Look around a chemistry lab and almost nothing is truly pure — air, milk, sea water and brass are all mixtures. The CDS exam loves this chapter because it rewards clear thinking, not heavy calculation. In the next few minutes you will learn what separates a mixture from a compound and which technique cleans up which mixture, fast.
Why this topic is worth your marks
In the CDS and OTA General Science paper, Mixtures and Separation Techniques appears almost every year. The questions are short, factual and need no formula sheet — you simply have to know what each method does and why. That makes this one of the best return-on-effort chapters for a graduate aiming to clear the cut-off, especially when time is tight in the final weeks of revision.
The topic also feeds into other areas: water purification, the petroleum industry, blood analysis and even forensic science all rest on separation methods. So a single afternoon here pays back across the whole paper. You will also find that these ideas reappear in everyday general-knowledge questions about how salt is made, how cream is taken off milk, or how a refinery turns crude oil into petrol and diesel.
The whole chapter rests on one simple idea: the substances in a mixture keep their own physical properties, and we exploit a difference in one of those properties — size, weight, magnetism, solubility, boiling point or density — to pull them apart. Once you fix that idea in your mind, every method in this chapter starts to feel obvious rather than something to be memorised blindly.
Examiners test application: given a mixture, choose the right technique. Memorising definitions alone is not enough — learn the property each method exploits.
Pure substances versus mixtures
A pure substance is made of only one kind of particle and has a fixed composition. Examples: pure gold, distilled water, oxygen gas, common salt (sodium chloride). It has a sharp, fixed melting and boiling point.
A mixture contains two or more substances that are simply mixed, not chemically combined. Its composition can vary, and its properties are a blend of its components. Air, milk, soil and lemonade are all mixtures.
The simplest test is this: ask whether the substance can be split into simpler substances by an ordinary physical method such as filtering, heating gently, or using a magnet. If it can, it is a mixture. If it cannot — if you would need a chemical reaction to break it down — then it is a compound or an element, that is, a pure substance. Sea water is a mixture because boiling it leaves salt behind; sodium chloride is a pure compound because no amount of physical effort will split it into sodium and chlorine.
Pure substances are of two kinds — elements (one type of atom, e.g. Fe, O2) and compounds (two or more elements chemically bonded in a fixed ratio, e.g. H2O, CO2). A mixture is not a pure substance.
Mixture or compound? The crucial difference
This is a classic CDS trap. In a compound, elements combine chemically in a fixed mass ratio, energy is absorbed or released, and the product has entirely new properties. In a mixture, substances keep their own properties and can be separated by simple physical means.
- Iron + sulphur, just mixed → a mixture; a magnet still pulls out the iron.
- Iron + sulphur, heated → iron sulphide (FeS), a compound; the magnet now does nothing.
Students call water a mixture of hydrogen and oxygen. Wrong — water is a compound. Hydrogen and oxygen are chemically bonded in a fixed 2:1 ratio and cannot be separated by physical methods.
Homogeneous and heterogeneous mixtures
Mixtures come in two families based on how uniform they are.
Homogeneous mixtures
The composition is uniform throughout and the components cannot be seen separately. A true solution such as salt in water, sugar syrup, air or an alloy like brass is homogeneous. Particle size is below 1 nanometre.
Heterogeneous mixtures
The composition is not uniform; you can often see the different parts. Examples: sand and salt, oil and water, soil, a mixture of iron filings and sulphur.
Particle size order: true solution (< 1 nm, homogeneous, does not scatter light) → colloid (1–1000 nm, looks uniform but scatters light) → suspension (> 1000 nm, heterogeneous, particles settle).
Solutions, colloids and suspensions
A solution has a solvent (larger amount, e.g. water) and a solute (smaller amount, e.g. salt). The dissolved particles are too small to see or filter out. Concentration is the amount of solute in a given amount of solution.
A colloid looks homogeneous but its particles are big enough to scatter a beam of light — the Tyndall effect. Milk, fog, smoke, blood and jelly are colloids. Colloid particles do not settle and pass through filter paper.
A suspension is heterogeneous: chalk in water or muddy water. Its particles are visible, settle on standing and are trapped by filter paper.
Why does this three-way split matter for the exam? Because the size of the particles decides which separation method will work. You can filter out a suspension but never a true solution; you can see a beam of light pass through a colloid but not through a clear solution. Many questions are really just asking you to place a given example — muddy water, milk, sugar solution — into the correct box and then read off its behaviour. Get the classification right and the rest of the answer follows automatically.
If a question mentions a beam of light visible through a liquid (a torch through fog or a projector beam in a dusty hall), the answer is the Tyndall effect, which proves the medium is a colloid.
Everyday separation methods
Each method exploits a difference in a physical property. Match the property to the mixture.
- Handpicking — large, visibly different solids, e.g. stones from rice.
- Winnowing — separates lighter husk from heavier grain using wind.
- Sieving — separates solids of different particle sizes, e.g. flour from bran.
- Magnetic separation — pulls out magnetic material like iron from a non-magnetic mixture.
- Sedimentation and decantation — heavier insoluble solid settles, then the liquid is poured off.
- Filtration — separates an insoluble solid from a liquid using filter paper, e.g. tea leaves from tea.
Notice how each of these depends on a single, easy-to-spot property. Handpicking and sieving use a difference in size; winnowing and sedimentation use a difference in weight or density; magnetic separation uses a magnetic property that only one component has. When a CDS question describes a real-life scene — a farmer cleaning grain, a mason sifting sand — identify which property differs between the components, and the method names itself.
Filtration deserves special attention because it is the workhorse of the laboratory. The liquid that passes through is the filtrate and the solid left on the paper is the residue. It works only when the solid is genuinely insoluble; a dissolved solid such as salt will simply pass through with the water and must be recovered by evaporation instead.
Loading speeds up sedimentation: adding alum (a coagulant) makes fine clay particles in muddy water clump and settle faster — the basis of water treatment.
Evaporation and crystallisation
To recover a dissolved solid solute from its solution, you remove the solvent.
Evaporation heats the solution so the solvent vaporises and the solid is left behind. Sea water is evaporated in shallow pans to obtain common salt. The drawback: gentle solids may decompose or look impure.
Crystallisation is the gentler, purer technique. A hot saturated solution is allowed to cool slowly so pure crystals of the solute grow while impurities stay in the liquid (the mother liquor). This is how pure copper sulphate and sugar crystals are obtained.
Do not confuse the two. Evaporation gives a quick solid but may carry impurities; crystallisation gives slow-grown, high-purity crystals. CDS often asks which method gives a purer product — the answer is crystallisation.
Distillation and fractional distillation
When you want to keep the liquid too, distillation is the answer. The mixture is boiled, the vapour is led through a condenser and collected as pure liquid (the distillate).
Simple distillation
Separates a dissolved solid from a liquid, or two liquids whose boiling points differ widely (more than about 25°C). Example: pure water from salt water.
Fractional distillation
Separates two or more miscible liquids with close boiling points using a fractionating column, which gives repeated condensation and vaporisation. It separates the gases of air after liquefaction and the components of petroleum — petrol, kerosene and diesel — in a refinery.
Liquid boiling points difference: > 25°C → simple distillation. < 25°C and miscible → fractional distillation. Immiscible liquids (oil and water) → separating funnel, not distillation.
Separating funnel, chromatography and centrifugation
Separating funnel separates two immiscible liquids of different densities, such as oil and water, or kerosene and water. The denser lower layer is run off first through the stopcock.
Chromatography separates components of a mixture that move at different speeds on a medium (paper or a column). It is used to separate the dyes in ink, pigments in plant leaves and drugs in blood. The colour that travels farthest is the most soluble in the moving solvent.
Centrifugation spins a mixture at high speed so denser particles are flung outward and settle. It separates cream from milk, and blood plasma from blood cells in a pathology lab.
Centrifugation is, in effect, sedimentation made hundreds of times faster. Where gravity alone would take hours to settle fine particles, a spinning rotor produces a force many times stronger than gravity and the denser matter packs down within minutes. This is why it is the method of choice for colloidal and very fine mixtures that filter paper cannot trap — cream from milk, butter from curd, and the cellular part of blood from its watery plasma.
Chromatography, by contrast, is prized for separating substances present in tiny amounts and very similar to each other, which is exactly why forensic and medical laboratories rely on it. A drop of mixture is placed on the medium, a solvent creeps along, and each component travels a distance that depends on how strongly it sticks to the medium versus how readily it dissolves in the moving solvent.
Sublimation separates a solid that turns directly to vapour (camphor, ammonium chloride, naphthalene, iodine) from one that does not, e.g. salt mixed with ammonium chloride.
Worked example: choosing the right method
You are given a single mixture containing iron filings, common salt, sand and ammonium chloride. Outline a sequence of techniques to separate all four components.
Notice how each step uses a different physical property — magnetism, sublimation, solubility and then removal of the solvent. That ordered thinking is exactly what the examiner rewards.
Previous-year style question
Q. Which one of the following techniques is most suitable for separating two miscible liquids that have boiling points differing by only 10°C? (a) Simple distillation (b) Fractional distillation (c) Filtration (d) Use of a separating funnel
Answer: (b) Fractional distillation. When miscible liquids have boiling points that are close (a difference under about 25°C), simple distillation cannot separate them cleanly. The fractionating column provides repeated vaporisation and condensation, giving an effective separation — the same principle used in petroleum refining.
Filtration and the separating funnel deal with heterogeneous mixtures (solid+liquid, or immiscible liquids), so they are ruled out the moment the question says the liquids are miscible.
Quick revision
- Pure substance = element or compound, fixed composition; mixture = physically mixed, variable composition.
- Homogeneous (solution) vs heterogeneous; particle order: solution < colloid < suspension.
- Tyndall effect → colloid; suspensions settle and are filterable.
- Solid from liquid → evaporation or crystallisation (crystallisation is purer).
- Keep the liquid → distillation; close boiling points → fractional distillation.
- Immiscible liquids → separating funnel; sublimable solids → sublimation; colours/dyes → chromatography.
Drill five to ten previous-year questions on this chapter and the marks become almost automatic. Match the mixture to the property, and the property to the method.
Frequently asked questions
Is air a mixture or a compound?
Air is a homogeneous mixture of gases — mainly nitrogen and oxygen with smaller amounts of argon, carbon dioxide and water vapour. Its components are not chemically bonded and can be separated by fractional distillation of liquefied air.
What is the difference between evaporation and distillation?
Evaporation only recovers the dissolved solid and lets the solvent escape as vapour. Distillation boils the mixture and then condenses the vapour, so you recover the pure liquid as well.
Why can a colloid not be separated by filtration?
Colloidal particles are between 1 and 1000 nanometres — small enough to pass straight through ordinary filter paper. They are separated by centrifugation rather than filtration.
Which separation method is used in a petroleum refinery?
Fractional distillation. Crude oil is heated and its components — petrol, kerosene, diesel and others — separate out at different heights of the fractionating column according to their boiling points.
How do you separate two immiscible liquids like oil and water?
Use a separating funnel. The two liquids form distinct layers by density, and the heavier lower layer is run off through the stopcock first, leaving the lighter liquid behind.
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