Purification Techniques

1. Purification Techniques

2. LearningObjectives By the end of the lesson, students sbat: • Describe methods of separation and purification for the components of the following types of mixtures: • Solid-solid • Solid-liquid • Liquid-liquid (miscible and immiscible)

3. Learning Objectives • Techniques to be covered for separations and purification include: • Use of a suitable solvent, filtration and crystallisation or evaporation • Distillation and fractional distillation • Paper chromatography • Describe paper chromatography and interpret chromatograms • Deduce from the given melting and boiling point, the identities of substances and their purity

4. Purity of Substances • Puresubstances are made up of only one substance. • Mixtures are substances that contain two or more substances that are not chemically combined.

5. Purity of Substances • Pure solids have a fixed (i.e. exact and constant) boiling point and melting point. • In the case of pure water, ice will melt at 0 oC and water boils at 100 oC

6. Classification of Mixtures • Mixtures can be classified as a solution or a suspension. • When the solid dissolves in a liquid, a solution is obtained. • The solid that dissolves is known as the solute. • The substance that dissolves the solid is known as the solvent.

7. Classification of Mixtures Example • When salt dissolves in water, a salt solution is obtained. salt + water → salt-water mixture (solute) (solvent) (solution) • When the solid does not dissolve in the liquid, a suspension is obtained.

8. Methods of Purification

9. Magnetic Separation • To separate a magnetic substance from a mixture.

10. Filtration • To separate an insoluble solid from a mixture. • E.g. sand from seawater. • Upon filtration the mixture is separated into residue and filtrate. • Residue – the insoluble solid that remains on the filter paper. • Filtrate – the liquid that passes through the filter paper.

11. mixture evaporating dish wire gauze tripod stand Bunsen burner Figure 3 Setup for evaporation Evaporation to Dryness • To evaporate solvent from the solution, obtaining the soluble salt from the mixture. • E.g. to obtain salt from seawater.

12. Crystallisation • Steps include: • Evaporate until a saturated solution is obtained. • Cool, crystals will form. • Filter to obtain crystals. • Rinse with cold distilled water. • Dry by pressing between filter paper.

13. Wake Up Call 1 • Which method is most suitable for obtaining a pure, dry sample of sodium chloride from a mixture of solid sodium chloride and sand? • Heat the mixture gently and collect the substance which boils off. • Heat the mixture gently and collect the substance which melts. • Shake the mixture with water and distil off the liquid. • Shake the mixture with water, filter and evaporate the filtrate. Answer: D

14. Wake Up Call 1 • A mixture contains two compounds, copper(II) sulfate crystals and calcium carbonate. They have the following properties:

15. Wake Up Call 2 List the steps that should be taken to separate the mixture to obtain dry samples of copper(II) sulfate and calcium carbonate. Answer: • Add water and stir, copper(II) sulfate dissolves. • Filter to remove calcium carbonate (residue is calcium carbonate, filtrate is copper(II) sulfate solution. • Rinse calcium carbonate in distilled water. • Press calcium carbonate between filter paper to dry.

16. Wake Up Call 2 • Evaporate the filtrate until saturated • Cool, crystals will form. • Filter to obtain crystals. • Rinse the crystals with cold distilled water. • Press the crystals between filter paper or leave in air to dry.

17. Simple Distillation • To obtain a solvent from a mixture

18. retort stand water out thermometer condenser distilling flask mixture water in boiling chips distillate tripod stand heat wooden block Figure 4 Setup for simple distillation Simple Distillation • How it works: • Water in the distilling flask boils. • Water vapour rises, enters the condenser. • Water vapour is condensed back to water. • Pure water (distillate) is collected in a receiver. To separate a solvent from a mixture.

19. Things to note Thermometer: bulb is placed at the opening of the distilling flask (to measure the temperature of the pure vapour) Condenser: water enters the condenser from the bottom and exits through the top. Boiling chips: ensures smooth boiling

20. Paper Chromatography • Uses a solvent to separate a mixture into its components. • Applications • separate dyes in ink, • pigments in plants, • amino acids obtained from proteins, • to identify poisons (eg. pesticides) or drugs, and • to detect traces of banned substances in food

21. Procedure for Ascending Paper Chromatography – Separating Dyes in a Black Ink Draw a pencil line on a piece of chromatography paper about 2 cm from the end of the paper. Put a tiny spot of black ink on the pencil line. Allow the spot to dry. Concentrate the ink spot by dipping the ink two or three times on the same spot. Using a split cork, suspend the paper inside a boiling tube containing a small volume of solvent. Ensure the ink spot is about 1 cm above the level of the solvent. Remove the chromatography paper when the solvent has almost reached the top of the paper. Paper Chromatography

22. Paper Chromatography Things to note: • Paper chromatography works only if the sample is soluble in the solvent. • The starting line should be drawn using a pencil and not a pen. • Pencil lead is insolubleand does not contain any dyes. • The starting line should not be below the solvent level. • This is so that the sample will not fully dissolve into the solvent. • The solvents used are usually water and ethanol.

23. Paper Chromatography • How it works • A spot of food colouring is applied to the chromatography paper. • Chromatography paper is dipped into a solvent (usually ethanol or water). • Solvent is soaked up by the paper, it continues to travel up the paper, carrying the dyes along. • A dye that is less soluble in the solvent will not be carried far along the paper; a dye that is more soluble in the solvent will be carried far along the paper. • Coloured spots are left in different places on the paper at the end of the experiment.

24. Paper Chromatography Chromatogram Experimental setup pure substance

25. Paper Chromatography Experimental setup Chromatogram

26. Paper Chromatography Experimental setup Chromatogram

27. Interpreting a Chromatogram • The chromatogram shows 2 coloured spots above the starting line. • This means that • the sample is a mixture • it is made up of 2 other substances.

28. Interpreting a Chromatogram • The chromatogram shows 1 spot above the starting line. • This means that • the sample is pure (it is either an element or a compound) • it cannot be separated into other substances.

29. Interpreting a Chromatogram • The chromatogram shows that no spots above the starting line. This means that • the sample is insolublein the solvent used. • the solvent needs to be changed.

30. Wake Up Call 4 • For two substances to be separable by paper chromatography, it is necessary that A they are both liquids. B they are both soluble in the same solvent. C they have different densities. D they have different colours. Answer: B

31. Wake Up Call 4 • Dyes are used to colour some sweets. A student placed a drop of orange dye on a piece of filter paper. The end of the filter paper was then dipped into a beaker of water. The results are shown below.

32. Wake Up Call 4 • What is the name of the process shown in the diagram? Paper chromatography • Is the orange dye a mixture or a compound? Give a reason for you answer. Mixture. It separated into 2 colours.

33. yellow orange green red original drop of orange dye Wake Up Call 4 The results were compared to another known chromatogram of colours as follows: • What colours did the dye contain? • yellow, orange, red