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Edexcel IGCSE Chemistry
Topic 1: Principles of chemistryElements, compounds and mixtures
1.8 understand how to classify a substance asan element, compound ormixture
● Element =substance made from only one type of atom
● Compound = substance made from two or more elementsthat have reactedchemically with each other
o Consistsof 2 or more elementsor compoundsnot chemically combined
o Chemical propertiesof each substance in the mixture are unchanged
1.9 understand that a pure substance hasa fixed melting and boiling point,
but that a mixture may melt or boil over a range of temperatures
● Apure substance =a single element or compound, not mixed with any othersubstance
● In everyday language, a pure substance = substance that hashad nothing addedto it, so it isunadulterated and in itsnatural state, e.g. pure milk
● Pure substancesmelt and boil at specific temperatureso Thismeltingand boiling pointsdata can be used to distinguish pure
substances from mixtures(which melt over a range of temperaturesdueto them consisting of 2 or more elementsor compounds)
1.10 describe these experimental techniquesfor the separation of mixtures:simple distillation, fractional distillation, filtration, crystallisation, paperchromatography
● Simple distillationo Used to separate a pure liquid from a mixture of liquids
▪ Workswhen the liquidshave different boiling points▪ Commonly used to separate ethanol from water▪ (Taking the example of ethanol…) ethanol hasa lower bp than
water so it evaporatesfirst. The ethanol vapour is then cooled andcondensed inside the condenser to form a pure liquid.
▪ Sequence of events in distillation isas follows: heating ->evaporating -> cooling ->condensing
● Fractional distillationo The oil isheated in the fractionatingcolumn and the oil evaporatesand
condensesat a number of different temperatures.o The many hydrocarbonsin crude oil can be separated into fractionseach
of which containsmoleculeswith a similar number of carbon atoms
o The fractionatingcolumn workscontinuously, heated crude oil ispiped inat the bottom. The vaporised oil risesup the column and the variousfractionsare constantly tapped off at the different levelswhere theycondense.
o The fractionscan be processed to produce fuelsand feedstock for thepetrochemical industry.
● Filtrationo If you have produced e.g. a precipitate (which isan insoluble salt), you
would want to separate the salt/precipitate from the salt solution.▪ You would do thisby filtering the solution, leaving behind the
o If you were to have produced a soluble salt and you wanted to separatethissalt from the solution that it wasdissolved in
▪ You would first warm the solution in an open container, allowingthe solvent to evaporate, leavinga saturated solution
▪ Allow thissolution to cool▪ The solid will come out of the solution and crystalswill start to
grow, these can then be collected and allowed to dry● Paper chromatography
o Chromatography…▪ Used to separate mixturesand give information to help identify
substances▪ Involvesa stationary phase and a mobile phase▪ Separation dependson the distribution of substancesbetween
Paper Chromatography Analytical technique separating compounds by theirrelative speeds in a solvent as it spreads through paper.
The more soluble a substance is, the further up the paperit travels.
Separatesdifferent pigments in a coloured substance.
Pigment Solid, coloured substance
1.11 understand how a chromatogram provides information about the
composition of a mixture● see 1.10- separatesmixture into individual components, so revealsnumber of
components in mixture and these componentscan be identified usingRf values● Compounds in a mixture may separate into different spotsdependingon the
solvent but a pure compound will produce a single spot in all solvents
1.12 understand how to use the calculation of Rf valuesto identify the
componentsof a mixture● Rf value = distance moved by substance / distance moved by solvent ( /
representsa dividingsign)▪ Different compoundshave different Rf values in different
solvents, which can be used to help identify the compounds
1.13 practical: investigate paper chromatography using inks/ food