Pick the Right Trick and Any Mixture Comes Apart

Find a physical property in which the components differ, then choose the method that exploits it. Different sizes suggest sieving, a difference in magnetism suggests a magnet, and different boiling points suggest distillation.

That one question — what is different about them? — chooses the method every time. This page covers everything in the ICSE Class 7 Chemistry chapter's third part: separating solid-solid mixtures, solids from liquids, and liquid mixtures.

Because the components of a mixture are only physically mixed, so a physical difference is enough to pull them apart — no chemical reaction is needed.

The properties used in this chapter are size, weight, magnetism, solubility, the ability to sublime, density, particle size and boiling point.

So the selection runs like this:

- Different sizes — sieving.
- Different weights with wind available — winnowing.
- One component magnetic — magnetic separation.
- One component sublimes — sublimation.
- Solid insoluble in the liquid — filtration.
- Solid soluble in the liquid — evaporation or crystallisation.
- Liquids immiscible — separating funnel.
- Liquids miscible with different boiling points — distillation.

A kitchen already uses several: a tea strainer filters, a sieve separates flour from bran, and a winnowing basket separates husk from grain.

The reason this matters is that the components come out unchanged. Filtering muddy water gives back exactly the same mud and the same water, which is why separation belongs with physical changes rather than chemical ones.

Five methods cover solid-solid mixtures, each matched to a property.

Handpicking — used when the pieces are large enough and few enough to lift out by hand. Stones from rice or dal, and damaged grains from good ones.

Winnowing — used when the components differ in weight and a breeze is available. The mixture is dropped from a height; the lighter husk is carried sideways by the wind while the heavier grain falls straight down.

Sieving — used when the particle sizes differ. Flour is separated from bran with a sieve, and sand from pebbles at a building site.

Magnetic separation — used when one component is magnetic. Iron filings are removed from sulphur powder or from sand by moving a magnet over the mixture.

Sublimation — used when one component sublimes on heating and the other does not. A mixture of common salt and ammonium chloride is heated in a china dish covered with an inverted funnel; the ammonium chloride sublimes and re-solidifies on the cool funnel, leaving salt behind. Camphor, naphthalene and iodine behave the same way.

A thali of raw rice sorted before cooking shows handpicking, and the same rice bought at a mandi was winnowed earlier.

The limit of handpicking is worth stating: it works only when the quantities are small and the pieces distinguishable. Separating fine sand from fine salt by hand is impossible, which is why the other methods exist.

Six methods, chosen by whether the solid dissolves and how fine its particles are.

Sedimentation and decantation — let the heavier insoluble solid settle at the bottom (sedimentation), then pour off the clear liquid carefully (decantation). Used for muddy water.

Loading — used when the particles are too fine to settle on their own. A pinch of alum is added; the fine clay particles stick to it, become heavier and settle faster. This is how drinking water is clarified in many homes and waterworks.

Filtration — the mixture is poured through filter paper in a funnel. The insoluble solid stays on the paper as the residue, and the clear liquid passing through is the filtrate. Used for chalk and water, or tea leaves and tea.

Centrifugation — the mixture is spun rapidly, so the denser particles are thrown outward and collect at the bottom. Used when the particles are too fine even for filter paper, as in separating cream from milk.

Evaporation — the liquid is heated so it escapes as vapour, leaving the dissolved solid behind. This is how salt is obtained from sea water in coastal salt pans.

Crystallisation — the solution is heated to concentrate it, then cooled slowly so the solid separates as pure crystals. Preferred over evaporation when the solid would decompose on strong heating, or when pure, well-formed crystals are wanted, as for copper sulphate or alum.

Filtration and evaporation are often confused, and the test is one word: soluble. A soluble solid passes straight through filter paper with the liquid, so filtration cannot remove it — only evaporation or crystallisation will.

It depends on whether the liquids mix at all, and how close their boiling points are.

Separating funnel — used for two immiscible liquids of different densities, such as oil and water or kerosene and water. The mixture is poured into the funnel and left to form two layers; the stopcock is opened to run off the denser lower layer, then closed before the upper layer follows.

Simple distillation — used for a miscible liquid mixed with a dissolved solid, or for two liquids whose boiling points differ widely. The mixture is heated, the liquid with the lower boiling point vaporises, and the vapour is cooled in a condenser and collected as the distillate. Pure water is obtained from salt water this way.

Fractional distillation — used for two or more miscible liquids whose boiling points are close together. A fractionating column is fitted above the flask; the vapours condense and re-vaporise repeatedly as they climb it, so the components separate more sharply and come off one at a time. Petroleum is refined this way, and liquid air is separated into nitrogen and oxygen.

Chromatography — used to separate the dissolved components of a mixture that travel at different speeds through a medium. A drop of ink is placed on filter paper whose lower edge dips in water; as the water rises, the dyes spread to different heights and appear as separate bands.

The ink test is easy to try at home, and it shows that a black marker's ink is a mixture of several coloured dyes.

The distinction between the two distillations is the marked point: simple for widely different boiling points, fractional for close ones.