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SECTION 1:

Principles of

Chemistry

SEPARATIO

N METHODS

ELEMENTS COMPOUNDS AND MIXTURES

TYPES OF MIXTURES

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ELEMENTS – are substances which:

are made up of only one type of atom and cannot be broken down into anything simpler. can be found on the Periodic Table

There are two main kinds: Metals and Non-metals

COMPOUNDS are formed when two or more atoms/elements join together chemically, by a chemical bond have completely different properties from the elements which are used to make them up Cannot be separated very easily i.e. cannot be separated by physical means; they require lots of

energy normally in the form of electricity to separate it

MIXTURES Are formed when two or more elements/compounds are together but are NOT chemically joined In a mixture the original properties of the elements still remain Mixtures can be separated by physical means which requires little energy

SOLUTIONS Can be recognised as being transparent (you can see through it)

Solute – substances usually a solid which dissolves in a liquid to form a solutionA solute is always present in the lesser amount

Solvent – a substance usually a liquid which dissolves the solute to form a solutionA solvent is always present in a greater amount

Important Examples of Solutions Air Crude oil Sea water Ink

SEPARATING MIXTURES

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SUSPENSIONS Can be recognised as being opaque (i.e. you cannot see through them

Examples of suspensions include: Milk Emulsion paint Muddy water Salad dressing Insoluble precipitates in solution

SOLUTIONS

FILTRATION

Is used for separating an insoluble solid from a liquid The solution which passes through the filter paper is called the filtrate The solid left behind on the filter paper is called the residue The solid reside can be dried in an oven

CRYSTALLISATION

This process is used mainly in purification Impure salt crystals can be dissolved in a warmed solvent such as water Any insoluble impurities can be removed by filtration The filtrate is then cooled allowing pure salt crystals to form These crystals are then dried using two pieces of filter paper

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SIMPLE DISTILLATION

FRACTIONAL DISTILLATION

This method of separating mixtures is used to separate miscible substances, which have large differences between their boiling points

An example is salt water The salt water would be heated to 1000C so that the

water would boil off as a vapour in the round bottomed flask (evaporation)

The water vapour would then condense back to liquid

This type of distillation is used to separate miscible liquids of similar boiling points

An example is alcohol (boiling point (780C) and water (boiling point 1000C)

As the mixture is heated in the round bottomed flask, it will be the alcohol which vaporises first at 78OC

As the ethanol evaporates it travels up the fractionating column and then condenses to form pure alcohol.

SUSPENSIONSSEPARATING FUNNEL

Immiscible liquids do not dissolve together

They separate into different layers We can place them in a separating funnel;

allow time for the layers to form and then open the tap

The higher density liquid will form the lower layer and will run out first.

Once the tap is closed the liquid of lower

CHROMATOGRAPHY

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DECANTING

CENTRIFUGATION

This technique simply involves pouring off the solution/

It is suitable for solids that are dense – for example

If a suspension is spun very fast the larger particles (residue) will move to the bottom

The filtrate (liquid) at the top can just be poured off

This method is used to separate coloured substances like ink The colours separate if:

The pigments have different solubility’s in the solvent and/or The pigments have different degrees of attraction to the filter paper

METHOD

1. Spots of colours to be separated are placed on a pencil line about 2cm from the edge of a piece of filter paper (chromatogram)

2. Pencil is used for the line and not a pen so that the colour does not smudge or run and spoil the results

3. The dish of water (solvent) is placed so that its level is just below the pencil line

4. The chromatogram develops as the water rises up the paper

INTERPRETING CHROMATOGRAMS

Rf VALUES

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5. The most soluble substance will be transported furthest by the solvent.

The green food colouring is a mixture of yellow and blue

The black food colouring contains red, yellow and blue colours

SCAN DIAGRAM PG 9 – CHEMISTRY FOR IGCSE CAMBRIDGE

The mixture M has separated into 3 dyes Mixture M contains dyes A and C BUT NOT

dye B

Chromatography has two main uses: To separate mixtures of colours To identify substances in the mixture

To identify substances in the mixture we need to calculate Rf values The dye that is most soluble in the solvent and sticks to the paper the least will move up the

paper the quickest. We can work out the Rf values for each component in the mixture separated as follows:

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Once an Rf value has been calculated we can compare this value to reference values of known substances to identify the component

Example:

NOTE: Rf values are always less than 1

HOW DO IMPURITES AFFECT MELTING AND BOILING POINTS? Pure substances have very distinct melting and boiling points For example, pure water boils at 1000C However, if impurities are present the melting and boiling points are not sharp – the substance

melts and boils over a narrow range of temperatures Boling points increase by impurities Melting points are decreased by impurities

DIAGRAM SCAN PG 11

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