EasyChemistryHigh School (11–12)Organic Chemistry: Some Basic Principles and Techniques › Purification and Analysis of Organic CompoundsConceptualA-LevelIB
Name the best method of purification for each case, and give the reason…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Name the best method of purification for each case, and give the reason. (a) Naphthalene mixed with sand. (b) Aniline mixed with water. (c) Two liquids, one boiling at and the other at . (d) Glycerol that decomposes at its normal boiling point.
Need a reminder? The key ideaShow
Purification: sublimation (a solid that turns to vapour and back), crystallisation (hot saturated solution, then cooling), distillation (simple, fractional, under reduced pressure, steam) and differential extraction. Chromatography separates by different clinging to a stationary phase, and , the distance moved by the substance over the distance moved by the solvent front. In steam distillation, .
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) Sublimation. Naphthalene turns to vapour on heating and sand does not.
(b) Steam distillation. Aniline does not mix with water, and the two boil together below , so the aniline does not decompose.
(c) Fractional distillation. The boiling points are close, so a fractionating column is needed to separate them.
(d) Distillation under reduced pressure. A lower pressure lowers the boiling point, so the glycerol distils without decomposing.
Answer: (a) sublimation, (b) steam distillation, (c) fractional distillation, (d) reduced pressure.
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