KS3 BIOLOGY Quizzes KS3 CHEMISTRY Quizzes & Worksheets KS3 PHYSICS Quizzes
Custom Search

 Doc Brown's Chemistry KS4 science GCSE/IGCSE/O Level Chemistry Revision Notes


dissolving-stirring, filtering, evaporation, crystallising diagram of paper/thin layer chromatography at the end diagram of paper/thin layer chromatography at start

PART 2 Methods of separating mixtures are described e.g. ways of separating liquids, simple distillation, fractional distillation, paper chromatography, thin layer chromatography, filtration, evaporation, crystallisation, separating funnel, decantation, centrifuge, centrifuging, decanting-decantation, instrumental methods of analysis, glc gas-liquid chromatography, purifying substances - index of keywords-terms-phrases below

Part 1 Definitions in Chemistry, Elements, Compounds & Mixture pictures & Physical & Chemical Changes

Part 2 Methods of Separating Mixtures of Substances (this page)

Part 3 How to write equations, work out formula and name compounds

Alphabetical list of KEYWORDS for Parts 1-3: atom  *  balancing equations (work your way down the section carefully)  *  centrifuges/centrifuging  *  chemical reaction/change  *  chromatography (paper/thin layer)  *  compound  *  covalencycrystallisation  *  decanting/decantation  * displayed formula  *  distillation (simple/fractional)  *  element  *  equations  *  evaporation  *  filtration  *  formula  *  impure/pure  *  insoluble  *  ionic equations  *  ionic valency  *  iron-sulphur separation and heating experiment  *  magnet  *  mixture  *  molecule  *  naming compounds and ions  *  particle pictures of elements/compounds/mixtures  *  physical change  *  precipitation  *  products  *  pure substance  *  purification  *  reactants  *  sand/salt separation  *  separating funnel  *  separating mixtures  *  soluble/solution/solvent/solute  *  solvent extraction  *  symbols (for elements, formula, in equations)  *   state symbols  *  valency  * working out formulae  *


METHODS of SEPARATING MIXTURES and purifying substances

2.1 Simple Distillation

simple distillation diagramHow do you separate two liquids that are miscible? Miscible means they completely mix and do not form two layers.

Distillation involves 2 stages and both are physical state changes.

(1) The liquid or solution mixture is boiled to vaporise the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action.

(2) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.

This can be used to purify water because the dissolved solids have a much higher boiling point and will not evaporate with the steam, BUT it is too simple a method to separate a mixture of liquids especially if the boiling points are relatively close.


2.2 Fractional Distillation

fractional distillation diagram and theoryHow can you separate a complex mixture of liquids by a method of distillation? Simple distillation isn't good enough to do an efficient job of separating liquids with boiling points that may be relatively close together.

Fractional distillation involves 2 main stages and both are physical state changes. It can only work with liquids with different boiling points. However, this method only works if all the liquids in the mixture are miscible (e.g. alcohol/water, crude oil etc.) and do NOT separate out into layers like oil/water.

(1) The liquid or solution mixture is boiled to vaporise the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action. 

(2) The vapour passes up through a fractionating column, where the separation takes place (theory at the end). This column is not used in the simple distillation described above.

(3) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.

This can be used to separate alcohol from a fermented sugar solution.

It is used on a large scale to separate the components of crude oil, because the different hydrocarbons have different boiling and condensation points (see oil).


Imagine green liquid is a mixture of a blue liquid (boiling point 80oC) and a yellow liquid (boiling point 100oC), so we have a coloured diagram simulation of a colourless alcohol and water mixture! As the vapour from the boiling mixture enters the fractionating column it begins to cool and condense. The highest boiling or least volatile liquid tends to condense more i.e. the yellow liquid (water). The lower boiling more volatile blue liquid gets further up the column. Gradually up the column the blue and yellow separate from each other so that yellow condenses back into the flask and pure blue distils over to be collected. The 1st liquid, the lowest boiling point, is called the 1st fraction and each liquid distils over when the top of the column reaches its particular boiling point to give the 2nd, 3rd fraction etc.

To increase the separation efficiency of the tall fractionating column, it is usually packed with glass beads, short glass tubes or glass rings etc. which greatly increase the surface area for evaporation and condensation. 

In the distillation of crude oil the different fractions are condensed out at different points in a huge fractionating column. At the top are the very low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.


2.3 Paper or Thin Layer Chromatography

 diagram of paper/thin layer chromatography at the enddiagram of paper/thin layer chromatography at start diagram of paper/thin layer chromatography at the endThis method of separation is used to see what coloured materials make up e.g. a food dye analysis.

The material to be separated e.g. a food dye (6) is dissolved in a solvent and carefully spotted onto chromatography paper or a thin layer of a white mineral material on a glass sheet.  Alongside it are spotted known colours on a 'start line' (1-5).

The paper is carefully dipped into a solvent, which is absorbed into the paper and rises up it. The solvent may be water or an organic liquid like an alcohol (e.g. ethanol) or a hydrocarbon, so-called non-aqueous solvents. For accurate work the distance moved by the solvent is marked on carefully with a pencil and the distances moved by each 'centre' of the coloured spots is also measured. These can be compared with known substances BUT if so, the identical paper and solvent must be used (See Rf values below).

Due to different solubilities and different molecular 'adhesion' some colours move more than others up the paper, so effecting the separation of the different coloured molecules.

Any colour which horizontally matches another is likely to be the same molecule i.e. red (1 and 6), brown (3 and 6) and blue (4 and 6) match, showing these three are all in the food dye (6).

The distance a substance moves, compared to the distance the solvent front moves (top of grey area on 2nd diagram) is called the reference or Rf value and has a value of 0.0 (not moved - no good), to 1.0 (too soluble - no good either), but Rf ratio values between 0.1 and 0.9 can be useful for analysis and identification.

Rf = distance moved by dissolved substance (solute) / distance moved by solvent.

Some technical terms: The substances (solutes) to be analysed must dissolve in the solvent, which is called the mobile phase because it moves. The paper or thin layer of material on which the separation takes place is called the stationary or immobile phase because it doesn't move.

It is possible to analyse colourless mixture if the components can be made coloured e.g. protein can be broken down into amino acids and coloured purple by a chemical reagent called Ninhydrin and many colourless organic molecules fluoresce when ultra-violet light is shone on them. These are called locating agents.

Thin layer chromatography (t.l.c) is where a layer of paste is thinly and evenly spread on e.g. a glass plate. The paste consists of the solid immobile phase like aluminium oxide dispersed in a liquid such as water. The plate is allowed to dry and then used in the same way as paper chromatography.

Gas-liquid chromatography is described below


2.4 Three techniques used in a particular and separation and purification procedure

e.g. How can we separate a mixture of sand and salt?

or, how do we separate a salt from a salt preparation?


dissolving-stirring, filtering, evaporation, crystallisingFiltration use a filter paper or fine porous ceramic to separate a solid from a liquid. It works because the tiny dissolved particles are too small to be filtered BUT any insoluble 'non-dissolved' solid particles are too big to go through!

Evaporation means a liquid changing to a gas or vapour. In separation, its removing the liquid from a solution, usually to leave a solid. It can be done quickly with gentle heating or left out to 'dry up' slowly. The solid will almost certainly be less volatile than the solvent and will remain as a crystalline residue.

Crystallisation can mean a liquid substance changing to its solid form. However, the term usually means what happens when the liquid from a solution has evaporated to a point beyond the solubility limit. Then solid crystals will 'grow' out of the solution because the solution is too concentrated for all the solid to remain dissolved at that temperature. Crystallisation is often done from a hot concentrated solution, because most substance are more soluble the hotter the liquid. Consequently on cooling a hot concentrated solution, crystals form as the solubility gets less and less.

These separation methods are involved in e.g. (1) separation of a sand and salt mixture or (2) salt preparations

(1) The sand/salt mixture is stirred with water to dissolve the salt. The sand is filtered off and washed with pure water to remove remaining traces of salt solution. The salt solution (filtrate) is carefully heated in a dish to evaporate the water and eventually the salt crystals form. Here the solvent is water, but other mixtures can be separated using the same sequence of procedures using a different solvent. e.g. copper and sulphur can be separated using an organic solvent like tetrachloromethane which will dissolve the sulphur (hazardous chemical solvent).

(2a) When the water insoluble base (e.g. a metal oxide) is dissolved in an acid, the excess solid base is filtered off and the filtrate solution heated to evaporate the water to produce the salt crystals.

(2b) Two solutions of soluble substances are mixed and react to form an insoluble salt. The insoluble salt is filtered off to separate it from the solution, washed with pure water to remove any residual salt solution. The solid is then removed from the filter paper and dried to give the pure dry insoluble salt.

Some important words-phrases to do with the above procedures.

A solvent is a liquid that dissolves things.

The solute is the solid that dissolves in a solvent.

A solution is a mixture of a liquid with something dissolved in it.

The technique of solvent extraction involves using a liquid to dissolve a solid to separate it from a mixture (e.g. in purifying salt in the experiment described above.

A saturated solution is one in which no more substance will dissolve in the liquid.

Soluble means the substance (gas, liquid or solid) dissolves in a liquid to form a solution.

Insoluble a substance won't dissolve in a particular liquid.

Remember, a solid may dissolve in one liquid but not in another.

See how these methods are used in making salts


  2.5 Miscellaneous Separation Methods and other apparatus uses
Separating funnel

using a separating funnel

How can we separate two liquids that do not mix?

Distillation, described above is used to separate miscible liquids that dissolve in each other. If two liquids do NOT mix, they form two separate layers and are known as immiscible liquids (e.g. oil/water). This is illustrated in the diagram on the left, where the lower grey liquid will be more dense than the upper layer of the yellow liquid and shows how you can separate these two liquids using a separating funnel. (particle picture on gas-liquid-solid page)

1. The mixture is put in the separating funnel with the stopper on and the tap closed and the layers left to settle out. 2. The stopper is removed, and the tap is opened so that you can carefully run the lower grey layer off first into a beaker. 3. This leaves behind the upper yellow layer liquid, so separating the two immiscible liquids.


How can we separate pieces of iron from a mixture of solids? e.g. in scrap metal or domestic waste?

A magnet can be used to separate iron from a mixture with sulphur (see below). It is used in recycling to recover iron and steel from domestic waster i.e. the 'rubbish' is on a conveyer belt that passes a powerful magnet which pluck's out magnetic materials.


Methods of collecting gases are on a separate web page. Includes the preparation of ammonia, carbon dioxide, sulphur dioxide, hydrogen and a cracking experiment.

Use of U tube to collect things in e.g. condensing out water in a combustion investigation Testing for water and carbon dioxide, the products of burning/combustion of a candle/hydrocarbon (c) doc b
Burette and pipette

topTheir use is included in Salt Preparations and titrations with the Acids, Bases, Salts, pH page. A burette is used to measure volumes accurately and a pipette is used to separate out an accurate volume of a solution from a bulk container of the solution.


Decanting is the simplest possible way of separating a liquid (pure or a solution) from an insoluble solid which has a density greater than water (i.e. > 1.0 g/cm3). The solid-liquid mixture is allowed to stand e.g. in a beaker, until all the solid settles out to the bottom of the container. Then the liquid is carefully poured off to leave the insoluble solid behind. However it is inefficient e.g. a small amount of liquid is always left in the solid residue and very fine solid particles take some time to settle out and any disturbance of the liquid can mix them in with the liquid being poured off. Wine may be served in a decanter to leave the undesirable solids behind - no good for bits of cork though, they float!

Solvent Extraction

In its simplest form these techniques involve using a liquid to dissolve a solid to separate it from a mixture. The extraction of pure salt from a sand-salt mixture is a simple example of the technique.

For more complex examples see the advanced level chemistry page. Advanced level chemistry - solute distribution between two immiscible liquids, partition coefficient , calculations and uses

Centrifuges and centrifuging

How can we separate fine particles of an insoluble solid from a liquid?

Centrifuges are devices or apparatus that can be used to separate insoluble materials (usually a solid) from a liquid, where normal filtration does not work well e.g. a suspension of very fine (tiny) solid particles. The centrifuge consists of carriage or glass tube holder, mounted on an electrically motor driven vertical axle. The carriage holds the balanced glass tubes of equal amounts of the solid-liquid mixture in each tube, all tubes initially in a horizontal position before the motor is switched on.  The tube carriage is rotated at high speed safely in a fully enclosed container. Unbalanced tubes can break with the extra vibration and this situation has a 'knock on' effect, quite literally, as other tubes are likely to shatter with the erratic high speed unbalanced motion. High velocity glass fragments are not good for you! On rapid rotation of the carriage the tubes whirl round horizontally and the centrifugal force causes the more dense insoluble material particles to move outwards, separating from the liquid. When rotation ceases the solid particles end up at the 'bottom' of the glass tubes with the liquid above. After the centrifuging operation the liquid can be decanted off and the solid is left at the bottom of the glass tube. You might be interested in the solid, liquid or both products depending on the context. Centrifuges come in all sizes and centrifuge technology has many applications in the separation of mixtures and the purification of materials.

If [ ] represents the glass tubes, the horizontal rotation situation is shown below ..

[solid/liquid] <== axle | carriage ==> [liquid\solid]

Uses-applications: In biology cells can be separated from fluids. A waste 'sludge' can be treated e.g. removing toxic solids from contaminated water from an industrial process. Milk can be separated from whey. Edible oils, wines and spirits can be cleaned or 'clarified' of solid impurities. Expensive oils and other fluids used as lubricants in machining metal parts in industry become contaminated with tiny metal fragments. The larger pieces of metal are easily removed by filtration or sedimentation (allowing to settle out) but the very fine metal particles can only be removed by using a centrifuge. This is likely to be a cheaper option than buying more machine fluid AND reducing pollution since the fluid is recycled leaving less waste to dispose of.


2.6 Why are Instrumental methods of detection and separation are useful?

Typical chemical tests are on a separate web page  *  Mass spectrometry

Instead of testing for chemicals using standard laboratory equipment such as test tubes etc. Special instruments have been developed to carry out such testing. These are quick, accurate and can be used on very small samples.

  • Elements and compounds can also be detected and identified using a variety of instrumental methods. Some instrumental methods are suited to identify elements while other instrumental methods are suited to the identification of compounds.

  • Instrumental methods are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small.

  • Mass spectroscopy can be used to identify elements and their relative ratio of isotopes and for molecules it can help to determine a molecular structure (its expensive, and nmr is much better for molecular structure analysis - especially organic molecules, see below).

    • The atoms or molecules are vapourised and converted to positive ions (based on a single atom or molecular fragment)  by bombardment with high energy electrons. The gaseous ions (e.g. Na+ or CH3+ etc.) are analysed according to their mass in a powerful magnetic field. 

  • Atomic emission spectroscopy can be used to identify elements and analyse element mixtures

    • Basically atomic spectroscopy is about 'exciting atoms' with heat or electrical energy until they emit the absorbed energy as visible light. You see this effect when fireworks go off, most of the colour comes from the 'excited' metal atoms in the salts added to the explosive powder mixture.

    • In a simple way flame colour tests in the school laboratory are used to identify elements e.g. sodium is yellow, barium green etc. BUT these colours are formed from many specific frequencies of visible light added together, so how do you sort out e.g. two shades of greens from copper or barium?

    • The answer is that detailed analysis of the different emitted frequencies of visible light (e.g. using a (c) doc b prism) gives a 'finger print pattern' by which to identify elements.

    • AND the greater the relative intensity of light frequency the more there is of that element.

    • So atomic spectroscopy is used to identify elements and analyse a mixture of elements or detect traces of elements in a solid or solution.

    • This analytical method has many applications e.g.

      • Its used in the steel industry to monitor the composition of steel as the molten mixtures are being made

      • Astrophysicists can identify elements in distant stars from the light emitted.

      • Tiny traces of metal ions can be detected in water e.g. for pollution monitoring.

  • Nuclear magnetic resonance spectroscopy (nmr) is one of the most powerful analytical tools for determining the molecular structure of an organic compound.

    • Its very expensive for routine analysis but is invaluable in designing and analysing new molecules or finding the structure of natural molecules that the drug industry might find useful in developing new pharmaceutical products.

  • topInfra-red spectroscopy can help to determine molecular structure and identify an organic compound.

    • Each molecule has a 'fingerprint' pattern of absorption of different infrared frequencies. Can be used to determine alcohol concentrations in breath!

  • Ultra-violet spectroscopy can be used to the determine purity or concentration of solution of a substance that absorbs uv light.

  • Gas-liquid chromatography (gc/glc/g.l.c.) can be used to analyse liquid mixtures which can be vapourised (e.g. petrol, blood for alcohol content). The instrument is called a gas chromatograph.

    • a picture of 'glc': diagram a gas chromatogram and the resulting chromatograph

    • A sample of the substance under investigation is injected and vapourised into a tube containing a carrier gas (called the mobile phase, it moves). The gas carries the vaporised substance through a long 'separating' tube or column wound around inside a thermostated oven.

    • The substances in the mixture are partially absorbed by an absorbent material held in the or column (called the immobile phase or stationary phase, which doesn't move), but only temporarily. However different substances are held back, or 'retained', for different times so that the mixture separates out in the carrier gas stream.

      • There is a dynamic equilibrium between the stationary and mobile phases and the separation of the components of a mixture by chromatography depends on the distribution of the components in the sample between the mobile and stationary phases.

      • The column is filled with a porous solid so gas can get through but passes over a large surface area OR it is coated in a very high boiling organic liquid which can also provide a large absorbing surface but still allows gas flow.

    • The gases emerge from the oven into a detector system which electronically records the different signal as each substance comes through. A printout or computer display of the results from the gas chromatograph, called the gas chromatogram, shows a series of peaks in the graph line imposed on a steady baseline when only the carrier gas is passing through the detector.

    • The time it takes for a substance to come through is called the retention time and is unique for each substance for a particular set of conditions (flow rate, length of separating column, nature of separating column material, temperature etc.). Generally speaking, the greater the molecular mass of the mixture molecule, the longer the retention time. This is because the component molecule - immobile phase intermolecular force of attraction increases with the size of the component molecule, so it is absorbed/retained temporarily a bit more strongly (see right of diagram).

    • topThe height of the peak, or more strictly speaking, the area under the peak, is proportional to the amount of that particular substance in the mixture.

    • Therefore it is possible to identify components in a mixture and calculate their relative proportions in the mixture.

    • The chromatogram shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in petrol (in reality it is far more complicated with dozens of hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions i.e. hexane >pentane > heptane.

    • The retention time order follows the trend of increasing molecular mass gives increasing retention time i.e. in time heptane C7H16 > C6H14 > C5H12

    • The gas chromatographic instrument can be calibrated with known amounts of known substances.

    • Don't confuse with 'non-instrumental' paper/thin layer chromatography.

  • Industry requires rapid and accurate methods for the analysis of its products. There have also been increasing demands from society for safe and reliable monitoring of our health and environment. The development of modem instrumental methods has been aided by the rapid progress in technologies such as electronics and computing.

  • Various factors have influenced the development of instrumental methods. With modern methods you get ...

    • greater sensitivity i.e. smaller amounts of material can be used OR much smaller amounts of a trace element or compound can be detected in a bulk mixture (drug testing of athletes)

    • more accurate data (perhaps analysed by computer)

    • automation of analysis, multi-samples efficiently analysed

    • a greater range of analytical techniques, today's laboratory is far more versatile these days

    • greater reliability and consistency once the instrument is set up and procedures in place and checked.

keywords: centrifuges/centrifuging * glc g.l.c chromatography (paper/thin layer) * crystallisation * decanting/decantation * distillation (simple/fractional) * evaporation * filtration * formula * impure/pure * insoluble * mixture * molecule * naming compounds and ions * precipitation * products * pure substance * purification * sand/salt separation * separating funnel * separating mixtures * soluble solution solvent solute * solvent extraction

See other web page for:

(c) doc b KS3 Science GCSE/IGCSE Chemistry States of Matter - kinetic particle theory of Gases, Liquids and Solids revision notes

(c) doc b KS3-GCSE/IGCSE Types of Chemical Reaction revision notes

(c) doc b GCSE/IGCSE Chemical Bonding revision notes (ionic, covalent, metallic etc.)


GCSE balancing and completing equation exercises:

(1) multiple choice * (2) number/word multi-fill

GCSE 'name and formula' of a compound quizzes

(1) pick the name given the formula * (2) pick the formula given the chemical name

GCSE/IGCSE formula quiz given the name, type in the formula

GCSE/IGCSE name quiz given the formula, type in the name

KS3 Science-GCSE/IGCSE Elements, Compounds & Mixtures m/c QUIZ

KS3 Science-GCSE/IGCSE element name/symbol quiz

Revision KS4 Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry O Level Chemistry (revise courses equal to US grade 8, grade 9 grade 10) KS3 science revision notes

topWebsite content copyright Dr W P Brown 2000-2013 All rights reserved on revision notes, puzzles, quizzes, worksheets, x-words etc. * Copying of website material is not permitted

Teach yourself chemistry online ALPHABETICAL SITE INDEX for chemistry

Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

BIG SITE HELP SEARCH Enter several specific words/formula etc.