0-5 Separation Technique

V. Separation Technique

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Topic

V.

Separation Technique

Reading Assignment

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 202 – 215

Objectives

0.5 - 0.6 Methods of separation Test of purity

Notes

A. Methods of separation Most naturally occurring materials are mixtures. e.g. sea water is a mixture of water and salt, air is a mixture of different kinds of gas. Usually, a material must be purified before it can be used. e.g. sea water is not drinkable but distilled water is. Different kinds of purification technique / separation methods are required to purify different mixture. They also provide us the opportunity to experience different kinds of laboratory technique. In a mixture, different substances still retain their own physical properties. By using these differences in physical properties, different components may be separated. e.g. salt solution - salt NaCl (b.p. 1413 ºC) and water H2O (100 ºC) have very different boiling points. Therefore, they can be separated by either evaporation, distillation or fractional distillation. However, before you can choose an appropriate method, you must be very familiar with the properties of individual components in the mixture. Method of separation

Apparatus required

Filtration

Essential :

Filter funnel, filter paper

Decantation

Essential : Optional : Essential :

Nil Glass rod Centrifuge, centrifuging tube Separating funnel

Centrifugation Using separating funnel Evaporation

Distillation

Essential : Essential : Optional : Essential :

Fractional distillation

Essential :

Sublimation

Essential :

Bunsen burner, evaporating dish / basin / Watch glass Steam bath Bunsen burner, water condenser, thermometer Bunsen burner, water condenser, thermometer, fractionating column Evaporating basin, funnel

The difference in physical properties of individual component that the separation rely on. Particle size of solid Big difference in density Small difference in density Density of immiscible liquid Big difference in boiling point Big difference in boiling point Small difference in boiling point Sublimation point of solid

Applicable example

water (small particles) and sand (big particles) water (low density) and sand (high density) water (low density) and flour (high density) water (higher density) and peanut oil (lower density) water (low b.p. 100 ºC) and salt (high m.p. 1413 ºC) water (low b.p. 100 ºC) and salt (high m.p. 1413 ºC) alcohol (low b.p. 78 ºC) and water (high b.p. 100 ºC) iodine (low s.p. 187 ºC) and sand (high s.p. 2230 ºC)

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Method of separation

Apparatus required

Crystallization

Essential : Optional :

Nil Steam bath

Paper chromatography

Essential :

Filter paper, a suitable carrying solvent

The difference in physical properties of individual component that the separation rely on. Solubility

Solubility in the solvent

Applicable example

sugar (high solubility when hot, low solubility when cold) Colour A is more soluble in water but Colour B is less soluble in water.

Sometimes, more than one method would be used together to separate a mixture. For example, to separate a mixture of sand and salt, dissolution, filtration and evaporation would be required to obtain the pure salt. 1.

Filtration

The holes on a filter paper is so small that it can trap any particles bigger than bacteria. The big particles will be retained on the top of the filter paper as residue. The solution passing through the filter paper is called filtrate.

2.

Decantation

Since sand is denser than water, upon standing, sand will settle at the bottom of the beaker. The water can then be decanted carefully from the beaker. Decantation is only a kind of preliminary purification. It cannot purify the water completely.

V. Separation Technique 3.

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Centrifugation

Although mud is denser than water, the difference is so small that mud does not settle down by itself upon standing. A centrifuge creates a gravity larger than normal. By putting the mud solution into a rotating centrifuge, the mud particles will settle down at the bottom. 4.

Using separating funnel

Separating funnel is used to separate two immiscible liquids with different density.

5.

Evaporation

Since salt (1413 ºC) has a much higher boiling point than water (100 ºC), when salt solution is heated, only the water will evaporate and the salt will be left behind.

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Distillation

Distillation is similar to evaporation. The only difference is that the vapour is collected by condensation. The bulb of the thermometer is fixed just next to the delivery tube to measure the temperature of the vapour to be condensed. The condensed liquid is called distillate. Antibumping granules are usually added to ensure smoother heating.

Distillation setup using Quickfit apparatus 7.

Fractional distillation

A mixture of water and alcohol cannot be separated satisfactorily by simple distillation. This is because they have only small difference in boiling point. (b.p. of water 100 ºC, b.p. of alcohol 78ºC) When the mixture is heated, say 70ºC, both water and alcohol will evaporate. The presence of the fractionating column provides cool surfaces to condense the less volatile component (water) in the vapour. Eventually, only the more volatile component (alcohol) can reach the top of the fractionating column and pass through the condenser.

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Sublimation

A few solids turn directly into gas upon heating without going through the liquid state. This kind of solid can be separated from the mixture by heating the mixture above the sublimation point of the solid and providing a cool surface for it to condense. In a mixture of iodine and sand, iodine has a sublimation point 187ºC and sand has a sublimation point 2230 ºC. Upon heating, only iodine will sublime. Other examples : ammonium chloride, iron(III) chloride. 9.

Crystallization A. Crystallization by cooling of saturated solution

The amount of solute that can be dissolved is depending on the temperature of the solvent and the volume of solvent. In general, the higher the temperature and the larger the volume of the solvent, more solute can be dissolved. A solution which holds the maximium amount of solute that it can hold is called saturated solution. If a saturated solution is cooled down or evaporated, the solution will not be able to hold the original amount of solute and the solute will crystal out. Meanwhile, the impurities in the solution remain in solution form and will not mix with the pure crystal. Pure crystal will be obtained.

B. Crystallization by cooling of saturated solution

Furthermore, the slower the rate of crystallization, the bigger will be the crystal.

10. Paper chromatography Paper chromatography can be used to separate a mixture of dyes. Because different dyes have different solubility in a given solvent, they will move at different speeds in the rising solvent. The chromatogram obtained can be used to identify the components in a mixture. DNA fingerprinting is a technique using chromatography.

Chromatogram

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B. Testing of purity 1.

Determination of melting point

A pure substance has a sharp melting point but a mixture does not melts sharply. According to this behaviour, the purity of a substance can be tested. Furthermore, the presence of impurity will also lower the melting point of the mixture. Example 1 Pure ice melts at 0ºC but a mixture of salt and ice melts at about -20ºC to 25ºC . This phenomenon is called depression of melting point by an impurity. Example 2 Pure octadecan-1-ol melts at 58ºC and pure napth-1-ol melts at 120ºC. Experimentally, pure octadecan-1-ol will start to melts at 57.5ºC and when the temperature reaches 58.5ºC, all solid will be melted. If it is mixed with a little napth-1-ol, the mixture will start to melts at 50ºC and will melts completely at 55ºC. By knowing whether a substance melts sharply or not, the purity of the substance can be tested. A pure substance only has a very narrow melting range. 2.

Determination of boiling point

Unlike melting point determination, a sharp boiling point may not represent a pure sample. (This will be discussed in A-Level). However, boiling point can be used as a kind of positive confirmation of the identity of an unknown sample. For example, if the boiling point of the liquid is 103ºC, the liquid must not be water.

Glossary

mixture physical properties filtration residue filtrate decantation centrifugation centrifuge centrifuging tube separating funnel immiscible evaporation evaporating basin distillation antibumping granules water condenser Quickfit apparatus fractional distillation volatile fractionating column sublimation sublimation point ammonium chloride iron(III) chloride solvent extraction saturated solution solubility crystallization paper chromatography chromatograph adsorption activated charcoal

V. Separation Technique

Past Paper Questions 90 30 A

93 42 B

30 Which of the following methods can be used to obtain sodium chloride from a solid mixture containing sodium chloride and ammonium chloride? A. heating B. adding water and filtering C. adding sodium hydroxide solution and filtering D. adding silver nitrate solution and filtering

42

Which of the following solids can be purified using the above set-up ? A. potassium iodide B. ammonium chloride C. lead(II) bromide D. sodium hydrogencarbonate 98 26 B

26 Some physical properties of a compound X are listed below: melting point : 82°C boiling point : 221 °C solubility in water: soluble Which of the following is the most appropriate method to obtain X from a solution of X in water ? A. decantation B. crystallization C. fractional distillation D. paper chromatography

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