Secondary 3 Science (Chemistry) Chap 3: Separation Techniques Notes

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1 Secondary 3 Science (Chemistry) Chap 3: Separation Techniques Notes Name:... Class:..... Date: Self-Checklist: Tick off each item after you have understood it Paper chromatography and interpreting chromatograms. Methods of separations and purification for the components of solid-solid, solid-liquid and liquid-liquid mixtures. Techniques for separations and purification, including the use of a suitable solvent, filtration and crystallisation or evaporation, distillation and fractional distillation. I. Chromatography Chromatography is a technique of using a solvent to separate a mixture into its components. Chromatography Mixture to be separated Solvent e.g. ethanol How does chromatography work? - For example, we need to determine the purity of a food colouring (which may be made up of different dyes). - A spot of the food colouring is applied to the chromatography paper. - The chromatography paper is dipped in ethanol (the solvent). - Ethanol travels up the paper, carrying the dyes along. - A dye that is in the solvent will travel further. 1

2 How do we interpret the result of chromatography? The chromatography paper with the separated components is called a. The chromatogram shows that the food colouring is a mixture of two dyes. It is not pure. If there is only one spot on the chromatogram, it means the substance is. Chromatography Techniques Draw a pencil line at about 2 cm from one end of the chromatography paper. Why is the pencil used in drawing a line in paper chromatography and not a ballpoint pen? Make a very fine spot of one of the ballpoint ink on the pencil line. Dip the paper into a solvent; solvent travels up the paper. When we dip the chromatography paper into the solvent, why is it important to keep the solvent level below the pencil line? Inks dissolve in the solvent and move up the paper. Dyes move up at different speeds are thus separated. Dyes are then identified. Uses of chromatography: 1. to separate the components in a sample, 2. to identify the number of components in a sample, 3. to identify the components present in a sample, 4. to determine the purity of a sample. II. Separation Techniques Besides chromatography, there are other techniques to separate mixtures. You will learn how to separate the following: 1) A insoluble solid from a liquid 2) Solids from liquids -soluble solids from a solution 3) Solid from solid mixture 4) A liquid from a solution 5) Miscible and immiscible liquids 2

3 1) Separating Insoluble Solid from Liquid A. FILTRATION We can use filtration to separate solid particles from a liquid (often it is water). Examples of small solid particles include sand, clay, dust particles and precipitates. 2) Separating Soluble Solid from Liquid A. EVAPORATION TO DRYNESS To recover salt from salt solution, we evaporate the solution to dryness. salt solution evaporating dish B) CRYSTALLIZATION When a substance decomposes on heating or can lose its water of crystallization, we do not evaporate it to dryness. Instead, we crystallize it. Example of substance that decomposes on heating: Example of substance that gives of water to become powder on heating: How to purify by crystallization? In crystallization, water is removed by heating the solution. Heating is stopped at the stage when a hot solution is formed. If the resulting solution is allowed to cool to room temperature, the dissolved solid will be formed as pure crystals. 3

4 How do we test for a saturated solution? A clean glass rod can be used to test whether a solution is saturated. It is dipped into the solution and removed. There will be a small amount of solution on the rod. If small crystals form on the rod as the solution cools, the solution is saturated. We say the solution is at its point or point. 3) Separating Solids (e.g. salt and sand) Method: By adding a suitable solvent and. Procedure: 1. Pour some distilled water into the mixture of common salt and sand. Stir the mixture. 2. Filter the mixture. 3. Wash the residue with a little distilled water to remove all the salt solution from it. The residue is sand. 4. Evaporate the filtrate to dryness. Salt is recovered. How do we use a magnet to separate solids? Some metals are. We can use this property to separate these metals (e.g. iron, nickel, cobalt, steel) from mixtures. 4) Separating a Liquid from a Solution When a solid dissolves in a solvent, a is formed. Examples of solutions are salt solution and sugar solution. A pure solvent can be separated from a solution by simple distillation. Distillation is the process of a liquid and the vapour. SIMPLE DISTILLATION 2. In the condenser, water vapour condenses. 3. Pure water is collected as the. 1. In the distillation flask, water boils and enters condenser. 4

5 Thermometer: The thermometer should be placed beside the side arm of the distillation flask. It should not dip into the solution. Why? Condenser: The condenser consists of two tubes: an inner tube and an outer water jacket. Cold running water enters from the bottom of the condenser and leaves from the top. Boiling chips are used to smooth the boiling. If the distillate is volatile, the receiver can be put in a large container filled with ice. This helps to keep the temperature of the distillate low so that it remains in the liquid state. 5) Separating Miscible Liquids: Fractional Distillation Unlike oil and water, ethanol and water mix together completely to form a solution. They are said to be. If two liquids are miscible, they must be separated by a technique called. Comparing Simple vs Fractional Distillation A fractionating column is attached to the round-bottomed flask and the condenser for fractional distillation. Many glass beads in the fractionating column provide a for vapour to condense on. An example of a mixture of 2 liquids (miscible liquids): The boiling point of ethanol is 78 C while the boiling point of water is 100 C. In fractional distillation, the liquid with boiling point (ethanol) will be collected first. 2. Vapour of the liquid with higher boiling point condense along the fractionating column and re-enter the roundbottomed flask. 3. Ethanol distils over. In the condenser, ethanol vapour condenses. 1. In the distillation flask, the mixture of ethanol and water boils and both liquids may distil together. 4. Ethanol is collected as the. 5

6 How does the temperature change as a solution of ethanol and water undergoes fractional distillation? 2. At 78 C, ethanol distils over. The temperature remains constant until all the ethanol has distilled out of the round-bottomed flask. Temperature / o C 3. The temperature then increases until 100 C. At 100 C, water distils over. 1. The temperature of The temperature remains the mixture increases unchanged as water is as it is heated. being distilled. Time / s Industrial Applications Fractional distillation is used in industries to obtain, and from. Fractional distillation can be used to separate such as. is formed when glucose solution undergoes fermentation in the presence of. We can separate ethanol from glucose solution by fractional distillation. 6