Chemistry · General Principles Of Organic Chemistry · NEET
Crystallisation purifies a solid using solubility. You dissolve the impure solid in a hot solvent where it dissolves a lot when hot but very little when cold. On cooling, the pure compound forms crystals and comes out, while soluble impurities stay dissolved in the liquid (called the mother liquor). It only works if the compound and impurity have different solubilities in the chosen solvent.
Simple distillation separates a liquid from a non-volatile impurity, OR two liquids whose boiling points are FAR apart (like chloroform 61C and aniline 184C). Fractional distillation is for liquids whose boiling points are CLOSE together (like the fractions in crude oil). It uses a fractionating column so the vapour condenses and re-evaporates many times, giving a cleaner separation.
Steam distillation is for compounds that are steam-volatile (they evaporate with steam), immiscible with water, and would decompose if heated to their normal boiling point. Passing steam lets the compound distil over well below its own boiling point, so it does not burn or break down. Aniline, o-nitrophenol and essential oils are purified this way.
Rf (retention factor) = distance moved by the compound divided by distance moved by the solvent front. It is always less than 1 and is a fixed value for a compound in a given solvent, so it helps identify the compound. A component that sticks less to the stationary phase moves further and has a higher Rf.
Adsorption chromatography (column, TLC) works because components stick (adsorb) to a solid stationary phase by different amounts. Partition chromatography (paper) works because components distribute (partition) between a liquid stationary phase and a liquid mobile phase by different amounts. In paper chromatography the stationary phase is actually water trapped in the paper, so it is partition, not adsorption.
It is used for liquids that decompose at or below their boiling point. Lowering the pressure lowers the boiling point, so the liquid distils at a lower temperature and does not break down. Example: glycerol is separated from spent-lye by distillation under reduced pressure.
The most suitable method of separation of a 1:1 mixture of ortho- and para-nitrophenols is:
Paper chromatography is an example of:
Match List I (Mixture) with List II (Method of Separation). List I: A. CHCl3 + C6H5NH2, B. Crude oil in the petroleum industry, C. Glycerol from spent-lye, D. Aniline-water. List II: I. Distillation under reduced pressure, II. Steam distillation, III. Fractional distillation, IV. Simple distillation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Look at the state and property. Solid with different solubility -> crystallisation. Solid that vapourises directly -> sublimation. Liquids with far-apart boiling points -> simple distillation. Liquids with close boiling points -> fractional distillation. Heat-sensitive liquid -> distillation under reduced pressure. Steam-volatile compound -> steam distillation. Very close mixture or tiny amounts -> chromatography.
No. In sublimation a solid changes directly to vapour without becoming a liquid first, then re-forms as solid on cooling. Evaporation is a liquid changing to vapour. Camphor, naphthalene and benzoic acid are purified by sublimation.
The mother liquor is the leftover solution after the pure crystals have separated out. It holds the dissolved soluble impurities, which is why the crystals that form are purer.
Fractional distillation. The many components of crude oil have close boiling points, so a fractionating column is used to separate them into fractions like petrol, kerosene and diesel.
