Edexcel IGCSE Chemistry

Topic 1: Principles of chemistryElements, compounds and mixtures

Notes

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

● Amixture:

o Consistsof 2 or more elementsor compoundsnot chemically combined

together

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

precipitate● Crystallisation

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

the phases

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

colourings