NDA Chemistry · Matter and Its States
Separation Techniques
Mixtures are pulled apart by physical methods chosen to exploit a difference between the components — boiling point (distillation), density/immiscibility (separating funnel, centrifugation), or volatility and solubility (sublimation, evaporation, crystallization).
Why this matters
Seven PYQs, the joint-largest subtopic. The bank tests a single skill: match the mixture to the right separation method. Distillation and fractional distillation dominate (petrol/kerosene, petroleum refining, two miscible liquids), with centrifugation (blood, cream), chromatography (pigments), separating funnel (oil and water), sublimation (anthracene, camphor) and evaporation/crystallization (a non-volatile solute) filling the rest. Learn which property each method exploits and the match-list questions solve themselves.
Concept 1 of 2: Distillation, fractional distillation and the separating funnel
How you split two liquids depends on whether they mix. If they mix (miscible) and boil at different temperatures, distil them. If their boiling points are close, use fractional distillation. If they don't mix (immiscible, like oil and water), just let them settle into layers and run off the bottom one — a separating funnel.
Definition
The liquid-separation methods:
- Distillation — separates two miscible liquids (a homogeneous mixture) with a large boiling-point difference, or a liquid from a non-volatile solid. The liquid is boiled and the vapour condensed back. Use for acetone and water, or two miscible liquids in general.
- Fractional distillation — distillation with a fractionating column, for miscible liquids whose boiling points are close. Use for petrol and kerosene, and the refining of petroleum (crude oil → fractions).
- Separating funnel — separates two immiscible liquids of different density (they form layers). Use for water and kerosene oil, or oil and water.
| Method | Use it when… | Example |
|---|---|---|
| Distillation | Two miscible liquids, boiling points far apart | Acetone and water; two miscible liquids |
| Fractional distillation | Miscible liquids, boiling points close | Petrol and kerosene; refining petroleum Close boiling points → you need the fractionating column → fractional distillation. |
| Separating funnel | Two immiscible liquids (don't mix) | Water and kerosene oil; oil and water Immiscible = they form separate layers → separating funnel, not distillation. |
Miscible → (fractional) distillation by boiling point; immiscible → separating funnel by layers.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
NDA · Sep 2023 · Q115Easy
Example 1 · Matter and Its States · Separation Techniques
Kerosene and petrol mixture can be best separated by
Miscible vs immiscible decides the method
Two liquids that mix (miscible) are separated by distillation; two that don't mix (immiscible, like oil and water) are separated by a separating funnel. Picking distillation for oil-and-water is the classic error.
Close boiling points need the column
When miscible liquids have close boiling points (petrol/kerosene, crude-oil fractions), simple distillation fails — use fractional distillation (with the fractionating column).
Concept 2 of 2: Centrifugation, chromatography, sublimation, evaporation and crystallization
The rest of the toolbox, each tuned to one property. Spin to separate by density (centrifugation). Let components creep up paper at different rates to separate by adsorption (chromatography). Heat a mixture where one part sublimes (sublimation). Boil off the solvent (evaporation) or cool a hot solution to grow pure crystals (crystallization).
Definition
The remaining methods and the property each exploits:
- Centrifugation — spins a mixture so denser particles move out; separates suspended solids from a liquid. Use for cream from milk and blood tests (separating blood cells from plasma).
- Chromatography — separates components by their different rates of movement over an adsorbing medium. Use for pigments from a plant extract (and dyes in ink).
- Sublimation — separates a sublimable solid (turns straight to gas) from a non-sublimable one. Use for anthracene from a salt mixture, camphor/naphthalene from sand, iodine from sand.
- Evaporation — boils/dries off the solvent to leave a non-volatile solute behind (e.g. salt from salt water).
- Crystallization — cools a hot saturated solution so the pure solute crystallises out (purer than evaporation). Both evaporation and crystallization recover a non-volatile solid solute from its solution.
| Method | Separates | Example |
|---|---|---|
| Centrifugation | Suspended solid from liquid (by density, by spinning) | Cream from milk; blood tests Blood tests in diagnostic labs use centrifugation to spin cells from plasma. |
| Chromatography | Components by rate of movement / adsorption | Pigments from plant extract; ink dyes |
| Sublimation | A sublimable solid from a non-sublimable one | Anthracene from salt; camphor from sand Use sublimation only when ONE component sublimes (turns solid → gas). |
| Evaporation | Non-volatile solute from solvent (dry off solvent) | Salt from salt water |
| Crystallization | Pure solute crystals from a hot solution | Pure crystals from a non-volatile solute A non-volatile solid solute can be recovered by EITHER evaporation OR crystallization. |
Practice this conceptself-check · 5 quick reps
The same idea in a real exam question:
NDA · Sep 2023 · Q92Moderate
Example 2 · Matter and Its States · Separation Techniques
Match List I with List II and select the correct answer using the code given below the Lists:
Code: A-B-C-D
| List I (Process) | List II (Separation method) |
|---|---|
| A. Separation of acetone and water from their mixture | 1. Chromatography |
| B. Separation of water and kerosene oil from their mixture | 2. Centrifugation |
| C. Separation of cream from milk | 3. Distillation |
| D. Separation of pigments from plant extract | 4. Separating Funnel |
Non-volatile solute → evaporation OR crystallization
A non-volatile solid solute can be recovered from its solution by both evaporation and crystallization — so in a 'which process(es) work' question, the answer is often 'both'. Crystallization gives purer crystals.
Sublimation needs a sublimable component
Reach for sublimation only when one component turns solid → gas directly (camphor, naphthalene, iodine, ammonium chloride, anthracene). To separate anthracene from salt, the anthracene sublimes and the salt stays.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Distillation, fractional distillation and the separating funnel3 rows
| Method | Use it when… | Example |
|---|---|---|
| Distillation | Two miscible liquids, boiling points far apart | Acetone and water; two miscible liquids |
| Fractional distillation | Miscible liquids, boiling points close | Petrol and kerosene; refining petroleum Close boiling points → you need the fractionating column → fractional distillation. |
| Separating funnel | Two immiscible liquids (don't mix) | Water and kerosene oil; oil and water Immiscible = they form separate layers → separating funnel, not distillation. |
Miscible → (fractional) distillation by boiling point; immiscible → separating funnel by layers.
Centrifugation, chromatography, sublimation, evaporation and crystallization5 rows
| Method | Separates | Example |
|---|---|---|
| Centrifugation | Suspended solid from liquid (by density, by spinning) | Cream from milk; blood tests Blood tests in diagnostic labs use centrifugation to spin cells from plasma. |
| Chromatography | Components by rate of movement / adsorption | Pigments from plant extract; ink dyes |
| Sublimation | A sublimable solid from a non-sublimable one | Anthracene from salt; camphor from sand Use sublimation only when ONE component sublimes (turns solid → gas). |
| Evaporation | Non-volatile solute from solvent (dry off solvent) | Salt from salt water |
| Crystallization | Pure solute crystals from a hot solution | Pure crystals from a non-volatile solute A non-volatile solid solute can be recovered by EITHER evaporation OR crystallization. |
Watch out for (4)
- Miscible vs immiscible decides the method→ Distillation, fractional distillation and the separating funnel
- Close boiling points need the column→ Distillation, fractional distillation and the separating funnel
- Non-volatile solute → evaporation OR crystallization→ Centrifugation, chromatography, sublimation, evaporation and crystallization
- Sublimation needs a sublimable component→ Centrifugation, chromatography, sublimation, evaporation and crystallization
Test yourself on Matter and Its States
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.