Separation and Purification Techniques

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But what if… Sugar Sand How to separate them?

Lesson Objectives At the end of the lesson, students should be able to: Name the 3 methods in which we can test for pure substances Identify the effect of impurities on a substance. Choose suitable separation techniques to separate a given mixture. Explain the working principles behind each separation technique.

To test for Purity of a Substance Methods of Testing Observation Melting point If a substance is pure, it will melt at a fixed temperature. Boiling point If a substance is pure, it will boils at a fixed temperature. Use of chromatography If a substance is pure, it will show only one spot on the chromatogram.

Effect of impurities in a substance Attributes Effect of impurities Melting point A substance melts below its melting point. Boiling point A substance boils higher than its boiling point. Use of chromatography Gives rise to more than one spot on the chromatogram. Salt can be used to melt ice that freezes a car to the driveway by sprinkling a thin layer over the car and around the tires, and the salt will react chemically with the snow to melt it away.

Physical Methods of Separation Only separate the different substances in a mixture. No new substance is formed. The choice of separation technique depends on the nature of the mixture.

Common physical methods to separate a mixture Paper chromatography Filtration Crystallisation Simple and fractional distillation

Filtration Solid-Liquid Mixture

Filtration Solid-liquid mixture (insoluble solids) E.g. sand in water A filter paper is used because it contains very tiny pores. Mixture of solid and liquid

Applications NEWater – purification of water using microfiltration to remove impurities.

CRYSTALLISATION Separating soluble solids from a solution

Crystallisation To separate dissolved pure solid from a solution. Maximum amount of solid dissolved in a given solvent. 2. Saturated solution Evaporation of solution 3. Pure Crystals formed. 1. Heat the solution until saturated.

How to test for saturated solution? Dip a glass rod into the solution and removed. If crystals are formed on the glass rod, it means that the solution is saturated. This is the saturation point or crystallisation point.

Think!!! Why can’t we just evaporate the solution to dryness to obtain the crystals? For some substances, they will decompose when heated.  charring When water is removed, any soluble impurities will be left on the crystal  not pure.

Mixture of Sand and Sugar How do I get sand and sugar back? Filtration  Sand is the residue, filtrate is the sugar solution Crystallisation  pure sugar crystals Is the filtrate always a pure liquid?

Simple Distillation Separating liquid from a solution

Distillation To separate a liquid from a solution. E.g. salt solution, sugar solution Involves two physical state changes.

Set-up for distillation Main concept Boiling point of the liquid Bulb of thermometer placed beside the side arm of the distillation flask to ensure accurate measurement of boiling point. Liquid is heated until its boiling point and changes to vapour. 1 Vapour is cooled and changes to liquid (distillate). 2 The vapour is cooled in the condenser until the temperature falls below the boiling point when it condenses back into a liquid which is collected in a container. If the inlet is below the outlet then the water fills the entire volume of the condenser before getting to the outlet. If you set it up so the water inlet is above the outlet then the water will run down the side of the condenser and out again. This is all the work of gravity. The condenser will operate at it's most effective if it is completely full of water, this is because it works by heat transfer from the vapors in the condenser to the cool water in the condesner. Surface area to volume ratio is maximised in a full condenser and the larger the volume of water the more capacity for heat transfer, thus maximising cooling of the vapor. Mixture To ensure smooth boiling. Other examples: 1. Marble chips 2. Porcelain chips

Boiling chips To ensure smooth boiling During boiling of water, big air bubbles can be seen. These air bubbles causes ‘bumping’. Boiling chips can reduce this ‘bumping’ effect. Never add a boiling chip to a hot solvent, because it can cause immediate boiling over of the solution. If you forget to add a boiling chip before you begin, you must cool the solution before adding one to prevent product loss. Boiling chips cannot be re-used since the pores inside these stones become filled with liquid on cooling.

Temperature Profile Temperature remains unchanged until all the liquid boils off. Temperature as solution is heated

Disadvantage of Simple Distillation Unable to separate liquids who boiling point differ by less than 20°C. Use fractional distillation!

Fractional Distillation Separating miscible liquids

Fractional distillation For liquids that are miscible and having different boiling points. Miscible – mix together completely to form a solution. E.g. water and ethanol Liquid with lower boiling point distill over first. Liquids with higher boiling point will return back into the round-bottomed flask.

Set-up of fractional distillation Main concept Boiling point of the liquids where the liquid with the lowest boiling point will distill over first. Water outlet Thermometer Condenser Fractionating column Round-bottomed flask Water inlet Liquid-liquid mixture Distillate Boiling chips

During fractional distillation, The liquid with lowest boiling point will distill over to the condenser first. The vapour of liquids with higher boiling point condenses along the fractionating column and re-enter the round-bottomed flask.

Fractionating Column Packed with glass beads   Provide large surface area for evaporation and condensation of liquids with different boiling points  for effectively separation.

Temperature Profile Liquid with lowest boiling point will distill over first Second liquid distill over. First liquid distill over upon reaching its boiling point

Industrial Applications Separation of liquid air Separation of crude oil Separation of alcohol from fermented solution.

Think!!! What is the difference between distillation and fractional distillation? Simple distillation Fractional Distillation No fractionating column Has a fractionating column Separate liquids whose boiling points differs more than 20°C Able to separate liquids whose boiling point differs by less than 20°C

Think!! Is it possible to obtain salt from seawater? Yes However, industrially, we use reverse osmosis to obtain salt (Desalination). What is the reason? Distillation exhaust a lot of energy, resulting in high production cost.

TYPES of MIXTURES MIXTURE SUBSTANCES TO OBTAIN METHODS Salt mixed with broken glass pieces Salt crystals Evaporation Sea Water Pure Salt Desalination/ Distillation Copper (II) Sulphate solution Copper (II) Sulphate Crystals Crystallisation Ink Pure Water Paper Chromatography Wine Ethanol Fractional Distillation get students to think back and confirm on the type of methods they can apply to use after the end of introducing the methods 33

Check your Understanding Which process is used to separate Three water-soluble dyes? Two miscible liquids with boiling points of 78°C and 100°C? Water containing an insoluble solid? Water containing a dissolved solid? Chromatography Fractional distillation Filtration Crystallisation

Check your Understanding A mixture contains the following three liquids that are completely miscible:   The liquid can be separated by fractional distillation. State, with a reason, which liquid will distill over first. Propanone. It has the lowest boiling point among the 3 liquids. Name an industrial process that involves fractional distillation. Fractional distillation of crude oil Liquid Boiling point (°C ) propanone 48 Ethanol 78 Water 100

What did you learnt today? 3 methods to test for purity Effect of impurities in a substance 3 separation techniques and their working principles

To separate miscible liquids Summary Simple distillation Fractional distillation To separate miscible liquids To separate solvent from solution Crystallisation Filtration Separation technique To separate solids (only one is soluble in a solvent) To separate solid from a liquid To separate solid from a solution

Online Quiz http://www.sciencequiz.net/jcscience/jcchemis try/septechniques/septechniques1a.htm