Chemistry Paper 2 AQA NEW SPEC

• The rate of a chemical reaction is how fast the reactants are changed into the products.
• One of the slowest is the rusting of iron, chemical weathering like acid rain damage to limestone buildings.
• An example of a moderate speed reaction would be the metal magnesium reacting with an acid to produce a gentle stream of bubbles.
• Burning is a fast reaction, but explosions are even faster and release a lot of gas. Explosive reaction are all over in a fraction of a second.

• The steeper the line on the graph, the faster the rate of reaction.
• Over time, the line becomes less steep as the reactants are used up.
• The quickest reactions have the steepest lines and become flat in the least time.

• mean rate of reaction  = quantity of reactant used / time taken
• mean rate of reaction  = quantity of product formed / time taken 
• The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3 . The units of rate of reaction may be given as g/s or cm3 /s.

• the concentrations of reactants in solution
• the pressure of reacting gases, the surface area of solid reactants
• the temperature
• the presence of catalysts. 

• When the temperature is increased, the particles all move faster.
• If they're moving faster, they're going to collide more frequently.
• Also, the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen.

• If a solution is more concentrated, it means there are more particles knocking about in the same volume of water. 
• Similarly, when the pressure of a gas is increased, it means that the same number of particles occupies a smaller space.
• This makes collisions between the reactant particles more frequent.

• If one of the reactants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio.
• This means that for the same volume of the solid, the particles around it will have more area to work on - so there will be collisions more frequently.

• A catalyst is a substance that speeds up the reaction without being used up in the reaction itself. This means it's not part of the overall reaction equation.
• Different catalysts are needed for different reactions, but they all work by decreasing the activation energy, the minimum amount of energy required for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy.
• Enzymes are biological catalysts - they catalyse reactions in living things.
• Catalysts are usually transition metals.

mean rate of reaction  = quantity of reactant used/time taken

mean rate of reaction  = quantity of product formed/time taken

The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3 . The units of rate of reaction may be given as g/s or cm3 /s.

• You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (opaque).
• You can observe a mark through the solution and measure how long it takes for it to disappears - the faster the mark disappears, the quicker the reaction.
• If the reactants are coloured and the products are colourless, you can time how long it takes for the solution to lose or gain its colour.
• The results are very subjuctive - depends on who thinks the cross has disappeared.

• Measuring the speed of a reaction that produces a gas can be carried out using a mass balance.
• As the gas is released, the mass disappearing is measured on the balance.
• The quicker the reading on the balance drops, the faster the reaction.
• If you take measurements at regular intervals, you can plot a rate of reaction graph and find the rate quite easily.
• This is the most accurate of the three methods described because the mass balance is very accurate. But it has the disadvantage of releasing the gas straight into the room.

• This involves the use of a gas syringe to measure the volume of a gas given off. 
• The more gas given off during a given time interval, the faster the reaction. 
• Gas syringes usually give volumes accurate to the nearest cubic cm, so they're quite accurate. You can take measurements at regular intervals and plot a rate of reaction graph using this method too. You have to be quite careful though - if the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe.

• Start by adding a set volume of dilute HCl to a conical flask.
• Add some magnesium ribbon to the acid and quickly attach an empty gas syringe to the flask.
• Start the stopwatch. Take readings of the volume of gas in the gas syringe at regular intervals.
• Plot the results in a table.
• Plot a graph with time on the x-axis and volume of gas produced on the y-axis.

• Use a measuring cylinder to put 10 cm3 sodium thiosulfate solution into the conical flask.
• Use the measuring cylinder to then add 40 cm3 water.  This dilutes the sodium thiosulfate solution to a concentration of 8 g/dm3.  
• Put the conical flask on the black cross.
• Put 10 cm3 of dilute hydrochloric acid into the 10 cm3 measuring cylinder.
• Put this acid into the flask. At the same time swirl the flask gently and start the stopclock.
• Look down through the top of the flask.  Stop the clock when you can no longer see the cross.  
• Take care to avoid breathing in any sulfur dioxide fumes.
• Repeat steps 1‒5 four times, but in step 1 use:
• 20 cm3 sodium thiosulfate + 30 cm3 water (concentration 16 g/dm3)
• 30 cm3 sodium thiosulfate + 20 cm3 water (concentration 24 g/dm3)
• 40 cm3 sodium thiosulfate + 10 cm3 water (concentration 32 g/dm3)
• 50 cm3 sodium thiosulfate + no water (concentration 40 g/dm3)
• Repeat the experiment and calculate the mean time for each of the sodium thiosulfate concentrations.  Leave out anomalous values from your calculations.

• Mean rate of reaction = change in y / change in x.
• Draw a tangent to find the reaction rate at a particular point and then work out the gradient.

• Collision theory explains how various factors affect rates of reactions.
• According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
• The minimum amount of energy that particles must have to react is called the activation energy.
• Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.
• Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

A  + B  C  + D

• In some chemical reactions, the products of the reaction can react to produce the original reactants.
• As the reactants react, their concentrations fall - so the forward reaction will slow down. But as more and more products are made and their concentration rise, the backward reaction will speed up.
• After a while, the forward reaction will be going at exactly the same rate as the backwards one - the system is at equilibrium.
• At equilibrium, both reactions are still happening, but there's no overall effect. This means the concentrations of reactants and products have reached a balance and won't change.
• Equilibrium is only reached if the reversible reaction takes place in a 'closed system'. A closed system means that none of the reactants and products can escape and nothing else can get in.
• When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

• When a reaction's are at equilibrium, it doesn't mean the amounts of reactants and products are equal. 
• If the equilibrium lies to the right, the concentration of products is greater than that of the reactants.
• If the equilibirum lies to the right, the concentration of reactants is greater than that of the products.
• The position of equilibrium depends on:
• temperature
• pressure (gases)
• concentration of reactants and products.

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
• The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction.
• Example:
• Hydrated copper sulfate (blue)  anhydrous copper sulfate (white) + water.
• The top arrow -> endothermic
• The bottom arrow <- exothermic.
• If you heat blue hydrated copper sulfate crystals, it drives the water off and leaves white anhydrous copper sulfate powder. This is endothermic.
• If you then add a couple of drops of water to the white powder you get the blue crystals back again. This is exothermic.

• The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counter act that change.
• The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
• The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.

If the temperature of a system at equilibrium is increased:

• the relative amount of products at equilibrium increases for an endothermic reaction
• the relative amount of products at equilibrium decreases for an exothermic reaction.

If the temperature of a system at equilibrium is decreased:

• the relative amount of products at equilibrium decreases for an endothermic reaction
• the relative amount of products at equilibrium increases for an exothermic reaction.

If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. This means you'll get more products for the exothermic reaction and fewer for the endothermic reaction.

If you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it. This means you'll get more products for the endothermic reaction and fewer products for the exothermic reaction.

For gaseous reactions at equilibrium:

• an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
• a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

Changing the pressure only affects equilibriums involving gases. 

If you increase the pressure, the equilibrium tries to reduce it - it moves in the direction where there are fewer molecules of gas.

If you decrease the pressure, the equlibrium tries to increase it - it moves in the direction where there are more molecules of gas.

You can use the balanced symbol equation for a reaction to see which side has more molecules of gases.

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. So the system responds to bring itself back to equilibrium again.

If you increase the concentration of the reactants the system will try to decrease it by making more products.

If you decrease the concentration of the products the system will try to increase it again by reducing the amount of reactants.

A hydrocarbon is any compound formed from carbon and hydrogen atoms only.

• Alkanes are the simplest type of hydrocarbon.
• The general formula is: CnH2n+n
• The alkanes are a homologous series - a group of organic compounds that react in a similar way.
• Alkanes are saturated compounds - each carbon atom forms four single covalent bonds.
• The first four alkanes are methane, ethane, propane and butane.

• The shorter the carbon chain, the more runny the hydrocarbon is - the less viscous it is.
• The shorter the carbon chain, the more volatile the hydrocarbon is. More volatile means it turns into a gas at a lower temperature. So, the shorter the carbon chain, the lower the temperature at which that hydrocarbon vaporises or condenses - and the lower its boiling point.
• Also, the shorter the carbon chain, the more flammable the hydrocarbon is.
• The properties of hydrocarbons affect how they're used for fuels, e.g. short chain hydrocarbons with lower boiling points are used as 'bottled gases' - stored under pressure as liquids in bottles.

• Complete combustion of any hydrocarbon releases lots of energy. The only waste products are carbon dioxide and water vapour.
• Hydrocarbon + oxygen -> carbon dioxide + water.
• During combustion, both carbon and hydrogen from the hydrocarbon are oxidised. Oxidation can be defined as the gain of oxygen.
• Hydrocarbons are used as fuels due to the amount of energy released when they combust completely.

• Crude oil is a finite resource found in rocks.
• Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud.
• Crude oil is a mixture of a very large number of compounds.
• Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.

• The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
• The oil is heated until most of it has turned into gas. The gases enter a fractionating column.
• In the column, there's a temperature gradient (hot at the bottom and cooler upwards).
• The longer hydrocarbons have high boiling points. They condense back into liquids and drain out of the column early on, when they're near the bottom. The shorter hydrocarbons have lower boiling points. They condense and drain out much later on, near to the top of the column where it's cooler.
• You end up with the crude oil mixture being seperated out into different fractions. Each fraction contains a mixture of hydrocarbons that all contain a similar number of carbon atoms, so have similar boiling points.

• The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
• Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil.
• Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers, detergents.
• The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds.

• Short-chain hydrocarbons are flammable so make good fuels and are high in demand. However, long-chain hydrocarbons form thick gloopy liquids like tar which aren't that useful.
• As a result of this a lot of the longer alkane molecules produced from fractional distillation are turned into smaller, more useful ones by a process called cracking.
• As well as alkanes, cracking produces another type of hydrocarbon called alkenes. Alkenes are used as a starting material when making lots of other compounds and can be used to make polymers.
• Some of the products of cracking are useful as fuels, e.g. petrol for cars and paraffin for jet fuel.

• Cracking is a thermal decomposition reaction - breaking down molecules by heating them.
• The first step is to heat long-chain hydrocarbons to vaporise them (turn them into a gas).
• Then the vapour can be passed over a hot powdered aluminium oxide catalyst.
• The long-chain molecules split apart on the surface of the specks of catalysts - this is catalytic cracking.
• You can also crack hydrocarbons if you vaporise them, mix them with steam and then heat them to a very high temperature. This is known as steam cracking.

• Alkenes are hydrocarbons with a double carbon-carbon bond.
• The general formula for the homologous series of alkenes is CnH2n.
• Alkene molecules are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
• The C=C double bond can open up to make a single bond, allowing the two carbon atoms to bond with other atoms. This makes alkenes reactive, more than alkanes.
• The first four members of the homologous series of alkenes are ethene, propene, butene and pentene.

• In a large amount of oxygen, alkenes combust completely to produce only carbon dioxide and water.
• However, when you burn them in the air, they tend to undergo incomplete combustion. Carbon dioxide and water are still produced, but you can also get carbon and carbon monoxide which is a poisonous gas:
• Alkene + oxygen -> carbon monoxide + carbon dioxide + water.
• Incomplete combustion results in a smoky, yellow flame and less energy being released compared to complete combustion of the same compound.

• A functional group is a group of atoms in a molecule that determines how that molecule typically reacts.
• All alkenes have the functional group C=C, so they all react in similar ways. So you can suggest the products of a reaction based on your knowledge of how alkenes react in general.
• Most of the time, alkenes react via addition reactions. The C=C bond will open up to leave a single bond and a new atom is added to each carbon.

• Addition of hydrogen is known as hydrogenation.
• Hydrogen can react with C=C to open up the double bond and form the equivalent, saturated alkene. 
• The alkene is reacted with hydrogen in the presence of a catalyst.

• Alkenes will also react in addition reactions with halogens such as bromine, chlorine and iodine. 
• The molecules formed are saturated, with the C=C carbons becoming bonded to a halogen atom.
• For example, bromine and ethene react together to form dibromoethane.

• When orange bromine water is added to saturated compounds, like an alkane, no reaction will happen and it'll stay bright orange.
• If it's added to an alkene, the bromine will add across the double bond, making a colourless dibromo-compound - so the bromine water is decolourised.

• When alkenes react with steam, water is added across the double bond and an alcohol is formed.
• For example, ethanol can be made by mixing ethene with steam and then passing it over a catalyst.
• The conversion of ethene to ethanol is one way of making ethanol industrially. After the reaction has taken place, the reaction mixture is passed from the reactor into a condenser. Ethanol and water have a higher boiling point than ethene, so both condense whilst any unreacted ethene gas is recycled back into the reactor.
• The alcohol can then be purified from the mixture by fractional distillation.

• Polymers are long molecules formed when lots of small molecules called monomers join together. 
• This reaction is called polymerisation - usually needs high pressure and a catalyst.
• Plastics are made from polymers. They're usually carbon based and their monomers are often alkenes.

• From unsaturated monomers.
• The monomers that make up addition polymers have a double covalent bond.
• Lots of unsaturated monomer molecules (alkenes) can open up their double bonds and join together to form polymer chains. This is called addition polymerisation.
• When the monomers react in addition polymerisation reactions, the only product is the polymer, so an addition polymer contains exactly the same type and number of atoms as the monomers that formed it.

Drawing the displayed formula of an addition polymer from the displayed formula of its monomer is easy.

• Start by drawing the two alkene carbons, replace the double bond with a single bond and add an extra single bond to each of the carbons.
• Then fill in the rest of the groups in the same way that they surrounded the double bond in the monomer.
• Finally, stick a pair of brackets around the repeating bit, and put an 'n' after it (to show that there are lots of monomers.

The name of the polymer comes from the type of monomer its made from (e.g. chloroethene) and sticking 'poly' in front of it (e.g. poly(chloroethene)).

To get from the displayed formula of the polymer, get rid of the two bonds going out through the brackets and put a double bond between the carbons.

• Alcohols contain the functional group –OH.
• Methanol, ethanol, propanol and butanol are the first four members of a homologous series of alcohols.
• CH3CH2OH or the general formula: CnH2n+1OH.

• Alcohols are flammable. They undergo complete combustion in air to produce carbon dioxide and water.
• Methanol, ethanol, propanol and butanol are all soluble in water. Their solutions have a neutral pH.
• They also react with sodium. One of the products of this reaction is hydrogen.
• Alcohols can be oxidised by reacting with oxygen to produce a carboxylic acid. 
• Different alcohols form different carboxylic acids. For example, methanol is oxidised to give methanoic acid, while ethanol is oxidised to give ethanoic acid.

• Alcohols such as methanol and ethanol are used as solvents in industry. This is because they can dissolve most things water can dissolve, but they can also dissolve substances that water can't dissolve - e.g. hydrocarbons, oils and fats.
• The first 4 alcohols are used as fuels. For example, ethanol is used as a fuel in spirit burners - it burns fairly cleanly and it's non-smelly.
• Solvents and fuels.

• Fermentation uses an enzyme in yeast to convert sugars into ethanol. Carbon dioxide is also produced. The reaction occurs in solution so the ethanol produced is aqueous.
• Sugar -> (yeast over arrow) ethanol + carbon dioxide.
• Fermentation happens fastest at a temperature around 37 degrees celsius in slightly acidic conditions under anaerobic conditions.
• Under these conditions, the enzyme in yeast works best to convert the sugar into alcohol. If the conditions were different, for example a low lower pH/higher temperature or higher pH/lower temperature, the enzyme could be denatured or work at a much slower rate.

• Carboxylic acids have the functional group –COOH.
• The first four members of a homologous series of carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
• The structures of carboxylic acids can be represented in the following forms: CH3COOH

• They react like any other acid with carbonates to produce carbon dioxide, salt and water.
• The salts formed in these reactions end with -'anoate' -e.g. methanoic acid would form a methanoate and ethanoic acid would form an ethanoate.
• Ethanoic acid -> sodium ethanoate + carbon dioxide + water.
• Carboxylic acids can dissolve in water. When they dissolve, they ionise and release H+ ions resulting in an acidic solution. But, because they don't ionise completely (not all of the acid molecules release their H+ ions) they just form weak acidic solutions. This means that they have a higher pH (are less acidic) than aqueous soltuions of strong acids with the same concentration.

• Esters can be made from carboxylic acids and an alcohol.
• Esters have the functional group -COO.
• An acid catalyst is usually used (e.g. concentrated sulfuric acid).
• Alcohol + carboxylic acid -> (acid catalyst over arrow) ester + water.
• The only ester you need to know: ethyl ethanoate.
• Ethyl ethanoate can be made from ethanoic acid and ethanol with an acid catalyst.

• Polymers can be made by condensation polymerisation.
• Condensation polymerisation involves monomers which contain different functional groups.
• The monomers react together and bonds form between them, forming polymer chains.
• For each new bond that forms, a small molecule (e.g. water) is lost. 
• The simplest types of condensation polymers contain two different types of monomer, each with two of the same functional group.
• Diol + a dicarboxylic acid -> condensation polymer + water.
• E.g = 
• Ethane diol + hexanedioic acid -> polyester + water.

• Amino acids
• Proteins
• DNA molecules
• Simple sugars.

• Amino acids contain two different functional groups - a basic amino group (NH2) and an acidic carboxyl group (COOH).
• An example of an amino acid is glycine - the smallest and simplest amino acid possible.

• Amino acids can form polymers such as polypeptides via condensation polymerisation.
• The amino group of an amino acid can react with the acid group of another, and so on, to form a polymer chain. For every new bond that is formed, a new molecule of water is lost.
• One or more long-chains of polypeptides are known as proteins. Proteins have loads of important uses in the human body. For e.g. enzymes work as catalysts, haemoglobin transports oxygen, antibodies form part of the immune system, and the majority of body tissue is made from proteins.
• Polypeptides and proteins can contain different amino acids in their polymer chains. The order of the amino acids is what gives proteins their different properties and shapes.

• Made from nucleotide polymers.
• DNA is made of two polymer chains of monomers called 'nucleotides'. 
• The nucleotides contain a small molecule such as a base. GCAT.
• The bases on the different polymer chains pair up with each other and form cross links keeping the two strands of nucleotides together and giving the double helix structure.
• The order of the bases acts as a code for an organisms genes.

• Can form polymers.
• Sugars are small molecules that contain carbon, hydrogen and oxygen.
• Sugars can react together through polymerisation reactions to form larger polymers, e.g. starch, which living things use to store energy, and cellulose, which is found in plant cell walls.

Number of types of monomers:

• Addition polymerisation - only one monomer type containing a C=C bond.
• Condensation polymerisation - two monomer types containing two of the same functional groups OR one monomer type with two different functional groups.

Number of products:

• Addition polymerisation - only one product formed.
• Condensation polymerisation - two types of product - the polymer and the small molecule (e.g. water).

Functional groups involved in polymerisation:

• Addition polymerisation - carbon-carbon double bond in monomer.
• Condensation polymerisation - two reactive groups on each monomer.

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk. 

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

• A formulation is a mixture that has been designed as a useful product.
• Many products are complex mixtures in which each chemical has a particular purpose.
• Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.
• Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

• Pigments - gives the paint colour (for example titanium oxide is used as a pigment in white paints).
• Solvent - used to dissolve the other components and alter the viscosity (runniness).
• Binder (resin) - forms a film that holds the pigment in place after it has been painted on.
• Additives - added to further change the physical and chemical properties of the paint.

Depending on the purpose of the paint, the chemicals used and their amounts are changed so the paint produced is right for its job.

Chlorine:

• Chlorine bleaches damp blue litmus paper, turning it white. If you use blue litmus paper it may turn red for a moment - that's because a solution of chlorine is acidic.

Oxygen:

• If you put a glowing splint inside a test tube containing oxygen, the oxygen will relight a glowing splint.

Carbon dioxide:

• Bubbling carbon dioxide through (or shaking carbon dioxide with) an aqueous solution of calcium hydroxide (limewater) causes the solution to turn cloudy.

Hydrogen:

• If you hold a burning splint at the open end of a test tube containing hydrogen = squeaky pop. Comes from the hydrogen burning quickly with oxygen in air to form water.

• A mobile phase - where the molecules can move. This is always a liquid or gas.
• A stationary phase - where the molcules can't move. This can be solid or a really thick liquid.
• The mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves through it.
• How quickly a chemical moves on depends on how it's distributed between the phases - whether it spends more time in the mobile phase or the stationary phase.
• The chemicals that spend more time in the mobile phase than the stationary phase will move further through the stationary phase.
• The components in a mixture will normally separate through the stationary phase, so long as all the components spend different amounts of time in the mobile phase.
• The separated compounds form spots. The number of spots formed may change in different solvents as the distribution of the chemical will change depending on the solvent.

• In paper chromatography the stationary phase is the chromatography paper and the mobile phase is the solvent.
• The amount of time the molecules spend in each phase depends on two things:
• How soluble they are in the solvent.
• How attracted they are to the paper.
• Molecules with a higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase - and they'll be carried further up the paper.

• The result of chromatography analysis is called a chromatogram.
• Any Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.
• You can calculate Rf values by:
• Rf = distance travelled by substance / distance travelled by solvent.
• The further through the stationary phase a substance moves, the larger the Rf value.
• Chromatography is often carried out to see if a certain substance is present in a mixture. To do this, you run a pure sample of that substance alongside the unknown mixture. If the Rf  values of the reference and one of the spots in the mixture match, the substance may be present.

• The Rf value of a substance is dependent on the solvent you use - if you change the solvent the Rf value for the substance will change.
• You can test both the mixture and the reference in a number of different solvents.
• If the Rf value of the reference matches the Rf value of one of the spots in the mixture in all the solvents, then it's likely the reference compound is present in the mixture.
• If the spot in the mixture and the spot in the reference only have the same Rf value in some of the solvents, then the reference compound isn't present in the mixture.

• Dilute acid can help detect carbonates.
• Tests for sulfates with HCl and barium chloride.
• Test for halides with nitric acid and silver nitrates.

• Carbonates are substance that contain carbonate 2- ions.
• You can test for carbonate ions by using a pipette to add a few drops of dilute acid to a test tube containing your mystery substance.
• Then, you can connect this test tube to a test tube containing limewater. If carbonate ions are present, for eg. sodium carbonate and HCl:
• Sodium carbonate + HCl -> carbon dioxide + sodium chloride + water.

• To identify a sulfate use a pipette to add a couple drops of dilute HCl followed by a couple of drops of barium chloride solution to a test tube containing your mystery solution.
• If sulfate ions are present, a white precipitate of barium chloride will form:
• Ba 2+ + SO4 (2-) -> BaSO4.
• Hydrochloric acid is added to get rid of any traces of carbonate ions before you do the test. These would also produce a precipitate, so they'd confuse the results.

• To identify a halide ion, add a couple drops of dilute nitric acid (HNO3), followed by a couple drops of silver nitrate solution, AgNO3, to your mystery solution.
• Ag + Cl -> AgCl : a chloride gives a white precipitate of silver chloride.
• Ag + Br -> AgBr : a bromine gives a cream precipitate of silver bromide.
• Ag + I -> AgI : a iodine solution gives a yellow precipitate of silver iodide.

• Compounds of some metals produce a characteristic colour when heated in a flame.
• Lithium ions, Li+, produces a crimson flame.
• Sodium ions, Na+, produces a yellow flame.
• Potassium ions, K+, produces a lilac flame.
• Calcium ions, Ca 2+, produces a orange-red flame.
• Copper ions, Cu 2+, produces a green flame.

• To do the test, you need to clean a nichrome wire loop by rubbing with fine emery paper, then hold it in the blue flame of a bunsen burner. 
• The bunsen flame may change colour for a bit, but once it's blue again, the loop is clean.
• Then, dip the loop into the sample you want to test, then put it in the flame again. Record the colour of the flame.
• You can use these colours to detect and identify ions, however it only works for samples that contain a single metal ion. If the sample tested contains a mixture of metal ions, the flame colour of some ions may be hidden by the colours of others.

• Many metal hydroxides are insoluble and precipitate out of solution when formed, they have a characteristic colour.
• You add a few drops of sodium hydroxide solution to a solution of your mystery compound to form an insoluble hydroxide.
• Calcium, Ca 2+ = White precipitate. Calcium + hydroxide -> calcium hydroxide.
• Copper (II), Cu 2+ = Blue precipitate. Copper + hydroxide -> copper hydroxide.
• Iron (II), Fe 2+ = Green precipitate. Iron + hydroxide -> iron hydroxide
• Aluminium, Al 3+ = White at first but then dissolves in excess NaOH to form a colourless solution. Aluminium + hydroxide -> aluminium hydroxide.
• Magnesium, Mg 2+ = White precipitate. Magnesium + hydroxide -> magnesium hydroxide.

• During FES a sample is placed in a flame, as the ions heat up, the electrons become excited and move to a higher energy level. When the electrons drop back to their original energy levels, they release energy as light.
• The light passes through a spectroscope, which can detect different wavelengths of light to produce a light spectrum.
• The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement. 
• Since no two ions have the same charge and the same electron arrangement, different ions emit different patterns of wavelengths, and has a different line spectrum.
• The intensity of the spectrum indicates the concentration of that ion in solution.
• This means that line spectra can be used to identify ions in solution and calculate their concentrations.

• FES can also be used to identify different different ions in mixtures. 
• This makes it more useful than flame tests, which only work for substances that contain a single metal ion.

• Very sensitive - they can detect even the tiniest amounts of substances.
• Very fast and tests can be automated.
• Very accurate.

• Phase 1 - Volcanoes gave out gases.
• Phase 2 - Oceans, Algae and green plants absorbed carbon dioxide.
• Some carbon became trapped in fossil fuels and rocks.
• Phase 3 - Green plants and algae produced oxygen.

• The first billion years of earth's history were pretty explosive - the surface was covered in volcanoes and released lots of gases. This how the early atmosphere was formed.
• The early atmosphere was probably mostly made up of carbon dioxide with virtually no oxygen. This was quite like the atmospheres of Mars and Venus today. 
• Volcanic activity also released nitrogen which built up in the atmosphere over time as well as water vapour and small amounts of methane and ammonia.

• When the water vapour in the atmosphere condensed, they formed oceans.
• Lots of carbon dioxide was removed from the early atmosphere as it dissolved in the oceans.
• This dissolved carbon dioxide then went through a series of reactions to form carbonate precipitates that formed sediments on the seabed.
• Later, marine animals evolved. Their shells and skeletons contained some of these carbonates from the oceans.
• Green plants and algae evolved and absorbed some of the carbon dioxide so they could carry out photosynthesis.

• When plants, plankton and marine animals die, they fall to the seabed and get buried by layers of sediment.
• Over millions of years, they become compressed and form sedimentary rocks, oil and gas- trapping the carbon within them, helping to keep it out of the atmosphere.
• Things like coal, crude oil and natural gas that are made by the process are called fossil fuels.
• Crude oil and natural gas are formed from deposits of plankton. These fossil fuels form reservoirs under the seabed when they get trapped in rocks.
• Coal is a sedimentary rock made from thick plant deposits.
• Limestone is also a sedimentary rock. It is mostly made up of calcium carbonate deposits from the shell and skeletons of marine organisms. 

As well as absorbing all the carbon dioxide in the atmosphere, green plants and algae produce oxygen by photosynthesis:

6CO2 + 6H2O -> C6H12O6 + 6O2

• Algae first evolved around 2.7 billion years ago.
• Over the next billions of years or so, green plants also evolved.
• As oxygen levels built up in the atmosphere over time, more complex life like animals could evolve.
• Eventually, about 200 million years ago, the atmosphere reached a composition similar to how it is today: 
• Approximately 79% Nitrogen, 
• 20% Oxygen
• 1% Gases such as carbon dioxide, knoble gases and water vapour.

• Greenhouse gases like carbon dioxide, methane and water vapour act like an insulating layer in the earths atmosphere- this, among other factors, allows the earth to be warm enough to support life.
• All particles absorb certain frequencies of radiation. Greenhouse gases don't absorb the incoming short wavelength radiation from the sun- but they do absorb the long wave radiation that gets reflected back off the earth.
• They re-radiate it in all directions including back towards the earth.
• The long wave radiation is thermal radiation, so it results in warming of the surface of the earth, this is called the greenhouse effect.

• Deforrestation- fewer trees means carbon dioxide is removed from the atmosphere via photosynthesis.
• Burning fossils fuels- carbon that wasd locked up in these fuels is released as carbon dioxide.
• Agriculture- more farm animals produce more methane through their digestive processes.
• Creating waste- More landfill sites and more waste from agriculture means more carbon dioxide and methane released by decomposition of waste.

• The earths temperature does vary naturally.
• But recently, the average temperature of the earths surface has been increasing by amounts that are greater than we would expect to see naturally. Most scientist agree that the extra carbon dioxide from human activity is causing this increase, and that increasing global temperature will lead to temperature change.
• Evidence for this has been peer reviewed so you know this information is reliable.
• Unfortunately, it is hard to fully understand the earths climate- this is because it's so complex, and there are so many variables, that its very hard to make a model that isn't oversimplified.
• This has led to speculation about the link between carbon dioxide and climate change, particularly in the media where stories may be biased or only some of the information is given.

• An increase in global temperature could lead to polar ice caps melting - causing a rise in sea levels, increased flooding in coastal areas and coastal erosion.
• Changes in rainfall patterns (the amount, timing and distribution) may cause some regions to get too much or too little water. This, along with changes in temperature, may affect the availability of certain regions to produce food.
• The frequency and severity of storms may also increase.
• Changes in temperature and the amount of water available in a habitat may affect wild species, leading to differences in their distribution.

• Carbon footprints are a measure of the amount of carbon dioxide and other greenhouse gases released over the full life cycle of something.
• That something could be a service (e.g. school bus), an event (e.g. the Olympics), a product (e.g. a toastie maker) - almost anything.
• Measuring the total carbon footprint of something is very hard, nearly impossible.
• That's because there were so many different factors to consider  - for example, you would have to count the emissions released as a result from manufacturing al the parts of you product and in making it, not to mention the emissions produced when you actually use it and finally dispose of it.
• Still, a rough calculation can give a good idea of what the worst emitters are, so that people can avoid them in the future.

• Renewable energy sources or nuclear energy could be used instead of fossil fuels.
• Using more efficient processes could conserve energy and cut waste. Lots of waste decomposes to release methane, so this will reduce methane emisisons.
• Governments could tax companies or individuals based on the amount of carbon dioxide they emit - e.g. taxing cars based on the amount of carbon dioxide they emit over a set distance could mean that people choose to buy cars that are more fuel-efficient and so less polluting.
• Governments can also put a cap on emissions of all greenhouse gases that companies can make - then licences for emissions up to that cap.
• There's also technology that captures carbon dioxide produced by burning fossil fuels before it's released into the atmosphere - it can then be stored deep underground in cracks in the rock, such as old oil wells.

• It's easy enough saying that we should cut emissions, but actually doing it - that's a different story.
• For a start, there's still a lot of work to be done on alternative technologies that result in lower carbon dioxide emissions. For examples:
• Carbon capture and storage is a relatively new idea. At the moment the technology is still at the developmental stage.
• Many renewable energy technologies, e.g. solar panels, are still quite expensive. More development should make them cheaper, so they can be used more widely.
• A lot of governments are also worried that making changes to reduce carbon dioxide emissions could have an impact on the economic growth of their countries - which could be bad for people's well-being. This particularly important for countries that are still developing.
• Because not everyone is on board, it's hard to make international agreements to reduce emissions. Most countries don't want to sacrifice their economic development if they think that other's won't do the same.
• It's not just governments though - individuals (particuarly those in developed countries) need to make changes to their lifestyle.

• It's not only governments, individuals need to make changes to their lifestyles for example:
• choosing to cycle or walk instead of using a car.
• reducing how much they use air travel.
• doing anything that saves energy at home, e.g. turning heating down.
• But it might be hard to get people to make changes if they don't want to and if there isn't enough education provided about why the changes are necessary and how to make them.

• Fossil fuels such as crude oil and coal, contain hydrocarbons. During combustion, the carbon and hydrogen in these compounds are oxidised, so that carbon dioxide and water vapour are released back into the atmosphere.
• When there's plenty of oxygen = complete combustion.
• If there's not enough oxygen, some of the fuel doesn't burn - this is called incomplete combustion. Under these condtions, solid particles (particulates), made up of soot (carbon) and unburned hydrocarbons are released and carbon monoxide can be produced as well as carbon dioxide.

• If the particles are inhaled, they can get stuck in the lungs and cause damage. This can cause respiratory problems.
• They're bad for the environment as well. Particulates (and the clouds they help to produce), reflect sunlight back into space. This means that less light reaches the Earth, causing global dimming.

• Carbon monoxide (CO) is really dangerous because it can stop your blood from doing its job of carrying oxygen around the body.
• It does this by binding to the haemoglobin in your blood that normally carries oxygen, so less oxygen is transported around the body.
• A lack of oxygen in the blood can lead to fainting, a coma or even death.
• Carbon monoxide doesn't have any colour or smell, so it's hard to detect. This makes it even more dangerous.

• Sulfur dioxide is released during the combustion of fossil fuels, such as coal, that contain sulfur impurities - the sulfur in the fuel becomes oxidised.
• Nitrogen oxides are created from a reaction between the nitrogen and oxygen in the air, caused by the heat of burning. (This can happen in the internal combustion engines of cars).
• When these gases mix with water in clouds, they form dilute sulfuric acid or dilute nitric acid. This then falls as acid rain. 
• Acid rain kills plants and damages buildings and statues. It also corrodes metals.
• Not only that, but sulfur dioxide and nitrogen oxides can also be bad for human health - can cause respiratory problems if they're breathed in.

• Ceramics are non-metal solids with high melting points that aren't made from carbon-based compounds.
• Some ceramics can be made from clay.
• Clay is a soft material when it's dug up out of the ground, so it can be molded into different shapes.
• When it's fired at high temperatures, it hardens to form a clay ceramic.
• Its ability to be moulded when wet and then hardened to make clay makes clay dieal for making pottery and bricks.
• Another example of a ceramic is glass. Glass is generally transparent, can be moulded when hot and can be brittle when thin.
• Most glass made is soda-lime glass, which is made by heating a mixture of limestone, sand and sodium carbonate (soda) until it melts. When the mixture cools, it comes out as glass.
• Borosilicate glass has a higher melting point than soda-lime glass. It's made in the same way as soda-lime glass, using a mixture of sand and boron trioxide.

• Ceramics are non-metal solids with high melting points that aren't made from carbon-based compounds.
• Some ceramics can be made from clay.
• Clay is a soft material when it's dug up out of the ground, so it can be molded into different shapes.
• When it's fired at high temperatures, it hardens to form a clay ceramic.
• Its ability to be moulded when wet and then hardened to make clay makes clay dieal for making pottery and bricks.
• Another example of a ceramic is glass. Glass is generally transparent, can be moulded when hot and can be brittle when thin.
• Most glass made is soda-lime glass, which is made by heating a mixture of limestone, sand and sodium carbonate (soda) until it melts. When the mixture cools, it comes out as glass.
• Borosilicate glass has a higher melting point than soda-lime glass. It's made in the same way as soda-lime glass, using a mixture of sand and boron trioxide.

• Composites are made from one material embedded into another. Fibres or fragments of a material (known as reinforcement) are surrounded by a matrix acting as a binder.
• The properties of a composite depend on the properties of the materials it is made from. E.g.:
• Fibreglass consists of fibres and glass embedded in a matrix made of polymer (plastic). It has a low density (like plastic) but is very strong (like glass). It's used for things like skis, surfboards and boats.
• Carbon fibre composites also have a polymer matrix. The reinforcement is either made from long chains of carbon atoms bonded together (carbon fibres) or from carbon nanotubes. These composites are very strong and light so are used in aerospace and sports car manufacturing.
• Concrete is made from aggregate (any material made from fragments - usually sand and gravel are used in concrete) embedded in cement. It's very strong. This makes it ideal for use as building material, e.g. in skate parks.
• Wood is a natural composite of cellulose fibres held together by an organic polymer matrix.

• Two important things that can influence the properties of a polymer - how it's made and what it's made from. For e.g. the properties of poly(ethene) depend on the catalyst that was used and the reaction conditions (the temperature and pressure) that is was made under.
• Low density (LD) poly(ethene) is made from ethene at a moderate temperature under a high pressure and with a catalyst. It's flexible and is used for bags and bottles.
• High density (HD) poly(ethene) is also made from ethene but at a lower temperature and pressure with a different catalyst. It's more rigid and is used for water tanks and drainpipes.
• The monomers that a polymer is made from determines the type of bonds that form between the polymer chains. These weak bonds between the different molecule chain determine the properties of the polymer:
• Thermosetting polymers contain monomers that can form cross-links between the polymer chains, holding the chains together in a solid structure. Unlike thermosoftening polymers, these polymers don't soften when they're heated. Thermosetting polymers are strong, hard and rigid.
• Thermosoftening polymers contain individual polymer chains entwined together with weak forces between the chains. You can melt these polymers and remould them.

• Ceramics include glass and clay ceramics such as porcelain and bricks. They're insulators of heat, electricity, brittle (aren't very flexible and break easily) and stiff.
• Polymers are insulators of heat and electricity, they can be flexible (they can be bent without breaking) and be easily moulded. Polymers have made many applications including in clothing and insulators in electrical items.
• The properties of composites depend on the matrix/binder and the reinforcement used to make them, so they have many different uses. 
• Metals are generally malleable, good conductors of heat and electricity, ductile (they can be drawn into wires), shiny and stiff. Metals have many uses, including in electrical wires, car bodywork and cutlery.

• Alloys are made by adding another element to the metal. This disrupts the structure of the metal, making alloys harder than pure metals.
• For example, alloys of iron called steels are often used instead of pure iron. Steels are made by adding small amounts of carbon and sometimes other metals to pure iron.
• High carbon steel is strong but brittle. Can be used as blades for cutting tools and bridges.
• Low carbon steel is softer and more easily shaped. Can be used on car bodies.
• Steels containing chromium and nickel (stainless steels) are hard and resistant to corrosion. Can be used for cutlery, containers for corrosive substances. 

• Bronze is an alloy of copper and tin. Bronze is harder than copper. It's used to make medals, decorative ornaments and statues.
• Brass is an alloy of copper and zinc. Brass is more malleable than bronze and is used in situations where lower friction is required, such as in water taps and door fittings.
• Gold used as jewellery is usually an alloy with silver, copper and zinc. The proportion of gold in the alloy is measured in carats. 24 carat being 100% (pure gold), and 18 carat being 75% gold. 
• Aluminium alloys are low density, which is an important property in aircraft manufacture. But pure aluminium is too soft for making aeroplanes, so it's alloyed with small amounts of other metals, so it is stronger.

• Corrosion is the destruction of materials by chemical reactions with substances in the environment.
• Rusting is an example of corrosion.
• Both air and water are necessary for iron to rust. To show that water alone is not enough, you can put an iron nail in a boiling tube with just water and the nail won't rust. The water is boiled to remove the oxygen and oil is used to stop air from getting in. To show that oxygen alone isn''t enough, you can put an iron nail in a boiling tube with just air and the nail won't rust. Calcium chloride can be used to absorb any water from theair.
• The rust is actually the compound hydrated iron(III) oxide:
• iron + oxygen + water -> hydrated iron(III) oxide.
• Unfortunately, rust is a soft, crumbly solid that soon flakes off to leave more iron available to rust again. This means that, eventually, all the iron in an object corrodes away, even if it wasn't initially at the surface.
• Aluminium also corrodes when exposed to air. Unlike iron objects, things made from aluminium aren't completely destroyed by corrosion. This is because the aluminium oxide that forms when aluminium corrodes doesn't flake away. In fact, it forms a protective layer that sticks firmly to the aluminium below and stops any further reaction from taking place.

By using barriers to keep out the water and oxygen:

• Painting/coating with plastic - ideal for big and small structures alike. It can be decorative too.
• Electroplating - this uses electrolysis to reduce metal ions onto an iron electrode. It can be used to coat the iron with a layer of different material that won't be corroded away.
• Oiling/greasing - this has to be used when moving parts are involved, like on bike chains.

Another method is the sacrificial method. This involves placing a more reactive metal, such as zinc or magnesium with the iron. Water and oxygen then react with the sacrificial metal instead of the iron.

Some protection techniques use both of the methods above. For e.g.:

• An object can be galvanised by spraying it with a coating of zinc. The zinc layer is firstly protective, but if it's scratched, the zinc around the site of the scratch works as a sacrificial metal.

• Natural resources form without human input. They involve anything that comes from the earth, sea or air. For example, cottong for clothing or oil for fuel.
• Some of these natural products can be replaced by synthetic products or improved upon by man-made processes. For e.g. rubber is a natural product that can be extracted from the sap of a tree, however, man-made polymers have now been made which can replace rubber in uses such as tyres.
• Agriculture improves conditions where natural resources can be enhanced for our needs. E.g. the development of fertilisers have meant that we can produce a high yield of crops.

• Renewable resources reform at a similar rate to, or faster than we use them.
• For e.g. timber is a renewable resource as trees can be planted following a harvest and only takes a few years to regrow. Other examples of renewable resources include fresh water and food.
• Finite resources aren't formed quickly enough to be considered replaceable.
• Finite resources include fossil fuels and nuclear fuels such as uranium and plutonium. Minerals and metals found in ores in the earth are also non-renewable materials.
• After they've been extracted, many finite resources undergo man-made processes to provide fuels and materials necessary for modern life. E.g. crude oil can undergo fractional distillation to produce materials such as petrol, and metal ores can be reduced to produce pure metals.

• Many modern materials are made from raw, finite resources.
• People have to balance the social, economic and environmental effects of extracting the finite resources.
• For e.g. mining metal ores is good because useful products can be made. It also provides local people with jobs and brings money into the area. However, mining ores is bad for the environment as it uses loads of energy, scars the landscape, produces a lot of waste and destroys habitats.

• Sustainable development - an approach to development that takes into account the needs of the present society wthout damaging the lives of the future generations and environment.
• Not all resources are renewable so it's unsustainable to keep using them.
• As well as using resources, extracting resources can be unsustainable due to the amount of energy used and waste product produced. Processing the resources into useful materials, such as glass or bricks, can be unsustainable too as the processes often use energy that's made from finite resources.
• If people reduce how much they use of a finite resource, that resource is likely to last longer. Reducing usage of these resources will also reduce the use of anything needed to produce them.
• We can't stop using finite resources altogether, but chemists can develop and adapt processes that use lower amounts of finite resources and reduce damage to the environment. For example, chemists have developed catalysts that reduce the amount of energy required for certain industrial processes.

• Copper is a finite resource. One way of improving its sustainability is by extracting it from low-grade ores (ores without much copper in):
• Bioleaching - bacteria are used to convert copper compounds in the ore into soluble copper compounds, spearating out the copper from the ore in the process. The leachate (solution produced by the process) contains copper ions, which can be extracted, e.g. by electrolysis or displacement with a more reactive metal, e.g. scrap iron.
• Phytomining - involves growing plants in soil that contains copper. The plants can't use or get rid of the copper, so it gradually builds up in the leaves. The plants can be harvested, dried and burned in a furnace. The ash contains soluble copper compounds from whichn copper can be extracted by electrolysis or displacement using scrap iron.
• Traditional methods of copper mining are pretty damaging to the environment. These new methods of extraction have a much smaller impact, but the disadvantage is that they're incredibly slow.

• Mining and extracting metals takes a lot of energy, most of which comes from burning fossil fuels.
• Recycling metals often uses much less energy than is needed to mine and extract new metal, conserves the finite amount of each metal in the earth and cuts down on the amount of waste getting sent to landfill.
• Metals are usually recycled by melting them and casting them into the shape of the new product.
• Depending on what the metal will be used for after recycling, the amount of separation required for recyclable metals can change. For e.g.:
• Waste steel and iron can be kept together as they can both be added to iron in a blast furnace to reduce the amount of iron ore required.

• Glass bottles can often be reused without reshaping.
• Other forms of glass can't be reused so they're recycled instead. Usually the glass is separated by colour and chemical compositions before being recycled.
• The glass is crushed and then melted to be reshaped for use in glass products such as bottles or jars. It also might be used for a different purpose such as insulating glass wool for wall insulation in homes.

• Extracting the raw materials needed for a products can damage the local environment, for example, mining metals. Extraction can also result in pollution due to the amounts of energy needed.
• Raw materials often need to be processed to extract the desired materials and this often needs large amounts of energy. For example, extracting metals from ores or fractional distillation of crude oil.

• Manufacturing products and their packaging can use a lot of energy of pollution. For example, harmful fumes such as CO or hydrogen chloride.
• You also need to think about any waste products and how to dispose of them. The chemical reactions used to make compounds from their raw materials can produce waste products. Some waste can be turned into other useful chemicals, reducing the amount that ends up polluting the environment.

• The use of a product can damage the environment. For example, burning fuels releases greenhouse gases and other harmful substances. Fertilisers can leach into streams and then rivers causing damage to ecosystems.
• How long a product is used for or how many uses it gets is also a factor - products that need lots of energy to produce but are used for ages may mean less waste in the long run.

• Products are often disposed of in landfill sites. This takes up space and pollutes land and water. E.g. paint may wash off a product in landfill and pollute a river.
• Energy is used to transport waste to landfill, which causes pollutants to be released into the atmoshphere.
• Products may be incinerated (burnt), which causes air pollution.

• The life cycle assessment looks at every stage in a products life to asses the impacts it would have on the environment.

• Raw materials - plastic bag = crude oil and paper bag = timber.
• Manufacturing and packaging - plastic bag =  the compounds needed to make the plastic are extracted from crude oil by fractional distillation, followed by cracking and then polymerisation. Waste is reduced as the other fractions of crude oil have other uses. Paper bag = pulped timber is processes using lots of energy. Lots of waste is made.
• Using the product - plastic = can be reused. Can be used for other things as well as shopping, for example bin liners. Paper = used only once.
• Product disposal - plastic = recyclable but not biodegradable and will take up space in landfill and pollute land. Paper = biodegradable, non-toxic and can be recycled.

The life cycle assessents have shown that even though plastic bags aren't biodegradable, they take less energy to make and have a longer lifespan than paper bags, so they may be less harmful to the environment.

• The use of energy, some natural resources and the amount of certain types of waste produced by a product over it's lifetime can be easily quantified. But the effect of some pollutants is harder to give a numerical value to. E.g. it's difficult to apply a value to the negative visual effects of the plastic bags in the environment compared to paper ones.
• So, producing an LCA is not an objective method as it takes into account the values of the person carrying out the assessment. This means that LCAs can be biased.
• Selective LCAs, which only show some of the impacts of a product on the environment, can also be biased as they can be written to deliberately support the claims of a company in order to give them positive advertising.

• Water of appropriate quality is essential for life.
• For humans, drinking water should have sufficiently low levels of dissolved salts and microbes.
• Water that is safe to drink is called potable water.
• Potable water is not pure water in the chemical sense because it contains dissolved substances.
• The methods used to produce potable water depend on available supplies of water and local conditions.

• Rainwater is a type of fresh water. Fresh water is water that doesn't have much dissolved in it. 
• When it rains, water can either collect as surface water (in lakes, reservoirs and rivers) or groundwater (in rocks called aquifiers that trap water underground).
• In the United Kingdom (UK), rain provides water with low levels of dissolved substances (fresh water) that collects in the ground and in lakes and rivers, and most potable water is produced by:
• choosing an appropriate source of fresh water
• passing the water through filter beds - a wire mesh screens out large twigs etc., and then gravel and sand beds filter out any other solid bits.
• sterilising - the water is sterilised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or ultraviolet light.

• There isn't enough surface or groundwater and instead sea water must be treated by desalination to provide potable water. In these countries, distillation can be used to desalinate water.
• Sea water can also be treated by processes that use membranes - like reverse osmosis. The salty water is passed through a membrane that only allows water molecules to pass through.
• Ions and larger molecuels are trapped by the mebrane and are separated from the water.
• Both distillation and reverse osmosis need loads of energy, so theyre really expensive and not practical for producing large quantities of fresh water.

• First, test the pH of the water using a pH meter. If the pH is too high or too low, you'll need to neutralise it by adding the acid or the alkali. Use a pH meter to tell when the solution is between 6.5 and 8.5.
• You should also test the water for the presence of sodium chloride (salt in seawater):
• To test for sodium ions, do a flame test on a small sample. If sodium ions are present the flame should turn yellow.
• To test for chloride ions, take another sample of your water and add a few drops of dilute nitric acid, followed by a few drops of silver nitrate solution. If chloride ions are present, a white precipitate will form.
• To distill the water, power the salty water into a distillation apparatus. Heat the flask from below. The water will boil and form steam, leaving any dissolved salts in the flask. The steam will condense back into liquid water in the condenser and can be collected as it runs out.
• Then, retest the distilled water for sodium chloride to check it has been removed. Also, retest the pH of the water with a pH meter to check that the pH is between 6.5 and 8.5.

• We use water for lots of things at home - like having a bath, going to the toilet, doing the washing-up etc. When you flush this water down the drain, it goes into the sewers and towards sewer treatment plants.
• Agricultural systems also produce a lot of waste water including nutrient run-off from fields and slurry from animal farms.
• Sewage from domestic or agricultural resources has to be treated to remove any organic matter and harmful microbes before it can be put back into fresh water sources like rivers or lakes. Otherwise, it would make them very polluted and would pose health risks.
• Industrial processes, such as the Haber Process also produce a lot of waste water that has to be collected and treated.
• As well as organic matter, industrial waste water can also contain harmful chemicals - so it has to undergo additional stages of treatment before it is safe to release it into the environment.

• 1) Screening - before being treated, the sewage is screened. This involves removing any large bits of material (like twigs or plastic bags) as well as any grit.
• 2) Sedimentation - then it's allowed to stand in a settlement tank and undergoes sedimentation - the heavier suspended solids sink to the bottom to produce sludge while the lighter, effluent floats on the top.
• 3) Aerobic digestion - the effluent in the settlement tank is removed and treated by biological aerobic digestion. This is when air is pumped through the water to encourage aerobic bacteria to break down any organic matter - including other microbes in the water.
• 4) Anaerobic digestion - the sludge from the bottom of the settlement tank is also removed and transferred into large tanks. Here, it gets broken down by bacteria in a process called anaerobic respiration.
• 5) Gas digested and waste produced - anaerobic digestion breaks down the organic matter in the sludge, releasing methane gas in the process. The methane gas can be used as an energy source and the remaining digested waste can be used as a fertiliser.
• For waste water containing toxic substance, additional stages of treatment may involve adding chemicals (e.g. to precipitate metals), UV radiation or using membranes.

• Requires more processes than treating fresh water but uses less energy than the desalination of salt water, so could be used as an alternative in areas where there's not much fresh water. For example, Singapore is treating waste water and recycling it back into drinking supplies. However, some people don't like the idea of drinkng water that used to be sewage.

• The Haber process is used to manufacture ammonia, which can be used to produce nitrogen-based fertilisers.
• The raw materials for the Haber process are nitrogen and hydrogen.
• Nitrogen (N2) + hydrogen (3H2) -> (reversible reaction) ammonia (2NH3).
• The nitrogen is easily obtained from the air, which is 78%.
• The hydrogen mainly comes from reacting methane (from natural gas) with steam to form hydrogen and carbon dioxide.
• Industrial conditions: pressure = 200 atmospheres, temperature = 450 degrees celsius and catalyst = iron.
• Because the reaction is reversible, some of the ammonia produced converts back into hydrogen and nitrogen again. It eventually reaches a dynamic equilibrium.
• The ammonia formed is a gas, but as it cools in the condenser, it liquefies and is removed. The unused hydrogen and nitrogen are recycled, so nothing is wasted.
• The ammonia produced can then be used to make ammonium nitrate - a very nitrogen-rich fertiliser.

• The forward reaction in the Haber Process is exothermic. That means that increasing the temperature will move the equilibrium the wrong way - away from ammonia and towards nitrogen and hydrogen. So the yield of ammonia would be greater at a lower temperature.
• The trouble with lower temperatures mean a slower rate of reaction and so equilibrium is reached more slowly. 
• The 450 degrees celsius is a compromise between maximum yiled and speed of reaction. It's better to wait just 20 seconds for a 10% yield thant to have 60 seconds for a 20% yield.
• Higher pressures move the position of equilibrium towards the products since there are 4 molecules of gas on the left hand side and two on the right hand side. Increasing pressure increases the % yield and rate of reaction.
• So the pressure is set as high as possible, without making the process too expensive or too dangerous to build and maintain. Hence the 200 atmopheres.
• And finally, the iron catalysts make the reaction go faster, but doesn't affect the yield.

• Farmers used to use manure to fertilise fields. Formulated fertilisers are better as they're more widely available, easier to use, don't smell and have just enough of each nutrient so more crops can be grown.
• The three main essential elements in fertilisers are nitrogen, phosphorous and potassium. If plants don't get enough of these elements, their growth and life processes are affected. These elemets may be missing from the soil if they've been used up by a previous crop.
• Fertilisers replace these missing elements or provide more of them. This helps to increase the crop yield, as crops can grow faster and bigger. For e.g. fertilisers add more nitrogen to the soil. Plants use this extra nitrogen to make plant proteins - allowing the plants to grow faster. This increases productivity.
• NPK fertilisers are formulations containing salts of nitrogen, phosphorous and potassium in the right percentages of the elements.

• Ammonia can be reacted with oxygen and water in a series of reactions to make nitric acid. 
• You can also react ammonia with acids, including nitric acid, to get ammonium salts, which can be used as fertilisers.
• Ammonia and nitric acid react together to produce ammonium nitrate - this is an especially good compound to use in a fertiliser because it has nitrogen from two sources.
• Ammonia + nitric acid -> ammonium nitrate.

• In industry: 
• The reaction is carried out in giant vats, at high concentrations resulting in a very exothermic reaction.
• The heat released is used to evaporate water from the mixture to make a very concentrated ammonium nitrate product.
• In the lab:
• The reaction is carried out on a much smaller scale, by titration and crystallisation.
• The reactants are at a much lower concentration than in industry, so less heat is produced by the reaction and it's safer for a person to carry out.
• After the titration, the mixture then needs to be crystallised to give pure ammonium nitrate crystals.
• Crystallisation isn't used in industry because it's very slow.

• Potassium chloride and potassium sulphate can be mined and used as a source of potassium.
• Phosphate rock is also mined. However, because the phosphate salts in the rock are insoluble, plants can't directly absorb them and use them as nutrients.
• Reacting phosphate rock with a number of different types of acids produces soluble phosphates:
• Reaction with nitric acid produces phosphoric acid and calcium nitrate.
• Reaction with sulfuric acid produces calcium sulfate and calcium phosphate (this mixture is known as single superphosphate). 
• Reaction with phosphoric acid only produces calcium phosphate (the product of this reaction can be called triple superphosphate).