Purification and Separation

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(Read up chapter before class as homework)  Determining purity  A pure substance is made up of only 1 substance. (In other words, it is not mixed with other substances)  We use chromatography or check a substance's melting point and boiling point to determine if the substance is pure. Pure solids have exact and fixed melting point. (A pure solid will melt completely at one temperature) Impurities lower the melting point. The greater the amount of impurities, the lower the melting point of the substance. Impurities also cause melting to take place over a range of temperatures. Likewise, pure liquids have exact and fixed boiling point. Impurities cause the boiling point to increase. The greater the amount of impurities, the higher the melting point of the substance. The liquid will also boil over a range of temperatures.  If pressure acting on the liquid is increased, the boiling point of the liquid is raised and vice versa.   CHROMATOGRAPHY   Chromatography is the technique of using a solvent to separate a mixture into its components. (eg. Separate dyes in ink, pigments in plants, amino acids obtained from proteins, to identify poisons or drugs and to detect traces of banned substances in food.)  The chromatography paper with the separated components is called a chromatogram. If there is only 1 spot on the chromatogram, it means that the substance is pure.  The ratio between the distance travelled by the substance and the distance travelled by the solvent is a constant. The ratio is called the Rf value. Rf value = Distance travelled by substance divided by the distance travelled by solvent.  Chromatography can also be used for colourless substance too such as amino acids. We can apply a locating agent to separate and analyse colourless substances. Uses of chromatography: separate the components in a sample, identify the number of components in a sample, identify the components present in a sample, determine if a sample is pure.  Separating a solid from a liquid DECANATING  Used to separate a dense, insoluble solid from a liquid. (eg. pebbles from water) FILTRATION  Used to separate small solid particles which are insoluble from a liquid (eg. sand, clay, dust particles, and precipitates) A solid can be separated from a liquid by filtration as the filter paper acts as a seive. A liquid can pass through the pores of the filter paper but a solid cannot. Upon filtration, the solid that remains on the filter paper is called the residue. The liquid that passes through the filter paper is called filtrate. EVAPORATION TO DRYNESS AND CRYSTALLISATION Used to separate substance that dissolve in water to form solutions from a liquid (eg. common salt) ONLY FOR SUBSTANCES THAT DO NOT DECOMPOSE UPON HEATING  CRYSTALLISATION   For substances that decompose upon strong heating Water is removed by heating of the solution. Heating is stopped at the stage when a hot saturated solution is formed. If the resulting solution is allowed to cool to room temperature, the dissolved solid will be formed as pure crystals.  A clean glass rod is used to test if 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. The solution is at saturation point/crystallisation point.  Separating solids   DISSOLVING AND FILTRATION      Only when one solid is soluble and the other is insoluble.  Pour some distilled water into the mixture of common table salt and sand. Stir and warm the mixture (dissolve). Next, pour the warm mixture into a filter funnel lined with filter paper. Collect the filtrate in a conical flask. Wash the residue with a little distilled water to remove all the salt solution from it. The residue is sand. Pour the filtrate into an evaporating dish and evaporate the filtrate to dryness. The white solid left in the evaporating dish is salt.  OTHERS  We can use a magnet to separate magnetic materials from non-magnetic materials.  Sublimation - separating a solid that sublimes on heating.  Separating a liquid from a solution  SIMPLE DISTILLATION   A pure solvent (eg. water) can be separated from a solution by simple distillation. The liquid that distils over is called the distillate. The solid that remains in the distillation flask is called the residue.  Note!  The thermometer should be placed beside the side arm of the distillation flask. It should NOT be dipped into the solution. This ensures that the thermometer measures the boiling point of the substance being distilled.  The condenser consists of 2 tubes, an inner tube and an outer water jacket. Cold running water is allowed to enter the water jacket from the bottom of the condenser and leave from the top.  The condenser slopes downwards so that the pure solvent formed can run into the receiver.  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.  Uses of simple distillation - 1) recover a solvent from a non-volatile solute, which has a high boiling point. 2) Separate mixture of liquids with different boiling points. (for liquids whose boiling points differ by more than 20°C) Separating liquids  SEPARATING FUNNEL   For immiscible liquids. Liquids that do not dissolve in each other are discribed as immicible. Note! Allow the liquid to separate completely. The denser liquid will be the bottom layer. Open the tap of the funnel to allow the bottom layer to drain into the beaker.  FRACTIONAL DISTILLATION  For misicible liquids  There is a column called fractionating column. It is attached to the round-bottomed flask and the condenser. They are filled with many glass beads that provide a large surface area for vapour to condense on. Some fractionating columns are also filled with plates or a spiral.   The liquid with the lowest boiling point will distil over to the condenser first. The vapour of the liquids with higher boiling points condense along the fractionating column and re-enter the round bottomed flask.

Introduction Learning objectives  - To explain the working principles involved in some separation techniques such as filtration, magnetic attraction, simple distillation, crystallization, evaporation, sublimation and the use of separating funnel - To explain how the techniques are used to separate the mixture into its components - To suggest suitable purification techniques, given information about the substances in a mixture. - To apply the various separation techniques in everyday life and industries In nature, almost all substances are in the form of a mixture.  Why is it essential to have pure substances at every instance?               Answer: Mixtures need to be separated into pure substances for characterisation, identification, produce of useful substances such as medicines.  Determine purity by melting and boiling points or to use chromatography. Chromatography  The solvent need not be only water! It can be gas (gas chromatography)! Pure dye only contains one spot while impure dye contains more than one spot Must have a small and concentrated dot so that the individual dyes will not be faint Retention Factor =Rf Unknown substances can be identified by their Rf values Separate and identify compounds in a mixture  ADSORB VS ABSORB  [Watch video on Chromatography and do experiment]  Simple distillation  Liquid (solvent) changes into a gas (boiling). Gas is pure (other substances are left behind) Gas condensed to pure liquid is (called distillate) on cooling Fractional distillation  More efficient than simple distillation Separate miscible liquids with different boiling points and their boiling points are close together. Example of miscible liquids are alcohol and water.   Refer to w/s for compare and contrast of fractional distillation and simple distillation  In the laboratory, the apparatus is essential the same as in simple distillation, except that a fractionating column is introduced between the distillation flask and the condenser. It is this fractionating column which does the actual separation.  The upper part of the column is cooler than the lower portion. Thus only vapours with the same temperature as the upper portion are allowed to pass on to the condenser, whereas vapours with higher boiling points would condense as they enter the upper portion and flow back into the distillation flask. As a result, the liquid with the lowest boiling point will always be distilled over first, followed by the liquid with the next lowest boiling point and so on, until all the fractions in the mixture have distilled over. Use of distillation - Desalination Plant: Obtain Pure Drinking Water Crystallisation Separate a dissolve solid from a solution. This method is commonly used to separate the heat-labile solutes from their solutions.  You need a saturated solution to carry out crystallisation. Saturated solution: solution containing a maximum amount of solute dissolved in a given volume of solvent at a particular temperature. A concentrated solution is not necessarily a saturated solution.