3. Separation by Evaporation
The changing of a liquid into vapours (or gas) is called evaporation.Evaporation is used to separate a solid substance that has dissolved in water (or any other liquid). The dissolved substance is left as a solid residue when all the water (or liquid) has evaporated. The use of process of evaporation for separating a mixture is based on the fact that liquids vaporise easily whereas solids do not vaporise easily. Though evaporation of a liquid can take place even at room temperature but it is very slow at room temperature.Evaporation can be made quicker by heating the solution.
If we have a mixture of common salt and water, then we cannot separate common salt from water by filtration or centrifugation. This is because common salt is completely dissolved m water and not insoluble "it We can recover common salt from salt-water mixture (or salt solution) by the process of evaporation.
The common salt dissolved in water can be separated by the process of eyaporation. This is done as follows : The solution of common salt and water is taken in a china dish and heated gently by using burner [see Figure 70(a)]. The water present in salt solution will form water vapours and escape atmosphere. When all the water present in the solution of common salt and water gets evaporated, then common salt is left behind in the china dish as a white solid [see Figure 70(b)].
The process of evaporation is used on a large scale to obtain common salt from sea-water. Sea-water trapped in shallow lakes (called lagoons) and allowed to stand there. The heat of sun gradually evaporates water in the shallow lakes and common salt is left behind as a solid.When a sugar solution is evaporated carefully, then water is eliminated and solid sugar is left behind. We can also separate copper sulphate (blue vitriol) from its solution by the process of evaporation.The substances like potash alum and potassium nitrate, etc.,are also separated from their water solutions (called aqueous solutions) by the process of evaporation. It should be noted that during the evaporation of a 'water solution we get the 'dissolved solid substance' but water cannot be recovered in this method. Water evaporates into the atmosphere. When a solution of sulphur in carbon disulphide is evaporated, then carbon disulphide vaporises and solid sulphur is left behind. It should be clear by now that evaporation is used for recovering dissolved solid substances from liquid mixtures (or solutions) but the liquid itself cannot be recovered by this method. The liquid vaporises and gets lost to the air.
The blue ink (or black ink) used in fountain pens is a liquid mixture.It is a mixture of a 'dye' in water. This dye is the coloured component of the ink (which gives it a blue or black colour).We can separate the "coloured component of ink' (or 'dye' from ink) by the process of evaporation. Please note that for evaporating ink, we do not heat the china dish containing ink directly over the flame (as we did in the case of salt solution in Figure 70). This is because the 'dye' obtained from ink can get decomposed by the high temperature produced by the direct heating with a burner. We use a 'water bath' for evaporating ink.A simple water bath can be made by filling a beaker half with water. We will now describe the experiment to obtain 'dye' from the ink.
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Matter in Our Surroundings - Notes
1. 7. Metals are Solids at the Room Temperature
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6. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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8. 11. Non-Metals Have Many Different Colours.
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9. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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16. 8. Metals Generally Have High Melting Points and Boiling Points.
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19. 8. Non-Metals Have Comparatively Low Melting Points and Boiling Points
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23. To Study the Properties of a Suspension
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24. Effect of Temperature and Pressure on Solubility
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25. Chemical Formula for daily use material
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26. 1. Non-Metals are Not Malleable. Non-Metals are Brittle.
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29. Pure Substances : Elements and Compounds
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31. Solutions, Suspensions And Colloids
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35. 1. Separation by a Suitable Solvent
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40. 4. Non-Metals are Not Lustrous (Not Shiny). They are Dull in Appearance.
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44. 1. Separation by Fractional Distillation
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45. 3. Metals are Good Conductors of Heat and Electricity.
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49. To Separate the Salt-Water Mixture (or Salt-Solution)
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50. Comparison Among the Properties of Metals and Non-Metals
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