Separation Techniques Dissolution & Filtration Evaporation Crystallisation Sublimation Using a separating funnel Distillation Fractional distillation Chromatography

Dissolution Process whereby a suitable solvent is used to dissolve a substance present in a mixture. E.g. Adding water to a mixture of salt and chalk allows the salt to dissolve.

Filtration Method used to separate large insoluble solid particles from a liquid / solution / gas. E.g. Sand and salt solution Dust from air Apparatus used : Filter funnel Filter paper Beaker / Conical flask

The filter paper has tiny pores that allow small solvent / liquid / solute particles to flow through but trap larger solid particles. The solid particles trapped within the filter paper is called the residue, while the solution / liquid which passes through the filter paper is called the filtrate.

Evaporation Involves the removal of solvent molecules in solution by heating a solution until only the solute particles remain i.e. evaporation to dryness . This is only done if the solute is involatile / does not decompose on heating. e.g. sodium chloride Or involves the removal of solvent molecules in a solution by heating a solution until it is saturated, followed by crystallisation. Crystals of the salt are obtained.

Experimental set-up : Salt solution Beaker Water Heat

Crystallisation Method of obtaining crystals of a salt by cooling a hot, saturated salt solution. If the saturated solution is cooled very fast, small crystals are formed. On slow cooling, large crystals are formed. To form large & well-shaped crystals, a cold saturated solution is allowed to evaporate slowly.

Crystallisation is one of the purest means of purification. Crystallisation is used in : - Obtaining salt from sea-water. - Obtaining sulphur. - Obtaining pure sugar from industries. - Obtaining very pure silicon used in computer chips.

Sublimation Involves the conversion of a solid into a vapour without the solid first melting. It is used to obtain solids which sublime from a mixture of solids. The mixture of solids should be dry. Solids that sublime are : iodine, naphthalene, iron (III) chloride, ammonium chloride, solid carbon dioxide & alumininum chloride, sulphur

Experimental Set-up : Heat gently

Question Describe how you would separate a mixture of ammonium chloride, sodium chloride and sand. Carry out sublimation until all the ammonium chloride is collected on the inverted filter funnel. Scrape and collect these crystals. The remaining mixture contains sodium chloride and sand.

Add sufficient / excess water to the mixture to dissolve the sodium chloride. Stir / warm. Filter the mixture. The residue is sand. The filtrate is sodium chloride solution. Rinse the residue with distilled water. Dry the residue between pieces of dry filter paper to obtain pure dry sand. Evaporate the filtrate to dryness to obtain pure dry sodium chloride.

Using a Separating Funnel Used to separate a mixture of two or more liquids that are immiscible e.g. oil and water. The denser liquid settles at the bottom of the funnel.

Experimental set-up :

Distillation Used to separate a mixture of miscible liquids whose boiling points are far apart OR To separate a solvent from a solution. e.g. pure water from sea water

Apparatus : Distillation flask Thermometer Liebig condenser Boiling stones The solvent molecules evaporate on heating and move into the condenser. The gaseous solvent molecules condense to form liquid within the condenser. The liquid flows out & is collected. This liquid is called the distillate.

The Distillation Apparatus

The thermometer reading remains constant as the liquid distils off. The thermometer reading shows the boiling point of the distillate.

Cool water flows into the condenser from the bottom and flows out from the top of the condenser. This is to ensure efficient condensation of hot vapour as the water jacket of the condenser is completely filled with cool water and residual vapour can be condensed before it leaves the condenser. Boiling stones are used to ensure smooth boiling of the solution mixture.

Fractional Distillation Used to separate a mixture of miscible liquids whose boiling points are fairly close e.g. ethanol and water Apparatus : Round Bottom flask Fractionating column Thermometer Liebig condenser Boiling stones

The solvent molecules evaporate on heating and move into the fractionating column. Cool glass beads in the column allow the gaseous solvent molecules to condense to form liquid within the column. As the temperature in the column rises, the liquid with the lowest boiling point vaporizes and distils off. The other liquids are trapped by condensation within the column. The liquid with the lower boiling point distils off and collects as distillate. The temperature rises and the next liquid distils off.

Chromatography Used to separate and identify components of a complex mixture. Advantages : Very accurate – trace quantities of impurities can be detected Very complex mixtures can be analysed Only small amounts of the mixture is required for analysis

Technique is based on DIFFERENTIAL SOLUBILITY of the solutes present in the mixture within the solvent used. The component MOST soluble in the solvent moves the furthest away from the start line. The start line is marked in pencil as pencil lead is insoluble in solvents. Ink, if used, may separate out & affect the results of chromatography.

The spots marked (at the start line) should be above the solvent level to prevent the spots from dissolving in the solvent prior to the start of the experiment. Longer strips of filter paper allow a clearer separation of spots i.e. spots will be further apart from each other. Spots must be as small as possible to prevent overlapping of spots.

Analysis of a chromatogram : The tube or vessel used to carry out chromatography is usually stoppered to prevent evaporation of the solvent used. Analysis of a chromatogram : Single spot formed implies a pure substance Two or more spots formed imply presence of a mixture. If original spot does not move above the start line, the substances in the spot are insoluble in the solvent used.

Separation of Screened Methyl Orange Paper Chromatography Separation of Screened Methyl Orange

Paper Chromatography Identify the colours X and Y based on the results of the previous slide.

Rf is a value used to identify the components in a mixture. The Rf value of a component is fixed for a particular solvent. The Rf value may vary if different solvents are used. Rf of a spot = Distance travelled by spot from start line Distance travelled by solvent front from start line

Applications of chromatography: In urine analysis – to detect drugs used to enhance performance at sports. In food industry – to ensure safety standards of artificial dyes used in food.