Separation Techniques

1. Separation Techniques LSS 1 Term 3

2. Mixtures and Pure Substances • Recap: How do we define mixture? • A mixture contains two or more constituent substances which are not chemically combined together. • A mixture retains the properties of its constituents. • Examples include: air, seawater, alloys like brass, bronze and steel

3. Is water a mixture? • Water from sea? • Mixture of sand, salt, dead matter, etc. • Water from tap? • Solution of small amounts of minerals • Click here to find out what’s in our tap water. • Deionized water? • Purified water formed by removing ions present in water source • Click here to find out more.

4. Pure substances • Pure substance is a single substance not mixed with anything else. • Most things around us are not pure substances. • Eg. Gold used in jewellery • Purification processes need to be used to obtain pure substances.

5. Separation Techniques • Separation techniques refers to the physical methods that can be used to separate the constituents of mixtures. • Sometimes, a combination of separation techniques will be used to obtain pure substances. • Separation techniques are essentially methods of purification.

6. Separation Techniques • Filtration • Evaporation (to dryness) • Distillation (Simple and Fractional) • Chromatography • Magnetic Attraction/ Separation • Crystallization • Separating immiscible liquids

7. Discussion • With the help of diagrams, describe the following separation techniques and the kind of mixture (solid-solid, solid-liquid, etc.) it can be used to separate: • Filtration • Distillation • Evaporation to dryness

8. Separating Immiscible Liquids • ‘Miscibility’ describes how well two substances mix together. • Two liquids are immiscible if they do not mix with each other. • This will give a two-layered liquid mixture. • Question: Which liquid will be on top of the other? • Example: Oil and water

9. Separating Immiscible Liquids • To separate two immiscible liquids, a separating funnel is used.

10. Separating Immiscible Liquids • Separating Petrol from Water • Immiscible liquid mixture is poured into separating funnel. • Allow some time for the liquid mixture to settle and form two distinct layers. • Open tap to allow the lower water layer to run out into a beaker. • The tap is closed as the last drop of water runs out. • The tap is then opened to allow petrol to run out into another beaker.

11. Limitations of Evaporation to Dryness as a purification method • Many substances decompose when heated strongly. • Eg. Sugar decompose to give water and carbon (black substance) on heating strongly. • Most crystals give off water to become powders when heated. • Soluble impurities are left behind with the crystal/powder form of the substance when water is evaporated off.

12. Crystallization • The best method for separating a pure solid sample from its solution, i.e. to obtain pure crystals of the solute from the solution it is dissolved in.

13. Procedure of Crystallization • Heat solution until most of the solvent has evaporated off. A hot saturated solution is obtained. • Allow saturated solution to cool. On cooling, solute appears as pure crystals while impurities stay in the solution. • Filter and Dry crystals by pressing it between pieces of filter paper.

14. Procedure of Crystallization

15. Applications of Crystallization • Used to purify crystals and substances that decompose on strong heating • Industrial Application: • Used in production of silicon wafers for microchips (the Czochralski Method) • Extraction of pure chemicals like sodium chloride and potassium chloride from the mixture of dissolved chemicals from Dead Sea using fractional crystallization.

16. Magnetic Attraction • Magnets can be used to separate a magnetic metals from a mixture of solids. • Examples of magnetic metals: iron, nickel, cobalt. Steel is a magnetic alloy containing iron. • Method widely used in removal of magnetic materials from domestic waste for recycling.

17. Further Applications of Magnetic Attraction • Magnetic Filters to remove metallic particles from lubricating oil in cars and machineries

18. Simple vs Fractional Distillation • Simple distillation is used to separate solvent from a non-volatile solute (i.e. solute with high boiling point). Eg. Water from salt solution. • Fractional distillation is used to separate mixture of miscible liquids, eg. Mixture of ethanol and water.

19. Fractional Distillation

20. How fractionating column works • May be filled with glass beads, plates or a spiral. • These provide a large surface area for vapour to condense on. • Liquid with lowest boiling point will distill over to the condenser first • Vapours of liquids with higher boiling points condense along fractionating column and re-enters the round-bottomed flask

21. How ethanol makes its way to condenser ahead of water • Animation • Bp of ethanol = 78oC, Bp of water = 100oC • Vapour from boiling mixture contains larger percentage of lower boiling point ethanol. • As vapour mixture moves up fractionating column, it repeatedly condenses and evaporates inside the column. • Each time the mixture evaporates, percentage of lower boiling point ethanol increases.

22. How ethanol makes its way to condenser ahead of water • By the time vapour reaches top of fractionating column, it has become almost pure ethanol • This vapour then passes into the condenser where it is cooled, and condenses into liquid ethanol. • Thermometer will show temperature of 78oC until all ethanol has distilled off.

23. Chromatography

24. Chromatography procedure • http://www.sambal.co.uk/chromatography.html