S1sc Separation of Mixtures Notes.pdf

Learning @ The Pinnacle (i) Bring a magnet close to a container of magnetic and non-magnetic objects. S1 Sc – Separation of Mixtures...

Learning @ The Pinnacle (i) Bring a magnet close to a container of magnetic and non-magnetic objects. S1 Sc – Separation of Mixtures (ii) Move the magnet gently around all parts of the mixture. Name ________________________________ Date : ____________ (iii) The magnetic objects will be attracted to the magnet. 1. Separating mixture (iv) The non-magnetic objects will not be attracted to the magnet and they will remain in the container. (a) There are physical methods which can be used to separate a mixture to obtain pure substances again. (d) We can use magnetic separation to obtain iron filings from a mixture of iron filings and sulfur powder. (b) the choice for selecting a suitable physical method is determined by the type or nature of the mixture. (e) Magnetic separation is also used in the mining industry to obtain iron from other non-magnetic metals in metal ores. Type of Mixtures Examples Two different solid substances mixed together such 4. Filtration Solid-solid as sand with salt or iron filings with sulfur powder (a) Filtration is used to separate an insoluble solid from a liquid Solid-liquid Salt solution, sugar solution mixture. It is suitable to separate a solid-solid mixture (of which one Liquid-liquid Alcoholic solution, crude oil solid is soluble in the solvent used such as sand with table salt dissolved in water) or a solid-liquid mixture (such as sand in water) Quantity of Sample containing very little amount of components which contains insoluble solids. components such as food dyes, food colouring and inks (b) The separation is first carried out by dissolving the mixture in water. The soluble solid (such as table salt) dissolves in the water while the insoluble solid (such as sand) remains undissolved in water. 2. There are many physical methods to separate mixtures. The methods are: (c) The solid left on the filter paper is called the residue. The liquid (a) Magnetic Separation - to separate magnetic objects from non- that drains through the filter paper is called the filtrate. magnetic objects. (d) For example, to separate a mixture of table salt and sand, the mixture is (b) Filtration - to separate solid from liquid. first dissolved in water and stirred well until the table salt dissolves in the water. The sand remains undissolved in the water. We will then (c) Evaporation - to obtain a solid from its solution. separate the mixture by filtration as shown below. (d) Crystallisation - to obtain crystals from its solution. (e) Simple and Fractional Distillation - to separate mixtures of liquids of different boiling points. mixture of table salt, (f) Chromatography - to separate and identify the constituents of a sand, and water mixture. residue (sand) 3. Magnetic Separation (a) Magnetic Separation is used to obtain magnetic objects among a mixture of magnetic and non-magnetic objects. filtrate (salt solution) (b) The four types of magnetic materials are iron, steel, nickel and cobalt. (c) The steps used to carry out magnetic separation are as follows: (e) To obtain the table salt from the salt solution, a method called evaporation is used. 5. Evaporation 7. Simple distillation (a) Evaporation is used to obtain pure soluble solid from a solution (a) Simple distillation is used to separate a liquid from a solution which is heat-stable. containing dissolved solids. (b) The solution is heated to dryness. The solid remains as crystallised (b) The liquid in the solution is heated until its boiling point and changes residue after the solvent has evaporated completely. into a vapour. (c) For example, sodium chloride crystals (table salt) are heat-stable which (c) The vapour is then cooled by a condenser and changes back into a can be obtained by evaporation to dryness. After water has evaporated pure liquid which is collected in the receiving flask as a distillate. completely, sodium chloride crystals are obtained. (d) The impurities which have much higher boiling points remain in the flask. (e) It is important to ensure that the bulb of the thermometer must be evaporating dish placed at the opening of the condenser so that it can measure the boiling point of the substance that is being distilled accurately. salt solution (f) During the period of time when the pure liquid is being distilled, the thermometer shows a constant temperature being the boiling point of sodium chloride the substance that is distilled. heat heat to dryness crystals (g) The boiling chips/stones or porcelain chips are added to ensure smooth boiling. (h) Use a conical flask as the receiving flask instead of a wide-mouth beaker to prevent spillage of the pure liquid during the distillation process. 6. Crystallisation (i) The water inlet pipe is placed at the lower end of the condenser (a) Crystallisation is used to obtain pure soluble solid from a solution nearer to the receiving flask while the water outlet pipe is placed at which is not heat-stable. the higher end of the condenser nearer to the distillation flask. This ensures a smooth thermal gradient to allow the rate of condensation to (b) The solution is first heated until it becomes saturated. It is then allowed take place uniformly throughout the condenser. to cool slowly so that crystals can be formed. (c) Slow cooling often produces large crystal while fast cooling usually produces small crystal. thermometer (d) If a compound is not heat-stable, strong heating to dryness would water out decompose it. (e) For example, sugar crystals are not heat-stable. They can only be obtained by crystallisation. Evaporating sugar solution to dryness will distillation decompose the sugar crystals into black carbon. flask (f) The steps of obtaining crystals from crystallization are: condenser (i) Dissolve the solid in a solvent to produce a solution. solution (ii) Heat the solution to evaporate most of the solvent. (iii) Allow the hot solution to cool. The solid appears as pure crystals. receiving flask boiling (g) Crystallization can be used to obtain white sugar crystals and silicon water in chips crystals. heat distillate 8. Fractional distillation the fastest is the most soluble and the slowest one is the least soluble. (a) Fractional distillation is used to separate a mixture of two or more The mixture is thus separated. miscible liquids with close boiling points (about less than 25 °C). (c) There are three forms of chromatography namely paper, liquid and gas. (b) Miscible liquids are liquids that dissolve in each other completely to form However, only the paper chromatography will be studied in the syllabus. a solution. (d) In the paper chromatography, the coloured components will move up (c) Unlike simple distillation, fractional distillation has an additional the paper together with the solvent. Depending on their solubility in the apparatus called a tall fractionating column attached to the distillation given solvent, each coloured component will travel at a different speed flask and the condenser. up the chromatography paper, ending at a different distance up on the paper. The least soluble coloured component will travel the slowest, so (d) The liquid with the lowest boiling point is distilled first, followed by ending at the shortest distance from the starting line. The most soluble the next higher boiling point and so forth. colour component will travel the fastest thus ending at the farthest (e) For example, to separate ethanol and water. Ethanol will be distilled first distance. as it has a lower boiling point of 78 °C. This is then followed by water (e) A simple paper chromatography his shown below: which has a boiling point of 100 °C. lid (to prevent evaporation thermometer water out of the volatile solvent) starting line chromatography paper (to place the sample fractionating condenser and it is drawn in pencil sample column as pencil lead is not soluble in most solvent solvent distillation so will not interfere with (must be below the starting flask receiving the results of the line to prevent the sample flask experiment) from dissolving into the solution water in solvent) distillate (f) A chromatography paper with the separated components is called boiling a chromatogram. chips heat solvent front spot C (the final position travelled by (d) Fractional distillation is used largely in the following industries: (the most soluble the solvent) component in the solvent so (i) to obtain oxygen and nitrogen from liquid air, spot B it travels the farthest (ii) to obtain useful products such as petrol and kerosene from crude oil distance) (the moderately soluble (petroleum) component in the solvent) 9. Chromatography spot A (a) Chromatography is used to separate a liquid mixture of coloured (the least soluble component starting line components that have different solubility in the same solvent. in the solvent so it travels the shortest distance) (b) The more soluble the coloured component in the solvent, the faster it will travel and the further the distance travelled in the a chromatogram chromatography paper. As such, the coloured component that travels (g) The advantages of using chromatography: EXAMPLE (i) Gives quick and accurate analysis of sample A chromatography is carried out to analyse the sample X. The chromatogram (ii) Requires only a small amount of sample from the investigation is shown below. (iii) Able to separate complex mixtures such as food dyes or flavourings (h) The identity of the components can be determined by comparing to the standard samples. (i) Chromatography can be used:  to find out the colours of the ink.  to identify the colouring used in food products, e.g. ice-cream, sweets, etc.  to find out the coloured pigments in petals.  to detect the type of ink used in forging cheques Which components, A, B, C or D, is/are found in sample X?  to identify the dyes used in food. Solution Dyes A and B are present in the sample X. Note: The two spots of dye C do not match at all to the spots of the sample X. Dye D contains one spot which does not show up in the sample X. So the sample X does not contain dye D.