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

ELEMENTS, COMPOUNDS, MIXTURE  separation, CHEMICAL REACTIONS & EQUATIONS

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  *  covalency *  crystallisation  *  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  *

2.1 Simple Distillation

How 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.

How 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).

FRACTIONAL DISTILLATION THEORY:

Imagine green liquid is a mixture of a blue liquid (boiling point 80^oC) and a yellow liquid (boiling point 100^oC), 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.

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

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

Filtration 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

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.

 MAGNET

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.

Their 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!

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

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.