Separating Mixtures - Past Paper PDF

Mixtures and Separations TOPIC: SEPARATING MIXTURES Learning Outcomes 2.5 apply suitable separation techniques based on differences in properties of the components of mixtures; Page 68 1 Mixtures are impure substances. Many times it is necessary...

Mixtures and Separations TOPIC: SEPARATING MIXTURES Learning Outcomes 2.5 apply suitable separation techniques based on differences in properties of the components of mixtures; Page 68 1 Mixtures are impure substances. Many times it is necessary to separate a mixture in order to obtain its pure compounds. Some of Methods of Separating Mixtures are: Sublimation Centrifugation and Decantation Filtration Evaporation and Crystallization Simple distillation Separating funnel Solvent extraction Fractional distillation Paper chromatography  Sublimation ▪ Sublimation is the method used to separate a substance which sublimes from a mixture. ▪ Examples of substances which sublime are: iodine, ammonium salts, naphthalene and dry ice. ▪ The mixture is heated in an evaporating dish covered with a filter funnel. The substance which sublimes will be changed into a vapour and then formed back on the inside of the funnel.  Decantation This method can be used to separate suspensions. The mixture is left undisturbed until all the solid particles settle out. The liquid layer is then carefully poured or decanted off, leaving the solid residue 4 behind.  Filtration Filtration is another method used to separate a suspension (an insoluble solid from a liquid). An example would be to separate a mixture of sand and water. The solid (sand) left on the filter paper is called residue. The liquid (water) that has passed through the filter paper is called filtrate.  Evaporation and Crystallisation This method can be used to separate a solid/liquid solution. The mixture is heating in the evaporating dish. The liquid solvent boils off so that the solution becomes increasingly concentrated. When the solution is super saturated, crystals begin forming. If the solution is cooled, crystalisation (crystal formation) occurs. Slower cooling produces larger crystals. The crystals are washed and dried.  Simple distillation Distillation is the method used to obtain a pure solvent from a solution. E.g. pure water from seawater. It is done by heating the solution in a distillation flask and collecting the vapour that boils off. A condenser is used to condense the hot vapour and change it to a liquid. Both components of the mixture are obtained. Separating Funnel This method is used to separate a mixture of immiscible liquids. The mixture is placed into a separating funnel and allowed to settle into two layers. The liquid of highest density sinks to the bottom of the funnel. Each layer can then be run off and collected separately via the tap at the bottom of the apparatus.  Solvent extraction This method is used to separate mixtures where one component dissolves in a particular solvent, and the other does not, e.g. a mixture of iodine and sodium chloride. Iodine dissolves in the solvent tetrachloromethane, while sodium chloride does not. The mixture can be separated as follow: Add tetrachloroethane to the mixture. The iodine dissolves while the sodium chloride remains as a suspension. Filter the mixture to remove the sodium chloride. Evaporate the filtrate to obtain the iodine. 9  Fractional Distillation Fractional distillation is used to separate miscible liquids of similar boiling points. The boiling point of water and ethanol are 100oC and 78oC respectively. As a mixture is heated, it boils. A vapour of ethanol and water rises up into the fractionating column. The fractional column provides a large cool surface for condensation to take place. Water (higher B.P.) trickles back into the flask. Vapor rises up the column consisting of purer ethanol (lower B.P.)  Paper Chromatography Method used to separate and identify the different coloured components in dyes or inks. It is based on the principle that different substances have different solubilities in the same solvent. Substances adsorbed to a surface to a different extent The more soluble substance will get carried along faster by the solvent and move further ahead than the less soluble substances.  Set up for Paper Chromatography ▪ The starting line must be drawn in pencil, not ink. This is because unlike ink, pencil lead is insoluble in the solvent and will not interfere glass cover solvent front chromatography with the chromatogram. paper ▪ The spots of mixtures must be placed above the solvent starting line large level, so that they will not beaker immediately dissolve in the solvent solvent, and the solvent has time to slowly move up. ▪ The solvent front must be allowed to move as far up the paper as possible to ensure that all the dyes are separated.  The Retention Factor( Rf )value of a substance (10 cm) Rf value = distance moved by a substance distance moved by solvent front E.g. Rf value of red dye = 7 cm = 0.7 (7 cm) 10 cm  Paper Chromatography Worked example The chromatogram shows 3 single dyes red, green and blue, and also four unknown samples P, Q, R and S. Identify the dyes present in each of the samples, P, Q, R, S. Solution Sample P contains green dye and one unknown dye. Sample Q contains only blue dye. Sample R contains green, blue and red dyes. Sample S contains green and red dyes.  Fractional Crystallisation Fractional crystallization can be used to separate two dissolved substances which have different solubilities at different temperatures. A warm concentrated solution containing the two solutes is cooled, a larger proportion of the solute with the lower solubility crystallises out. The solute with the higher solubility remains in solution, the crystals still contain a small amount of the solute with higher solubility. So the procedure is repeats several times to improve the purity filtration is used to separate the crystals from the solution. 15  SUMMARY OF SEPARATION TECHNIQUES TYPE OF SEPARATION PRINCIPLE OF METHOD EXAMPLE MIXTURE METHOD USED SOLID/SOLID Sublimation One substance sublimes the other Ammonium does not. chloride/sodium chloride Solvent extraction Different solubility in a particular Iodine/ sodium solvent chloride LIQUID/LIQUID SOLID/LIQUID Decantation and Suspension Filtration Different particle sizes Chalk/water Evaporation/ Solution Crystallisation / Widely differing boiling points Copper Simple distillation sulphate/ water Differing solubility in a particular Colloids Chromatography solvent leading to differing speeds Screened of movement on chromatogram methyl orange LIQUID/LIQUID Miscible Fractional distillation Slightly differing boiling points Ethanol/water Immiscible 16 Separating Funnel Differing densities Oil/ water  Key points Explain the following terms: Chromatography, colloid, crystallisation, distillation, filtrate, fractional distillation, miscible, residue, separating funnel, simple distillation, immiscible, solution, solubility, sublimation, suspension. Identify different types of solutions. Describe and perform methods of separation. Choose an appropriate method for separation of a mixture, based on differences in properties of its components. 17  Quick check 1. State the method you will use to separate the following substances. (a) calcium carbonate from table salt (b) iodine from sodium chloride (c) table salt from seawater (d) sugar from sugar solution (e) pure water from sewage water (f) ethanol from beer (g) yellow dye from durian ice cream 2. Explain the following in chromatography. (a) Why is the starting line not drawn with ink or a ball point pen? (b) The spots of samples on the start line should be small. (c) What is the biggest advantage of chromatography? 3. A sample of ink was analysed using paper chromatography (see diagram above). Identify the dyes present in the ink.  Solution to Quick check 1. State the method you will use to separate the following substances. (a) dissolution, followed by filtration (b) sublimation (c) evaporation (d) crystallisation (e) distillation (f ) fractional distillation (g) chromatography 2. Explain the following in chromatography. (a) Ink contains dyes which could dissolve in the solvent and interfere with the chromatogram. (b) So that they would not smudge the paper. (c) It can detect and identify very small amounts of substances. 3. Blue and yellow dyes  Homework 1. Name the separation technique you would use to separate: a. mud from a mixture of mud and water b. oil from a mixture of oil and water c. distilled water from sea water d. the dyes in black ink. explain each of your choices. 2. Describe with the aid of a diagram how you would obtain ethanol from a mixture of ethanol and water. 3. Describe giving experimental details how you would separate the pigments in chlorophyll, starting with fresh leaves. 4. a. Describe how a mixture of sand and sodium chloride could be separated to give pure dry samples of each substances. b. Name the technique(s) used. 20  Homework 5. Two students working independently in different parts of the world discover a purple flower. They believe the flower contains the same pigment. How could they test their hypothesis? 6. Imagine you are shipwrecked on a desert island. The only source of drinking water is the sea. You have an empty gasoline can and matches from the lifeboat. Bamboo, coconuts and driftwood can be found on the island. Describe with the aid of a diagram how could you obtain fresh water from sea water. 21  Classwork #2 7. A student used chromatography to investigate the dyes in a food colouring. A series of dyes and extract of the food colouring were spotted on the paper at the points marked X. Ethanol was used as the solvent to carry the dyes up the paper. The chromatogram below shows the results. 22  Class work #2 (i) State which dye is the most soluble in ethanol. (ii) State which dye is not present in the food colouring. (iii) The Rf value of a dye is calculated by using the formula: Calculate the Rf value for Chad Yellow 8. Iodine is only slightly soluble in water. It forms a pale yellow solution when dissolves in water. However, it is much more soluble in cyclohexane, forming a purple solution when it dissolves. Cyclohexane is less dense than water and is immiscible with water. (i) Predict what will occur when cyclohexane is added to a solution of iodine in water and the mixture shaken and allowed to stand. (ii) Using a labeled diagram outline the steps to be taken when separating the mixture in i) above. 23