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AQA GCSE Science C1 notes
 

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    AQA GCSE Science C1 notes AQA GCSE Science C1 notes Document Transcript

    • AQAGCSE Science A New Unit 2 ChemistrySummary Notes Prepared and compiled by Steve Bishop
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesContentsC1.1 THE FUNDAMENTAL IDEAS IN CHEMISTRY ..................................................... 3 C1.1.1 ATOMS ........................................................................................................................................... 4 C1.1.2 THE PERIODIC TABLE ................................................................................................................ 6 C1.1.3 CHEMICAL REACTIONS ............................................................................................................. 7C1.2 LIMESTONE AND BUILDING MATERIALS .......................................................... 8 C1.2.1 CALCIUM CARBONATE .............................................................................................................. 9C1.3 METALS AND THEIR USES .....................................................................................11 C1.3.1 EXTRACTING METALS ............................................................................................................ 12 C1.3.2 ALLOYS ....................................................................................................................................... 15 C1.3.3 PROPERTIES AND USES OF METALS ................................................................................ 16C1.4 CRUDE OIL AND FUELS ...........................................................................................17 C1.4.1 CRUDE OIL ................................................................................................................................. 18 C1.4.2 HYDROCARBONS..................................................................................................................... 19 C1.4.3 HYDROCARBONS FUELS ....................................................................................................... 20C1.5 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 .................................................................................................................................... 24C1.6 PLANT OILS AND THEIR USES .............................................................................25 C1.6.1 VEGETABLE OILS ..................................................................................................................... 26 C1.6.2 EMULSIONS ............................................................................................................................... 27 C1.6.3 SATURATED AND UNSATURATED OILS ............................................................................ 28C1.7 CHANGES IN THE EARTH AND ITS ATMOSPHERE .....................................29 C1.7.1 THE EARTH’S CRUST ............................................................................................................. 30 C1.7.2 THE EARTH’S ATMOSPHERE ............................................................................................... 31 Page 2 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.1 The fundamental ideas in chemistryAtoms and elements are the building blocks of chemistry. Atoms contain protons,neutrons and electrons. When elements react they produce compounds.Key words/ conceptsAtomElementCompoundPeriodic tableNucleusProtonNeutronAtomic numberMass numberEnergy levelElectron shellNoble gasesUnreactiveStableMoleculesIonsCovalent bondsReactionReactantsProducts Page 3 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.1.1 Atomsa) All substances are made of atoms. A substance that is made of only one sort ofatom is called an element. There are about 100 different naturally occurringelements.Elements are shown in the periodic table.The groups contain elements with similar properties.The numbers above the columns show the group number.Li, Na, K, Rb, Cs, Fr are all in group I and have similar chemical properties.b) Atoms of each element are represented by a chemical symbol, eg O represents anatom of oxygen, and Na represents an atom of sodium.The periodic table above shows the names and the symbols of the elements.c) Atoms have a small central nucleus, which is made up of protons and neutronsand around which there are electrons.d) The relative electrical charges are: Name of particle Charge Proton +1 Neutron 0 Electron –1 Page 4 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notese) In an atom, the number of electrons is equal to the number of protons in thenucleus. Atoms have no overall electrical charge.f) All atoms of a particular element have the same number of protons. Atoms ofdifferent elements have different numbers of protons.g) The number of protons in an atom of an element is its atomic number. The sum of umber.the protons and neutrons in an atom is its mass number.Lithium has a mass number of 7, its atomic number is 3.It will have 3 protons.Mass number – atomic number = the number of neutrons7–3–4It will have 4 neutrons.For the atom to have no charge the number of electrons (-) = number of protons (+)So, there will be 3 electrons.h) Electrons occupy particular energy levels.Each electron in an atom is at a particular energy level (in a particular shell). Theelectrons in an atom occupy the lowest available energy levels (innermost availableshells).This element has 3 protons, 4 neutrons in its nucleus. The nucleus has a charge of +3, so there must be 3 electrons to make the atom neutral.It will have a mass number of 7 and an atomic number of 3.The first two electrons fill the first shell and so the third electron will be in the thirdshell.It has one electron in the ou shell so it will be in group 1 in the Periodic Table outerIt has two shells (or energy levels) so it is in period 2 of the Periodic Table. Page 5 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notes C1.1.2 The periodic table a) Elements in the same group in the periodic table have the same number of electrons in their highest energy level (outer electrons) and this gives them similar chemical properties. The periodic table was first developed by a Russi Russian Chemist Dmitri Mendeleev (1834-1907) at the end of the nineteenth century. At the time many elements were undiscovered and Mendelev was able to predict the properties of the yet to be discovered elements from the patterns in his Periodic Table. Group 0Period 2 Columns in the table are known as groups Rows are known as periods Groups go down, periods across. b) The elements in Group 0 of the periodic table are called the noble gases. They are unreactive because their atoms have stable arrangements of electrons They electrons. have the maximum number of electrons possible in their outer shell. The noble gases are: Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) Radon (Rn) The electron structure of neon is 2, 8: It has two shells (so it is in period 2) and 8 electrons in its outer shell so group 8 (though we call this group 0!). Page 6 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.1.3 Chemical reactionsa) When elements react, their atoms join with other atoms to form compounds. Thisinvolves giving, taking or sharing electrons to form ions or molecules.Compounds formed from metals and non-metals consist of ions.In ions the atoms transfer (give or take) electrons.Compounds formed from nonnon-metals consist of molecules.In molecules the atoms are held together by covalent bonds.In covalent bonds the atoms share electrons.b) Chemical reactions can be represented by word equations: carbon + oxygen → carbon dioxideor by symbol equations. C + O2 → CO2 reactants → productsThe chemicals before the arrow are the reactants. Those after the arrow are theproducts.c) No atoms are lost or made during a chemical reaction so the mass of the productsequals the mass of the reactants. Atoms are never destroyed or created.On the left there is:one atom of carbon (C) and two atoms in a molecule of oxygen (O2).This must balance with the right:CO2 is one atom of carbon combined with two of oxygenThere is the same number of carbon and oxygen atoms on the left as on the right, sothe equation is balanced. Page 7 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.2 Limestone and building materialsRocks provide essential building materials. Limestone is a naturally occurringresource 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 wordsLimestoneDecomposeThermal decompositionSaltCementConcreteMortarChemical compoundsCalcium carbonate CaCO3Calcium oxide CaOCarbon dioxide CO2Calcium hydroxide Ca(OH)2LimewaterCarbonates Page 8 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.2.1 Calcium carbonatea) Limestone, mainly composed of the compound calcium carbonate (CaCO3), isquarried and can be used as a building material.b) Calcium carbonate can be decomposed by heating (thermal decomposition) tomake calcium oxide and carbon dioxide.Calcium oxide is often called quicklime. The word equation is: calcium carbonate → calcium oxide + carbon dioxideAs a symbol equation: CaCO3 → CaO + CO2c) The carbonates of magnesium, copper, zinc, calcium and sodium decompose onheating in a similar way.d) Calcium oxide reacts with water to produce calcium hydroxide, which is an alkalithat 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)2The two by the bracket means that there are two OH molecules joined with the Caatom: Ca OH OHSlaked 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 newsubstances are formed. We can show this by using symbols (formulae) in theequations: calcium carbonate → calcium oxide + carbon dioxide CaCO3 → CaO + CO2This is a balanced equation; the number of calcium, oxygen and carbon atoms onthe left hand side balances the number on the right hand side.The number of atoms of each element is the same on both sides Page 9 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notese) A solution of calcium hydroxide in water (limewater) reacts with carbon dioxide toproduce calcium carbonate.Limewater is used as a test for carbon dioxide. As carbon dioxide is bubbled throughit, it will turn ‘milky’ as the insoluble calcium carbonate if formed.The equation is: CO2 + Ca(OH)2 → CaCO3 + H2Of) 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 waterThis 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 weakwhen it is pulled apart (tension).Limestone can also be used to make glass. Sand and sodium carbonate are alsoneeded to make glass.Making new quarries is always controversial.The advantages are: The disadvantages are: • jobs; • quarry can be an eyesore; • improved roads; • dust from the quarry; • more money in the area, • noise from blasting; leading to better facilities. • 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. Page 10 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.3 Metals and their usesMetals are very useful in our everyday lives. Ores are naturally occurring rocks thatprovide an economic starting point for the manufacture of metals. Iron ore is used tomake iron and steel. Copper can be easily extracted but copper-rich ores arebecoming 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 MetalsOres ElementsAlloys IronExtraction CopperCarbon AluminiumOxides TitaniumElectrolysis GoldFurnace Transition metalsSmeltingDepleted AlloysPhytomining SteelBioleachingElectrode CompoundsCorrosion Iron oxideConductor Page 11 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.3.1 Extracting metalsa) Ores contain enough metal to make it economical to extract the metal. Theeconomics of extraction may change over time.Two common ores are bauxite (aluminium ore, Al2O3) and haematite (iron ore Fe2O3) Bauxite Three different types of haematiteb) Ores are mined and may be concentrated before the metal is extracted andpurified.c) Unreactive metals such as gold are found in the Earth as the metal itself but mostmetals 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 byreduction with carbon.In simple terms reduction is the removal of oxygen: reduction Al2O3 Al reduction Fe2O3 Fe Page 12 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesIron oxide (haematite) can be reduced with carbon in the blast furnace to make ironas carbon is more reactive than iron. So carbon is able to reduce iron oxide.e) Metals that are more reactive than carbon, such as aluminium, are extracted byelectrolysis of molten compounds. The use of large amounts of energy in theextraction of these metals makes them expensive.f) Copper can be extracted from copper-rich ores by heating the ores in a furnace(smelting). The copper can be purified by electrolysis. The supply of copper copper-rich oresis limited. Copper is extracted from its ores by chemical processes that involve heat or electricity. Copper-rich ores are being depleted and traditional mining and extraction have major environmental impacts. Page 13 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesg) New ways of extracting copper from low grade ores are being researched to limitthe environmental impact of traditional mining.Copper can also be extracted by phytomining, or by bioleaching:• phytomining uses plants to absorb metal compounds when they grow. The plantsare burned to produce ash that contains the metal compounds. The metals can thenbe extracted from the ash.• bioleaching uses bacteria that can live by using the energy of the bond betweensulfur and copper. This separates the metal from the ore.It has the advantage that it is very energy efficient typically using only 30% to 50 % ofthe traditional method. It has the disadvantage that it is very slow.h) Copper can be obtained from solutions of copper salts by electrolysis or bydisplacement using scrap iron.During electrolysis positive ions move towards the negative electrode (cathode) (cathode).i) Aluminium and titanium cannot be extracted from their oxides by reduction withcarbon. Current methods of extraction are expensive because: • there are many stages in the processes • large amounts of energy are needed.j) We should recycle metals because extracting them uses limited resources and isexpensive in terms of energy and effects on the environment Page 14 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.3.2 Alloysa) Iron from the blast furnace contains about 96% iron. The impurities make it brittleand 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 stainlesssteels are resistant to corrosion. Percentage of Uses Type of steel carbon Mild steel Up to 0.25% Steel buildings, girders Medium carbon Car parts 0.25% to 0.45% steel Surgical instruments eg High carbon steel 0.45% to 1.50% scalpelc) Most metals in everyday use are alloys. Pure copper, gold, iron and aluminium aretoo soft for many uses and so are mixed with small amounts of similar metals tomake them harder for everyday use. Page 15 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.3.3 Properties and uses of metalsa) The elements in the central block of the periodic table are known as transitionmetals.Like other metals they are good conductors of heat and electricity and can be bent orhammered into shape. They are useful as structural materials and for making thingsthat 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 antibacterialc) Low density and resistance to corrosion make aluminium and titanium usefulmetals.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 aslight as steel, but it is equally strong. Page 16 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.4 Crude oil and fuelsCrude oil is derived from an ancient biomass found in rocks. Many useful materialscan be produced from crude oil. Crude oil can be fractionally distilled. Some of thefractions can be used as fuels. Biofuels are produced from plant material. There areadvantages 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 ChemicalsCrude oil ElementsCompound SulfurMixture CarbonDistillation HydrogenHydrocarbonsSaturated CompoundsAlkanes MethaneCovalent bond EthaneEvaporating PropaneCondense ButaneFractional distillation Carbon dioxideFractionating column Carbon monoxideParticulates Sulfur dioxideCombustion Nitrogen oxidesSoot EthanolBiofuels Page 17 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.4.1 Crude oila) Crude oil is a mixture of a very large number of compounds.b) A mixture consists of two or more elements or compounds not chemicallycombined together. The chemical properties of each substance in the mixture areunchanged. It is possible to separate the substances in a mixture by physicalmethods including distillation.c) Most of the compounds in crude oil consist of molecules made up of hydrogen andcarbon atoms only (hydrocarbons). Most of these are saturated hydrocarbons calledalkanes, which have the general formula CnH2n+2a) Alkane molecules can be represented in the following forms: n=1 n=2 n=3 n=4 Page 18 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.4.2 Hydrocarbonsb) The many hydrocarbons in crude oil may be separated into fractions, each ofwhich contains molecules with a similar number of carbon atoms, by evaporating theoil and allowing it to condense at a number of different temperatures.This process is fractional distillation.Candidates should know and understand the main processes in continuous fractionaldistillation in a fractionating column.c) Some properties of hydrocarbons depend on the size of their molecules. Number of Fraction Carbon Use atoms Petroleum 1 to 4 Heating and cooking gas Naphtha 5 to 9 Making other chemicals Petrol 5 to 10 Fuel for cars and light aeroplanes Kerosene 10 to 16 Fuel for jet or turbine (paraffin) aeroplanes Diesel 14 - 20 Diesel fuel or heating Oil 20 - 50 Lubricating oil Bitumen 50 or more Making roadsThese properties influence how hydrocarbons are used as fuels. Page 19 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.4.3 Hydrocarbons fuelsa) Most fuels, including coal, contain carbon and/or hydrogen and may also containsome 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 thecarbon and hydrogen in the fuels are oxidised.c) Sulfur dioxide and oxides of nitrogen cause acid rain, carbon dioxide causesglobal 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 examplein power stations.e) Biofuels, including biodiesel and ethanol, are produced from plant material.There are economic, ethical and environmental issues surrounding their use. Page 20 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.5 Other useful substances from crude oilFractions from the distillation of crude oil can be broken down (cracked) to makesmaller molecules including unsaturated hydrocarbons such as ethene. Unsaturatedhydrocarbons can be used to make polymers and ethene can be used to makeethanol. Ethanol can also be made by fermentation.Key words ChemicalsHydrocarbons EtheneCracked PropeneMolecules Bromine waterVapours Poly(ethane)Catalyst Poly(propene)Thermal decompositionAlkanesAlkenesDouble bondCrackingPolymersMonomersBiodegradableMicrobesHydrationCatalystFermentation Page 21 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.5.1 Obtaining useful substances from crude oila) Hydrocarbons can be cracked to produce smaller, more useful molecules. Thisprocess involves heating the hydrocarbons to vaporise them. The vapours are eitherpassed over a hot catalyst or mixed with steam and heated to a very hightemperature so that thermal decomposition reactions then occur.b) The products of cracking include alkanes and unsaturated hydrocarbons calledalkenes. Alkenes have the general formula CnH2nc) 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 igniteeasily. However, they can be broken down into shorter chain hydrocarbons whichare more useful. This process is called cracking.Cracking involves heating the hydrocarbons and passing the vapours produced overa hot catalyst.Some of the products of cracking are used as fuels, but others are used to makeplastics. Page 22 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.5.2 Polymersa) 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 verylarge molecules (polymers).For example: Candidates should be able to recognise the molecules involved inthese reactions in the forms shown in the subject content.They should be able to represent the formation of a polymer from a given alkenemonomer.b) Polymers have many useful applications and new uses are being developed, forexample: • 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 developedand 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 microbesand this can lead to problems with waste disposal. Knowledge of specific namedexamples is not required, but candidates should be aware of the problems that arecaused by landfill sites and by litter.d) Plastic bags are being made from polymers and cornstarch so that they breakdown more easily. Biodegradable plastics made from cornstarch have beendeveloped. Page 23 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.5.3 Ethanola) Ethanol can be produced by hydration of ethene with steam in the presence of acatalyst. ethene + steam → ethanol C2H4 + H2O → C2H5OH An ethanol plant.b) Ethanol can also be produced by fermentation with yeast, using renewableresources. This can be represented by: sugar → carbon dioxide + ethanol C6H12O6 → 2CO2 + 2C2H5OHUsing ethane to make ethanol needs non-renewable crude oil as its raw material,whereas fermentation uses renewable plant material. Page 24 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.6 Plant oils and their usesMany plants produce useful oils that can be converted into consumer productsincluding processed foods. Emulsions can be made and have a number of uses.Vegetable oils can be hardened to make margarine. Biodiesel fuel can be producedfrom vegetable oilsKey wordsEmulsionsBiodieselDistillationEmulsifiersHydrophilicHydrophobicSaturatedUnsaturatedDouble bondsCatalystHydrogenatedChemicalsBromine waterNickel Page 25 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.6.1 Vegetable oilsa) Some fruits, seeds and nuts are rich in oils that can be extracted. The plantmaterial 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 rapeb) Vegetable oils are important foods and fuels as they provide a lot of energy. Theyalso provide us with nutrients.c) Vegetable oils have higher boiling points than water and so can be used to cookfoods at higher temperatures than by boiling. This produces quicker cooking anddifferent flavours but increases the energy that the food releases when it is eaten. Page 26 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.6.2 Emulsionsa) 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 theirspecial properties. They provide better texture, coating ability and appearance, forexample in salad dressings, ice creams, cosmetics (such as face creams, bodylotions 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. Thetails ‘hate’ water but the heads ‘love’ it. They stop the oil and water in an emulsionseparating out into layers.The tails dissolve in oil making tiny droplets. The surface of each droplet is made upof the heads – the heads are charged and so will be repelled by other droplets. Inthis way they keep the droplets apart and stop them forming into two layers. Page 27 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.6.3 Saturated and unsaturated oilsa) Vegetable oils that are unsaturated contain double carbon–carbon bonds (C=C).These can be detected by reacting with bromine water. The coloured bromine waterwill decolourise if the oils are unsaturated.b) Vegetable oils that are unsaturated can be hardened by reacting them withhydrogen 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 roomtemperature, making them useful as spreads and in cakes and pastries. This processis known as hardening. Page 28 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.7 Changes in the Earth and its atmosphereKey words/ conceptsCoreMantleCrustAtmosphereTectonic platesConvection currentsRadioactive processesHydrocarbonsAlgaePhotosynthesisSedimentary rocksCarbonatesReservoirMarine environmentFossil fuelsChemicalsNitrogenOxygenCarbon dioxideNoble gasesMethaneAmmoniaOxygen Page 29 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.7.1 The Earth’s crusta) The Earthconsists of a core,mantle and crust,and is surroundedby theatmosphere. b) The Earth’s crust and theupper part of the mantle arecracked into a number of largepieces (tectonic plates).c) Convection currents withinthe Earth’s mantle driven byheat released by naturalradioactive processes causethe plates to move at relativespeeds of a few centimetresper year. The mantle is mostlysolid, but it is able to move slowly.d) The movements can be sudden and disastrous. Earthquakes and/or volcaniceruptions occur at the boundaries between tectonic plates. Page 30 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesC1.7.2 The Earth’s atmospherea) For 200 million years, the proportions of different gases in the atmosphere havebeen 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 volcanicactivity. This activity released the gases that formed the early atmosphere and watervapour 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 wasformed involves the interactionbetween hydrocarbons,ammonia and lightning. Page 31 of 32
    • AQA GCSE SCIENCE Chemistry Unit 2 summary notesf) 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 upin sedimentary rocks as carbonates and fossil fuels.Carbon dioxide dissolves in the oceans and that limestone was formed from theshells and skeletons of marine organisms. Fossil fuels contain carbon andhydrocarbons that are the remains of plants and animals.h) The oceans also act as a reservoir for carbon dioxide but increased amounts ofcarbon 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 levelof carbon dioxide in the atmosphere.j) Air is a mixture of gases with different boiling pointsand can be fractionally distilled to provide a source ofraw materials used in a variety of industrial processes. Page 32 of 32