Separation TechniquesSeparation Techniques

LSS 1 Term 3LSS 1 Term 3

Mixtures and Pure SubstancesMixtures 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

Is water a mixture?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.

Pure substancesPure 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.

Separation TechniquesSeparation 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.

Separation TechniquesSeparation Techniques

• Filtration

• Evaporation (to dryness)

• Distillation (Simple and Fractional)

• Chromatography

• Magnetic Attraction/ Separation

• Crystallization

• Separating immiscible liquids

DiscussionDiscussion

• 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

Separating Immiscible LiquidsSeparating 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

• To separate two immiscible liquids, a separating funnel is used.

Separating Immiscible LiquidsSeparating Immiscible Liquids

Separating Immiscible LiquidsSeparating Immiscible LiquidsSeparating 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.

Limitations of Evaporation to Limitations of Evaporation to Dryness as a purification methodDryness 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.

CrystallizationCrystallization

• 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.

Procedure of CrystallizationProcedure 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.

Procedure of CrystallizationProcedure of Crystallization

Applications of CrystallizationApplications 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.

Magnetic AttractionMagnetic 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.

Further Applications of Magnetic Further Applications of Magnetic AttractionAttraction

• Magnetic Filters to remove metallic particles from lubricating oil in cars and machineries

Simple vs Fractional DistillationSimple 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.

Fractional DistillationFractional Distillation

How fractionating column worksHow 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

How ethanol makes its way to How ethanol makes its way to condenser ahead of watercondenser 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.

How ethanol makes its way to How ethanol makes its way to condenser ahead of watercondenser 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.

ChromatographyChromatography

Chromatography procedureChromatography procedure

• http://www.sambal.co.uk/chromatography.html