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Methods of Purification

Methods of Purification

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Methods of Purification

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  1. Methods of Purification (a)Describe methods of separation for the components of the following types of mistures: solid-solid solid-liquid liquid-liquid(miscible) Techniques to be covered for separations and purification include: Use of a suitable solvent, filtration and crystallization or evaporation Distillation and fractional distillation Paper chromatography (b)

  2. Pure Substances & Mixtures A pure substance contains only one type of substance, and is not mixed with any other substance. Are the following pure substances or mixtures? • Clean tap water • 100% orange juice • Distilled water mixture mixture Pure substance

  3. Methods of Purification Some terms : • Solute - the solid that dissolves • Solvent- the liquid that does the dissolving • Solution- solid + solvent • Residue- the insoluble solid trapped in the filter paper • Filtrate - the liquid that passes through the filter paper • Sublimate - the condensed solid produced during sublimation

  4. Methods of Purification The particular method used for separating any given mixture depends on the nature (eg. solubility, physical state) of its constituents.

  5. mixture of solid and liquid residue filter funnel filter paper filtrate Filtration • Used to separate a solid from a liquid in which the solid is insoluble

  6. Principle of Filtration • The separation works only if the particles concerned are of different sizes, such as larger particles of an insoluble solid and smaller particles of a liquid or solution.

  7. Principle of Filtration The filter material acts as a sieve which allows the smaller particles to pass through and keeps the bigger particles behind.

  8. Principle of Filtration Can a mixture of common salt and water be separated by filtration? Why?

  9. Filtration - large scale use • Sand filters used in water treatment plants to remove solid impurities • Dissolved substances and bacteria will not be removed

  10. Decanting • Separates an insoluble solid from a liquid • liquid is carefully poured away from the solid which is usually heavier and settles at the bottom of the container.

  11. Evaporation (Heating )to Dryness • used to recover a soluble solid from its solution • Only for solids that will NOT decompose on heating. • Eg. sodium chloride (common salt) from salt solution

  12. Crystallisation • used to recover a soluble solid from its solution • for solids that decompose on heating • Eg. Copper(II) sulphate and most other salts Steps : • The solution is heated (evaporated) to saturation point OR ‘heated to remove most of the solvent’ • The saturated solution is left to cool; crystals are formed. • The crystals are removed by filtration. To purify the crystals, they can then be washed with cold distilled water and dried between filter papers.

  13. Crystallisation - the Principle behind • Substances are usually more soluble in hot water than cold water eg. more copper(II) sulphate will dissolve in water at 80ºC than at 30 ºC. • When the hot saturated solution is cooled, the cooled solution is unable to hold as much solute as when it was hot. The extra solute that cannot remain dissolved appears as crystals.

  14. Solubility & Crystallisation Solubility data for NH4Cl in grams/100 mL H2O • 30oC -  41.4 g •  50oC -  50.4 g •  70oC -  60.2 g •  90oC -  71.3 g • If the water was heated to 90°C, how much of the solute will be able to dissolve? • If this hot solution is then cooled to 30 °C, how much solute can the water contain now? • Hence what is the mass of crystals you would expect to obtain?

  15. Separating a mixture of solids Method used depends on the nature of the solids :- • If the mixture of solids behave differently in a particular solvent , that is, one component is soluble in it while the other is insoluble… carefully choose a solvent that will dissolve only one of the solids Egs. common salt and sand; naphthalene and sand Use water as solvent Use methylated spirits as solvent

  16. Separating a mixture of solids Basic Steps : Evaporation to dryness Dissolution Filtration OR Crystallisation

  17. Separating a mixture of solids (2) Sublimation used when one of the solid sublimes Eg. • mixture of iodine + copper; • mixture of ammonium chloride + sodium chloride

  18. Separating a mixture of solids (3) Other methods : • make use of the special properties of the substance Eg. mixture of iron filings and sulphur …. …use a magnet • Bring a magnet to the mixture. • The iron filings will be attracted to the magnet while the sulphur will be left behind.

  19. solute solvent solution Separating the Solvent from the Solution Crystallisation Distillation Evaporation to dryness

  20. Simple Distillation • Some Terms Used : • Distillate - the liquid that distils over • Miscible liquids - liquids that mix completely to form a single layer • Immiscible liquids - liquids that do not mix • A solvent can be separated from a solution and collected by simple distillation.

  21. Principle of Distillation A liquid boils and turns into vapour at its boiling point. When the vapour is condensed, the (pure) liquid is obtained again.

  22. Simple Distillation 3. the condenser is cold, sothe vapour condenses to liquidwater. 2. .. vapourise. The vapour rises up the flask thermometer flask sea water condenser Boiling chips 4. Pure water drips into the beaker. It is distilled water. 1. Solution is heated, causing the solvent to … distillate

  23. thermometer flask sea water condenser Boiling chips Simple Distillation Water out To maintain even boiling, with not too much bumping Water in

  24. Liebig Condenser Condensed vapour in liquid form (distillate) leaves Vapour enters Direction of water flow Water out Cold water in Water flows in anti-current to the flow of vapour.

  25. Liebig Condenser Condensed vapour in liquid form (distillate) leaves Vapour enters Direction of water flow Water out Cold water in This is to make sure the coldest part of the condenser is just before the vapour escapes.

  26. Simple Distillation Simple distillation can be used to obtain • from salt solution • from copper(II) sulphate solution • from a solution of sugar in ether water water ether Note : The liquid that distils over is called the distillate . The solid that remains in the flask is called the residue .

  27. thermometer flask sea water condenser Boiling chips distillate Simple Distillation Qns. : Where is the thermometer placed? What is the reason for this?

  28. thermometer flask sea water condenser Boiling chips distillate Simple Distillation Thermometer placed at the side arm of the flask so that it records the temperature of the vapour as it enters the condenser.

  29. Separating miscible liquids - Fractional distillation • miscible liquids can only be separated by fractional distillation if they have different boiling points . • Eg. mixture of ethanol and water.

  30. Separating miscible liquids - Fractional distillation thermometer Fractionating column Water out condenser flask distillate Water in Boiling chips mixture of ethanol and water

  31. Separating miscible liquids - Fractional distillation 3. Eventually, the liquid with the lower boiling point reaches the top and distils over. 2. The fractionating column is packed with glass beads to increase its surface area. Vaporisation followed by condensation takes place many times as the vapour is swept upwards. 1. When heated, the liquid with the lower boiling point will vaporize more readily.

  32. Separating miscible liquids - Fractional distillation 4. The temperature stays constant at 78°C. When all the ethanol has distilled over, the temperature reading rises above 78°C. At 100°C, water starts to distil over. 5. The receiver is changed to collect each distillate separately.

  33. Separating miscible liquids - Fractional distillation Sketch a graph of temperature versus time to show the changes in temperature readings throughout the distillation. temperature 100°C 78°C time

  34. Separating miscible liquids - Fractional distillation • Note : • The glass beads in the fractionating column provides a large surface area so that condensation occurs more readily. • The liquid with the lower boiling point distils over first, followed by the liquid with the next higher boiling point. glass beads

  35. Separating miscible liquids - Fractional distillation • Note : • If the liquids in the mixture have the same boiling point, fractional distillation is not possible. • If the difference in boiling point is great, fractional distillation occurs readily. glass beads

  36. Fractional distillation - applications • Fractional distillation can be used to separate :- • nitrogen and oxygen from liquid air • the components of crude oil • ethanol from fermented liquor

  37. Fractional distillation of crude oil

  38. Separating immiscible liquids • A separating funnel can be used to separate two immiscible liquids. • Eg. water and petrol. • This method can be used to separate : • a mixture of petrol and water • engine oil and water

  39. liquid + liquid solid + solid solid + liquid (Solid INSOLUBLE in liquid) The main points so far… Mixtures Filtration Residue (solid) Filtrate (liquid)

  40. liquid + liquid solid + solid solid + liquid (Solid SOLUBLE in liquid) The main points so far… Mixtures Simple Distillation Residue (solid/ solute) Distillate (liquid/ solvent)

  41. liquid + liquid YES solid + solution The main points so far… Mixtures solid + solid Difference in solubility? Add suitable solvent Filtration Crystals (soluble solid) Residue (insoluble solid) Filtrate (solution) Crystallisation

  42. liquid + liquid NO Egs. The main points so far… Mixtures solid + solid YES Difference in solubility? solid + solution Add suitable solvent Filtration Sublimation Crystallisation Other physical difference? Using magnet

  43. liquid + liquid The main points so far… Mixtures miscible liquids (different boiling pts.) Fractional distillation immiscible liquids Separating funnel