Every Separation Method Exploits One Difference and Nothing Else

Find the one property in which the components differ most, and choose the method built on that property.

Iron and sulphur differ in magnetism, so a magnet works. Sand and water differ in solubility, so filtration works. Alcohol and water differ only in boiling point, so nothing but distillation will do.

There is no general-purpose method, and there does not need to be. Every technique in the syllabus is a machine for exploiting exactly one difference — which means choosing correctly is a matter of naming the difference first. This page covers the third part of the ICSE Class 8 Chemistry chapter on elements, compounds and mixtures.

Five simple methods, each built on a different obvious difference.

Handpicking — exploits a difference in size, shape or colour when the unwanted part is few and visible. Removing stones, husk and damaged grains from rice or dal before cooking.

Winnowing — exploits a difference in weight, using moving air. Grain is dropped from a height and the breeze carries the light husk sideways while the heavy grain falls straight down. Used on every threshing floor.

Sieving — exploits a difference in particle size. Flour passes through the mesh while bran is held back; builders sieve sand from gravel the same way.

Magnetic separation — exploits magnetism. A magnet drawn over a mixture of iron filings and sulphur lifts the iron out; scrapyards separate iron from other metals with an electromagnet.

Sedimentation and decantation — exploits a difference in density where the solid is insoluble and heavier than the liquid. Muddy water left standing lets the mud settle as sediment, and the clear water above is then poured off, which is decantation. Rice is washed the same way.

Loading speeds sedimentation up. Adding a pinch of alum to muddy water makes the fine clay particles clump together into heavier lumps that settle far faster — which is a standard step in treating drinking water.

Why sedimentation alone is never enough. Decanting always leaves some suspended solid behind, and very fine particles may never settle at all. That is why decantation is normally followed by filtration, and why these two appear together in almost every practical question.

Three methods for solid-and-liquid mixtures, and they are not interchangeable.

Filtration exploits a difference in particle size, separating an insoluble solid from a liquid. The mixture is poured through a filter paper in a funnel: the liquid passes through as the filtrate and the solid is held back as the residue.

Used for sand from water, chalk from water, and tea leaves from tea — a kitchen strainer is a filter. It will not separate salt from water, because dissolved salt particles pass straight through the paper.

Evaporation exploits a difference in volatility, recovering a dissolved solid from its solution. The solution is heated so the liquid escapes as vapour, leaving the solid behind.

This is how common salt is obtained from sea water in coastal salt pans, using nothing but sunshine and wind. Note what it gives you: the solid is recovered and the liquid is lost.

Crystallisation exploits the fact that solubility falls as temperature falls. A hot saturated solution is allowed to cool slowly; as it cools it can no longer hold all the solute, and pure, well-shaped crystals separate out while the impurities stay dissolved in the liquid.

This is how pure copper sulphate crystals are obtained from an impure sample, and how sugar candy is made.

Why crystallisation beats evaporation for purity. Evaporation leaves behind everything that was dissolved, impurities included. Crystallisation leaves the impurities in the solution and takes only the crystals — so it gives a purer product. It is also gentler: some substances decompose if heated to dryness, and crystallisation never heats them that far.

So the choice runs: filtration if the solid is insoluble, evaporation if you want a dissolved solid quickly, and crystallisation if you want it pure.

When the components are both liquids, or when you want the liquid back rather than the solid.

Simple distillation exploits a large difference in boiling point, where one component is non-volatile or boils much higher. The solution is boiled, the vapour travels through a condenser where it is cooled back to liquid, and the pure liquid — the distillate — is collected in a receiver.

Used to get pure water from salt water or from muddy water. Unlike evaporation, distillation recovers the liquid, which is the whole point of a desalination plant.

Fractional distillation is used when two miscible liquids have boiling points that are close but different. A tall fractionating column packed with glass beads is fitted above the flask. Vapour rising through it condenses and re-evaporates many times over, so each pass enriches the vapour in the lower-boiling component, and the liquids come off one after another at their own temperatures.

Used for petroleum refining, where crude oil is split into petrol, kerosene and diesel; for separating alcohol and water; and for separating the gases of liquid air into oxygen and nitrogen.

A separating funnel exploits a difference in density between two immiscible liquids. The mixture is poured into a pear-shaped funnel with a tap at the bottom and left to stand until it forms two clear layers. Opening the tap runs off the lower, denser liquid first, and the tap is closed as the boundary reaches it.

Used for oil and water, kerosene and water, and to separate mustard oil from water after extraction.

The distinction that decides your answer. Ask whether the two liquids mix. If they form two layers — immiscible — use a separating funnel, and distillation would be pointless extra work. If they form a single clear liquid — miscible — a separating funnel is useless, and only fractional distillation can part them.

Three special methods for jobs the ordinary ones cannot do.

Sublimation exploits the fact that some solids turn directly into vapour on heating, with no liquid stage. The mixture is heated in a china dish covered with an inverted funnel; the subliming solid vaporises, and its vapour deposits as pure solid on the cool inner surface of the funnel, while the other component stays in the dish.

Used to separate camphor from sand, naphthalene from salt, or ammonium chloride from common salt — the subliming substance in each case being camphor, naphthalene and ammonium chloride. Iodine behaves the same way.

Chromatography exploits a difference in how strongly components are adsorbed on a surface and how readily they dissolve in a moving solvent. A spot of the mixture is placed near one end of a strip of filter paper, and the end is dipped in a solvent. As the solvent creeps up the paper, each component is carried along at its own rate — the more soluble and less strongly held ones travel furthest — and the single spot spreads into a row of separate coloured bands.

The classic demonstration separates the dyes in a black ink into distinct colours, which is proof that black ink is a mixture. It also separates the pigments in a leaf extract, and is used to detect drugs in blood or urine.

Why chromatography is special. It works on tiny quantities and on components present in traces, and it can separate substances so similar that no boiling point or density difference is usable. No other method in this chapter can do that.

Centrifugation exploits a difference in density under very rapid spinning. The mixture is whirled in a centrifuge; the denser component is thrown outwards to the bottom of the tube and the lighter one collects above. It does in seconds what sedimentation would take hours to do, and it works on particles too fine ever to settle on their own.

Used to separate cream from milk, to separate the cells from the plasma in blood, and in a washing machine's spin cycle, which throws water out of wet clothes.

How to tell centrifugation from sedimentation in a question. Both use density. Sedimentation waits for gravity; centrifugation supplies a much stronger effect by spinning, so it is chosen when the particles are very fine or the separation must be fast.