Separation of Mixtures

7,686
-1

Published on

Separation of Mixtures

Published in: Education, Technology, Business

Separation of Mixtures

  1. 1. Separating MIXTURES
  2. 2. Learning Objectives By the end of the lesson, students sbat: • Describe methods of separation and purification for the components of the following types of mixtures: 1. Solid-solid 2. Solid-liquid 3. Liquid-liquid (miscible and immiscible)
  3. 3. Learning Objectives • Techniques to be covered for separations and purification include: i. Use of a suitable solvent, filtration and crystallisation or evaporation ii. Distillation and fractional distillation iii. Paper chromatography • • Describe paper chromatography and interpret chromatograms Deduce from the given melting and boiling point, the identities of substances and their purity
  4. 4. Purity of Substances • Pure substances are made up of only one substance. • Mixtures are substances that contain two or more substances that are not chemically combined.
  5. 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. 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. 7. Sugar dissolves in the solvent Therefore resulting mixture is a SOLUTION.
  8. 8. 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.
  9. 9. Sand does NOT dissolve in the solvent Therefore resulting mixture is a SUSPENSION
  10. 10. Methods of Purification
  11. 11. Magnetic Separation • To separate a magnetic substance from a mixture.
  12. 12. 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.
  13. 13. Evaporation to Dryness • To evaporate solvent from the solution, obtaining the soluble salt from the mixture. • E.g. to obtain salt from seawater. mixture evaporating dish wire gauze tripod stand Bunsen burner Figure 3 Setup for evaporation
  14. 14. 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.
  15. 15. Wake Up Call 1 1. Which method is most suitable for obtaining a pure, dry sample of sodium chloride from a mixture of solid sodium chloride and sand? A. Heat the mixture gently and collect the substance which boils off. B. Heat the mixture gently and collect the substance which melts. C. Shake the mixture with water and distil off the liquid. D. Shake the mixture with water, filter and evaporate the filtrate.
  16. 16. Wake Up Call 1 2. A mixture contains two compounds, copper(II) sulfate crystals and calcium carbonate. They have the following properties: Name Properties Copper(II) sulfate •Soluble in water •Low melting point Calcium carbonate •Insoluble in water •High melting point
  17. 17. 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.
  18. 18. Simple Distillation • To obtain a solvent from a mixture
  19. 19. 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. water out thermometer retort stand condenser distilling flask mixture water in boiling chips distillate tripod stand heat Figure 4 Setup for simple distillation wooden block
  20. 20. 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
  21. 21. 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
  22. 22. Paper Chromatography • Procedure for Ascending Paper Chromatography – Separating Dyes in a Black Ink 1. Draw a pencil line on a piece of chromatography paper about 2 cm from the end of the paper. 2. 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. 3. 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. 4. Remove the chromatography paper when the solvent has almost reached the top of the paper.
  23. 23. 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 insoluble and 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.
  24. 24. 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.
  25. 25. Paper Chromatography Experimental setup Chromatogram pure substance
  26. 26. Paper Chromatography Experimental setup Chromatogram
  27. 27. Paper Chromatography Experimental setup Chromatogram
  28. 28. 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.
  29. 29. 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.
  30. 30. Interpreting a Chromatogram • The chromatogram shows that no spots above the starting line. This means that 1. the sample is insoluble in the solvent used. 2. the solvent needs to be changed.
  31. 31. Wake Up Call 4 1. For two substances to be separable by paper chromatography, it is necessary that A B C D they are both liquids. they are both soluble in the same solvent. they have different densities. they have different colours.
  32. 32. Wake Up Call 4 2. 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.
  33. 33. Wake Up Call 4 (a) What is the name of the process shown in the diagram? (b) Is the orange dye a mixture or a compound? Give a reason for you answer.
  34. 34. Wake Up Call 4 The results were compared to another known chromatogram of colours as follows: yellow orange green red original drop of orange dye What colours did the dye contain?

×