Aluminium and titanium carbon. Current methods of

• there are many stages• large amounts of energy

j) We should recycle metalsexpensive in terms of energy


extracting copper from low grade ores are being researched of traditional mining.

extracted by phytomining, or by bioleaching:

plants to absorb metal compounds when they grow.ash that contains the metal compounds. The metals

ash.

bacteria that can live by using the energy of the bondseparates the metal from the ore.

it is very energy efficient typically using only 30%has the disadvantage that it is very slow.

obtained from solutions of copper salts by electrolysis iron.

positive ions move towards the negative electrode (cathode)

cannot be extracted from their oxides by reductionof extraction are expensive because:

stages in the processes energy are needed.

metals because extracting them uses limited resourcesenergy and effects on the environment


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researched to limit

grow. The plants metals can then

bond between

30% to 50 % of

electrolysis or by

(cathode).

reduction with

resources and is


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C1.3.2 Alloys a) Iron from the blast furnace contains about 96% iron. The impurities make it brittle and so it has limited uses. Cast iron is useful because it is strong in compression. b) Most iron is converted into steels. Steels are alloys since they are mixtures (not compounds) of iron with carbon. Some steels contain other metals such as nickel, chromium and tungsten. Alloys can be designed to have properties for specific uses. Low-carbon steels are easily shaped, high-carbon steels are hard, and stainless steels are resistant to corrosion.

Type of steel Percentage of carbon

Uses

Mild steel Up to 0.25% Steel buildings, girders

Medium carbon steel

0.25% to 0.45% Car parts

High carbon steel 0.45% to 1.50% Surgical instruments eg scalpel

c) Most metals in everyday use are alloys. Pure copper, gold, iron and aluminium are too soft for many uses and so are mixed with small amounts of similar metals to make them harder for everyday use.


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C1.3.3 Properties and uses of metals a) The elements in the central block of the periodic table are known as transition metals.

Like other metals they are good conductors of heat and electricity and can be bent or hammered into shape. They are useful as structural materials and for making things that must allow heat or electricity to pass through them easily. b) Copper has properties that make it useful for electrical wiring and plumbing. Copper:

• is a good conductor of electricity and heat • can be bent but is hard enough to be used to make pipes or tanks • it can easily be joined together • does not react with water • it is non-magnetic • it is antibacterial

c) Low density and resistance to corrosion make aluminium and titanium useful metals. Aluminium

1) Low density and strength make aluminium ideal for construction of aircraft,

lightweight vehicles, and ladders. An alloy of aluminium called duralumin is

often used instead of pure aluminium because of its improved properties.

2) Easy shaping and corrosion resistance make aluminium a good material

for drink cans and roofing materials.

3) Corrosion resistance and low density leads to its use for greenhouses

and window frames.

4) Good conduction of heat leads to its use for boilers, cookers and

cookware.

5) Good conduction of electricity leads to its use for overhead power cables

hung from pylons (low density gives it an advantage over copper).

6) High reflectivity makes aluminium ideal for mirrors, reflectors and heat

resistant clothing for fire fighting.

Titanium is 60% heavier than aluminium, but it is much stronger. It is nearly half as light as steel, but it is equally strong.


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C1.4 Crude oil and fuels Crude oil is derived from an ancient biomass found in rocks. Many useful materials can be produced from crude oil. Crude oil can be fractionally distilled. Some of the fractions can be used as fuels. Biofuels are produced from plant material. There are advantages and disadvantages. You should be able to:

• evaluate the impact on the environment of burning hydrocarbon fuels

• consider and evaluate the social, economic and environmental impacts of the uses of fuels

• evaluate developments in the production and uses of better fuels, for example ethanol and hydrogen

• know and understand the benefits and disadvantages of ethanol and hydrogen as fuels in terms of: o use of renewable resources o storage and use of the fuels o their products of combustion.

• evaluate the benefits, drawbacks and risks of using plant materials to produce fuels.

Key words Crude oil Compound Mixture Distillation Hydrocarbons Saturated Alkanes Covalent bond Evaporating Condense Fractional distillation Fractionating column Particulates Combustion Soot Biofuels

Chemicals Elements Sulfur Carbon Hydrogen Compounds Methane Ethane Propane Butane Carbon dioxide Carbon monoxide Sulfur dioxide Nitrogen oxides Ethanol


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C1.4.1 Crude oil a) Crude oil is a mixture of a very large number of compounds. b) A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged. It is possible to separate the substances in a mixture by physical methods including distillation. c) Most of the compounds in crude oil consist of molecules made up of hydrogen and carbon atoms only (hydrocarbons). Most of these are saturated hydrocarbons called alkanes, which have the general formula CnH2n+2

a) Alkane molecules can be represented in the following forms: n = 1 n = 2 n = 3 n = 4

C1.4.2 Hydrocarbons b) The many hydrocarbonswhich contains molecules oil and allowing it to condense This process is fractional distillation.

Candidates should know anddistillation in a fractionating c) Some properties of hydrocarbons

These properties influence

Fraction Number

Carbon

atoms

Petroleum gas

1 to

Naphtha 5 toPetrol 5 to

Kerosene (paraffin)

10

Diesel 14 Oil 20 Bitumen 50


Hydrocarbons

hydrocarbons in crude oil may be separated into fractions, with a similar number of carbon atoms, by evaporating

condense at a number of different temperatures.

distillation.

and understand the main processes in continuousfractionating column.

hydrocarbons depend on the size of their molecules.

influence how hydrocarbons are used as fuels.

Number of

Carbon

atoms

Use

to 4 Heating and cooking

to 9 Making other chemicals to 10 Fuel for cars and light

aeroplanes to 16 Fuel for jet or turbine

aeroplanes - 20 Diesel fuel or heating - 50 Lubricating oil or more Making roads


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fractions, each of evaporating the

continuous fractional

molecules.


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C1.4.3 Hydrocarbons fuels a) Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur. The gases released into the atmosphere when a fuel burns may include:

• carbon dioxide

• water (vapour)

• carbon monoxide

• sulfur dioxide and

• oxides of nitrogen. Solid particles (particulates - sometimes called soot) may also be released.

b) The combustion of hydrocarbon fuels releases energy. During combustion the carbon and hydrogen in the fuels are oxidised. c) Sulfur dioxide and oxides of nitrogen cause acid rain, carbon dioxide causes global warming, and solid particles cause global dimming. d) Sulfur can be removed from fuels before they are burned, for example in vehicles. Sulfur dioxide can be removed from the waste gases after combustion, for example in power stations. e) Biofuels, including biodiesel and ethanol, are produced from plant material. There are economic, ethical and environmental issues surrounding their use.


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C1.5 Other useful substances from crude oil Fractions from the distillation of crude oil can be broken down (cracked) to make smaller molecules including unsaturated hydrocarbons such as ethene. Unsaturated hydrocarbons can be used to make polymers and ethene can be used to make ethanol. Ethanol can also be made by fermentation. Key words Hydrocarbons Cracked Molecules Vapours Catalyst Thermal decomposition Alkanes Alkenes Double bond Cracking Polymers Monomers Biodegradable Microbes Hydration Catalyst Fermentation

Chemicals Ethene Propene Bromine water Poly(ethane) Poly(propene)


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C1.5.1 Obtaining useful substances from crude oil a) Hydrocarbons can be cracked to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them. The vapours are either passed over a hot catalyst or mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur. b) The products of cracking include alkanes and unsaturated hydrocarbons called alkenes. Alkenes have the general formula CnH2n

c) Unsaturated hydrocarbon molecules can be represented in the following forms:

The = represents a double bond. d) Alkenes react with bromine water, turning it from orange to colourless.

e) Some of the products of cracking are useful as fuels.

The longer chain hydrocarbons are not as useful as fuels because they don’t ignite easily. However, they can be broken down into shorter chain hydrocarbons which are more useful. This process is called cracking.

Cracking involves heating the hydrocarbons and passing the vapours produced over a hot catalyst.

Some of the products of cracking are used as fuels, but others are used to make plastics.


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C1.5.2 Polymers a) Alkenes can be used to make polymers such as poly(ethene) and poly(propene). In these reactions, many small molecules (monomers) join together to form very large molecules (polymers). For example: Candidates should be able to recognise the molecules involved in these reactions in the forms shown in the subject content. They should be able to represent the formation of a polymer from a given alkene monomer. b) Polymers have many useful applications and new uses are being developed, for example:

• new packaging materials,

• waterproof coatings for fabrics,

• dental polymers, wound dressings,

• hydrogels,

• smart materials (including shape memory polymers). Candidates should consider the ways in which new materials are being developed and used, but will not need to recall the names of specific examples. c) Many polymers are not biodegradable, so they are not broken down by microbes and this can lead to problems with waste disposal. Knowledge of specific named examples is not required, but candidates should be aware of the problems that are caused by landfill sites and by litter. d) Plastic bags are being made from polymers and cornstarch so that they break down more easily. Biodegradable plastics made from cornstarch have been developed.


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C1.5.3 Ethanol a) Ethanol can be produced by hydration of ethene with steam in the presence of a catalyst.

ethene + steam → ethanol

C2H4 + H2O → C2H5OH

An ethanol plant.

b) Ethanol can also be produced by fermentation with yeast, using renewable resources. This can be represented by:

sugar → carbon dioxide + ethanol

C6H12O6 → 2CO2 + 2C2H5OH Using ethane to make ethanol needs non-renewable crude oil as its raw material, whereas fermentation uses renewable plant material.


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C1.6 Plant oils and their uses Many plants produce useful oils that can be converted into consumer products including processed foods. Emulsions can be made and have a number of uses. Vegetable oils can be hardened to make margarine. Biodiesel fuel can be produced from vegetable oils Key words Emulsions Biodiesel Distillation Emulsifiers Hydrophilic Hydrophobic Saturated Unsaturated Double bonds Catalyst Hydrogenated Chemicals Bromine water Nickel


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C1.6.1 Vegetable oils a) Some fruits, seeds and nuts are rich in oils that can be extracted. The plant material is crushed and the oil removed by pressing or in some cases by distillation. Water and other impurities are removed.

Lavender is used to make lavender oil

A field of oilseed rape

b) Vegetable oils are important foods and fuels as they provide a lot of energy. They also provide us with nutrients. c) Vegetable oils have higher boiling points than water and so can be used to cook foods at higher temperatures than by boiling. This produces quicker cooking and different flavours but increases the energy that the food releases when it is eaten.


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C1.6.2 Emulsions a) Oils do not dissolve in water. They can be used to produce emulsions. Emulsions are thicker than oil or water and have many uses that depend on their special properties. They provide better texture, coating ability and appearance, for example in salad dressings, ice creams, cosmetics (such as face creams, body lotions and lipsticks) and paints.

Milk is another example of an emulsion. It is mainly made up of animal fat and water. b) Emulsifiers have hydrophilic heads and hydrophobic tails, which are charged. The tails ‘hate’ water but the heads ‘love’ it. They stop the oil and water in an emulsion separating out into layers. The tails dissolve in oil making tiny droplets. The surface of each droplet is made up of the heads – the heads are charged and so will be repelled by other droplets. In this way they keep the droplets apart and stop them forming into two layers.


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C1.6.3 Saturated and unsaturated oils a) Vegetable oils that are unsaturated contain double carbon–carbon bonds (C=C). These can be detected by reacting with bromine water. The coloured bromine water will decolourise if the oils are unsaturated.

b) Vegetable oils that are unsaturated can be hardened by reacting them with hydrogen in the presence of a nickel catalyst at about 60°C. Hydrogen adds to the carbon–carbon double bonds.

The hydrogenated oils have higher melting points so they are solids at room temperature, making them useful as spreads and in cakes and pastries. This process is known as hardening.


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C1.7 Changes in the Earth and its atmosphere Key words/ concepts Core Mantle Crust Atmosphere Tectonic plates Convection currents Radioactive processes Hydrocarbons Algae Photosynthesis Sedimentary rocks Carbonates Reservoir Marine environment Fossil fuels Chemicals Nitrogen Oxygen Carbon dioxide Noble gases Methane Ammonia Oxygen


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C1.7.1 The Earth’s crust a) The Earth consists of a core, mantle and crust, and is surrounded by the atmosphere.

b) The Earth’s crust and the upper part of the mantle are cracked into a number of large pieces (tectonic plates).

c) Convection currents within the Earth’s mantle driven by heat released by natural radioactive processes cause the plates to move at relative speeds of a few centimetres per year. The mantle is mostly solid, but it is able to move slowly. d) The movements can be sudden and disastrous. Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates.


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C1.7.2 The Earth’s atmosphere a) For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today:

• about four-fifths (80%) nitrogen • about one-fifth (20%) oxygen • small proportions of various other gases, including carbon dioxide, water vapour and noble gases.

b) During the first billion years of the Earth’s existence there was intense volcanic activity. This activity released the gases that formed the early atmosphere and water vapour that condensed to form the oceans. c) There are several theories about how the atmosphere was formed.

One theory suggests that during this period the Earth’s atmosphere was mainly carbon dioxide and there would have been little or no oxygen gas (like the atmospheres of Mars and Venus today). There may also have been water vapour and small proportions of methane and ammonia.

d) There are many theories as to how life was formed billions of years ago. e) One theory as to how life was formed involves the interaction between hydrocarbons, ammonia and lightning.


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f) Plants and algae produced the oxygen that is now in the atmosphere. g) Most of the carbon from the carbon dioxide in the air gradually became locked up in sedimentary rocks as carbonates and fossil fuels. Carbon dioxide dissolves in the oceans and that limestone was formed from the shells and skeletons of marine organisms. Fossil fuels contain carbon and hydrocarbons that are the remains of plants and animals. h) The oceans also act as a reservoir for carbon dioxide but increased amounts of carbon dioxide absorbed by the oceans has an impact on the marine environment. i) Nowadays the release of carbon dioxide by burning fossil fuels increases the level of carbon dioxide in the atmosphere. j) Air is a mixture of gases with different boiling points and can be fractionally distilled to provide a source of raw materials used in a variety of industrial processes.

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AQA Science A Chemistry notes