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State Board · Class 11 · Chemistry

Lesson 3: Some Analytical Techniques

7 concepts · 56 practice questions with answers · Free to read

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Purifying solids: filtration

From daily life

A tea strainer separates leaves from liquid; the lab does the same with filter paper.

Simple definition

Filtration separates insoluble solid from a liquid using filter paper. Suction filtration uses a Büchner funnel and pump for speed and a drier residue.

FiltrationFiltration: simple and suction (Büchner)filter paperfiltrateresidueBüchner funnelsuction flaskvacuumSuction ⇒ faster, drier residue, good for fine solidsRemoves insoluble impurities

In the figure : Filtration.

Rule 1: Simple filtration — A folded filter paper in a funnel; insoluble impurity stays as residue and liquid passes as filtrate.

  • Easy : Filtering muddy water.
  • Medium : Powder + water.
  • Exam-type : Dry the residue.

Rule 2: Suction filtration — Büchner funnel, suction flask and pump; fine particles are filtered fast and air pulls the residue dry.

  • Easy : Collecting crystals.
  • Medium : Minutes instead of hours.
  • Exam-type : Reduced pressure in the flask.

Rule 3: Limits — Separates only insoluble solids; soluble mixtures need distillation or crystallisation.

  • Easy : Salt in water cannot be filtered out.
  • Medium : Sand + salt: filter, then evaporate.
  • Exam-type : Only suspended particles are removed.

Calculation: residue percentage

  • Step 1: 10.0 g of mixture is filtered.
  • Step 2: Dry residue 3.2 g.
  • Step 3: 3.2 ÷ 10.0 × 100 = 32 %.

CET level (MCQ)

  • CET 1) For suction filtration? 1) Büchner funnel 2) separating funnel 3) condenser 4) column — Answer: 1).
  • CET 2) A dissolved solid is recovered by? 1) filtering 2) evaporation 3) sieving 4) hand — Answer: 2).

Real-life use: Water treatment, lab preparation.

Remember

Filtration = insoluble; suction = fast and dry.

Common mistake

Thinking dissolved salt can be filtered out.

CET trap

Wet the filter paper first in suction filtration.

Shortcut

Memory clue: "filter = solids, distil = liquids".

Spot the difference

MethodSpeedResidue
Simpleslowwet
Suctionfastdrier

Practice questions

Easy
  1. 1What is filtration?
  2. 2Why suction filtration?
  3. 3What is a residue?
Medium
  1. 4What is the filtrate?
  2. 5How separate salt from water?
  3. 63.2 g residue in 10 g ⇒ %?
Challenging
  1. 7Separating sand and salt?
  2. 8Use of a Büchner funnel?
Show answers (try them yourself first)
  1. 1Separating insoluble particles.
  2. 2Speed and drier residue.
  3. 3Solid left on the paper.
  4. 4The liquid passing through.
  5. 5Evaporation or distillation.
  6. 632 %.
  7. 7Filter, then evaporate.
  8. 8Suction filtration.

Crystallisation and fractional crystallisation

From daily life

Rock candy forms as hot sugar solution cools slowly; that is crystallisation.

Simple definition

Crystallisation obtains pure crystals from a hot saturated solution on cooling. Fractional crystallisation separates two solutes by their solubility difference, one after the other.

CrystallisationCrystallisation: solubility curvesABtemperature →solubilityA is very soluble hot, little cold ⇒ crystals of A on coolingB changes little ⇒ stays dissolved (fractional crystallisation)e.g. pure sugar, alum

In the figure : Solubility curves.

Rule 1: Steps — Dissolve in hot solvent ⇒ saturate ⇒ filter hot ⇒ cool slowly ⇒ filter and dry the crystals.

  • Easy : Impure sugar ⇒ pure.
  • Medium : Alum crystals.
  • Exam-type : Slow cooling ⇒ large crystals.

Rule 2: Solubility and yield — Yield = hot solubility − cold solubility (per 100 g solvent).

  • Easy : 80 g (373 K), 20 g (293 K) ⇒ 60 g crystals.
  • Medium : Steeper curve ⇒ larger yield.
  • Exam-type : Flat curve ⇒ small yield.

Rule 3: Fractional crystallisation — Two solutes with different solubility: the less soluble crystallises first; repeat to separate fully.

  • Easy : KNO₃ + NaCl: cool ⇒ KNO₃ crystals.
  • Medium : NaCl stays in solution.
  • Exam-type : The mother liquor remains.

Calculation: yield

  • Step 1: 100 g water dissolves 80 g at 373 K.
  • Step 2: Only 20 g stays at 293 K.
  • Step 3: Crystals = 80 − 20 = 60 g.

CET level (MCQ)

  • CET 1) Crystallisation depends on? 1) boiling point 2) solubility 3) density 4) colour — Answer: 2).
  • CET 2) Large crystals come from? 1) fast cooling 2) slow cooling 3) stirring 4) boiling — Answer: 2).

Real-life use: Purifying sugar, salt and drugs.

Remember

Hot saturated ⇒ cool ⇒ crystals; slow = large.

Common mistake

Expecting large crystals from rapid cooling.

CET trap

Crystallisation purifies; the fractional version separates.

Shortcut

Memory clue: "dissolve hot, crystallise cold".

Spot the difference

ItemSimpleFractional
Solutesonetwo
Basissolubilitysolubility gap

Practice questions

Easy
  1. 1What is crystallisation?
  2. 2State the steps.
  3. 3Why cool slowly?
Medium
  1. 480 g / 20 g ⇒ yield?
  2. 5What is fractional?
  3. 6How separate KNO₃ and NaCl?
Challenging
  1. 7What is mother liquor?
  2. 8How are crystals dried?
Show answers (try them yourself first)
  1. 1Getting pure crystals from solution.
  2. 2Dissolve, filter, cool, filter.
  3. 3Large pure crystals.
  4. 460 g.
  5. 5Separation by solubility difference.
  6. 6Cool and collect KNO₃.
  7. 7Liquid left after crystallising.
  8. 8Press between filter papers or air-dry.

Simple distillation

From daily life

Rain comes from water vapour cooling to liquid; distillation does the same on a bench.

Simple definition

Distillation boils a liquid and condenses the vapour to collect the pure liquid. Simple distillation works when the boiling points differ widely (rule of thumb: more than about 40 K) or the impurity is non-volatile.

Simple distillationSimple distillationmixturethermometercondenserwater out ↑water in ↑receiverConditions: non-volatile impurity or a wide boiling-point gape.g. pure water from seawater; CHCl₃ (334 K) + aniline (457 K)

In the figure : Simple distillation.

Rule 1: Apparatus — Flask ⇒ thermometer ⇒ condenser (water in at the bottom, out at the top) ⇒ receiver. Add boiling chips.

  • Easy : The condenser jacket stays full.
  • Medium : The thermometer bulb sits at the side arm.
  • Exam-type : Chips keep boiling smooth.

Rule 2: When to use — Non-volatile impurities (seawater) or a wide boiling-point gap.

  • Easy : Seawater ⇒ pure water.
  • Medium : CHCl₃ (334 K) + aniline (457 K).
  • Exam-type : Salt water.

Rule 3: Principle — The lower-boiling component vaporises first; a pure liquid boils at a constant temperature.

  • Easy : Pure water holds 373 K.
  • Medium : A changing temperature ⇒ a mixture.
  • Exam-type : First fraction = lower boiling point.

Reasoning: why do CHCl₃ and aniline separate?

  • Step 1: Boiling points 334 K and 457 K.
  • Step 2: Gap = 123 K, very large ⇒ simple distillation is enough.
  • Step 3: Simple distillation suffices; CHCl₃ distils first.

CET level (MCQ)

  • CET 1) Water enters the condenser from? 1) top 2) bottom 3) both 4) none — Answer: 2).
  • CET 2) How do you separate two miscible liquids with close boiling points? 1) fractional distillation 2) simple distillation 3) filtration 4) crystallisation — Answer: 1).

Real-life use: Drinking water, spirit purification.

Remember

Simple: a wide b.p. gap; condenser water from the bottom.

Common mistake

Feeding water from the top; the jacket does not fill.

CET trap

Place the thermometer bulb at the vapour side arm.

Shortcut

Memory clue: "in at the bottom, out at the top".

Spot the difference

Methodb.p. gap
Simplewide
Fractionalsmall

Practice questions

Easy
  1. 1What is distillation?
  2. 2When use simple distillation?
  3. 3Which way does water flow?
Medium
  1. 4Why boiling chips?
  2. 5CHCl₃ + aniline?
  3. 6Which fraction first?
Challenging
  1. 7Temperature of a pure liquid?
  2. 8Purifying seawater?
Show answers (try them yourself first)
  1. 1Boil and condense vapour.
  2. 2A wide b.p. gap.
  3. 3From the bottom.
  4. 4To smooth boiling.
  5. 5Simple distillation.
  6. 6The lower-boiling one.
  7. 7Constant.
  8. 8Distillation.

Fractional, reduced-pressure and steam distillation

From daily life

Petrol, diesel and kerosene come from crude oil as fractions, again by distillation.

Simple definition

Fractional: boiling points close together (small gap), with a fractionating column. Reduced pressure: high-boiling or decomposing liquids. Steam: immiscible liquids that carry over with steam.

Fractional distillationFractional and reduced-pressure distillationfractionating columnhot bottom, cooler topReduced pressure: high-boiling liquidsGlycerol b.p. 563 K (decomposes)under reduced pressure ≈ 453 KSteam distillation: anilinecomes over with water below 373 Kfor immiscible, steam-volatile liquidsPetroleum fractions: fractional distillationclose b.p. ⇒ fractional; decomposes ⇒ reduced pressuree.g. benzene (353 K) + toluene (384 K)

In the figure : Fractional, reduced pressure, steam.

Rule 1: Fractional distillation — Repeated vaporisation-condensation in the column; the lower-boiling liquid collects at the top.

  • Easy : Benzene (353 K) + toluene (384 K).
  • Medium : Crude oil fractions.
  • Exam-type : Liquid air.

Rule 2: Reduced pressure — Lower pressure ⇒ lower boiling point ⇒ no decomposition. Glycerol decomposes at 563 K ⇒ distil near 453 K under reduced pressure.

  • Easy : Glycerol.
  • Medium : Sugar syrups.
  • Exam-type : Heat-labile drugs.

Rule 3: Steam distillation — Immiscible liquid + water ⇒ total vapour pressure is the sum ⇒ boiling below 373 K.

  • Easy : Aniline.
  • Medium : Clove and eucalyptus oil.
  • Exam-type : Natural fragrances.

Reasoning: why boil below 373 K in steam distillation?

  • Step 1: Two immiscible liquids ⇒ P(total) = P₁ + P₂.
  • Step 2: Boiling starts when P(total) equals atmospheric pressure.
  • Step 3: Each pressure is below atmospheric ⇒ temperature stays under 373 K.

CET level (MCQ)

  • CET 1) To purify glycerol? 1) simple 2) reduced pressure 3) filtration 4) extraction — Answer: 2).
  • CET 2) Aniline is purified by? 1) steam distillation 2) crystallisation 3) filtration 4) chromatography — Answer: 1).

Real-life use: Petroleum refining, essential oils.

Remember

Fractional = close b.p.; reduced pressure = decomposing; steam = immiscible.

Common mistake

Treating all three as the same case.

CET trap

Reduced pressure lowers the boiling point, never raises it.

Shortcut

Memory clue: "small gap ⇒ column; heat-sensitive ⇒ low pressure".

Spot the difference

TypeWhen
Fractionalclose b.p.
Reduced pressuredecomposes
Steamimmiscible

Practice questions

Easy
  1. 1When use fractional distillation?
  2. 2Why reduced pressure for glycerol?
  3. 3What is steam distillation?
Medium
  1. 4One example of each.
  2. 5How is crude oil separated?
  3. 6Function of the column?
Challenging
  1. 7Why below 373 K?
  2. 8Boiling point of aniline?
Show answers (try them yourself first)
  1. 1Boiling points close together.
  2. 2It decomposes at 563 K.
  3. 3Collecting an immiscible liquid with steam.
  4. 4Benzene–toluene, glycerol, aniline.
  5. 5Fractional distillation.
  6. 6Repeated vaporisation-condensation.
  7. 7Total pressure equals atmospheric.
  8. 8457 K.

Solvent extraction

From daily life

In tea making the colour moves from leaves into water — a solvent pulling out a component.

Simple definition

Extraction moves a compound from one solvent into another immiscible one. K = C(organic) ÷ C(aqueous).

Solvent extractionSolvent extractionetherwaterseparating funnelDistribution coefficient K = C(organic) ÷ C(aqueous)e.g. K = 4; 1 g in 100 mL water1 × 50 mL ether ⇒ 66.7 % extracted2 × 25 mL ether ⇒ 75 % extractedremaining = [V ÷ (V + Kv)]ⁿseveral small portions beat one largeorganic compound moves from water to ether/CHCl₃; solvents immisciblee.g. caffeine from tea, plant extractsSeparate the layer, evaporate ⇒ compound

In the figure : Extraction.

Rule 1: Principle — Distribution law: K is constant at a fixed temperature. Organic compounds dissolve more in ether.

  • Easy : K = 4 ⇒ ether holds 4×.
  • Medium : Shake in a separating funnel.
  • Exam-type : Let layers settle.

Rule 2: Once or many times — Fraction left = [V ÷ (V + Kv)]ⁿ; several small portions extract more.

  • Easy : 1 × 50 mL ⇒ 66.7 %.
  • Medium : 2 × 25 mL ⇒ 75 %.
  • Exam-type : 4 × 12.5 mL ⇒ 80 %.

Rule 3: Procedure — Aqueous solution + ether ⇒ shake ⇒ separate ⇒ evaporate the ether ⇒ compound. Ether is flammable: no flame.

  • Easy : Caffeine.
  • Medium : Medicinal oils.
  • Exam-type : Benzoic acid.

Calculation: 100 mL aqueous, K = 4, 50 mL ether

  • Step 1: Left = V ÷ (V + Kv) = 100 ÷ (100 + 200) = 0.333.
  • Step 2: Extracted = 66.7 %.
  • Step 3: 2 × 25 mL ⇒ left (100 ÷ 200)² = 0.25 ⇒ 75 %.

CET level (MCQ)

  • CET 1) Apparatus for extraction? 1) separating funnel 2) condenser 3) Büchner 4) column — Answer: 1).
  • CET 2) More efficient? 1) one large 2) several small 3) same 4) none — Answer: 2).

Real-life use: Caffeine extraction, herbal medicine.

Remember

K = C(org)/C(aq); several small portions.

Common mistake

Using miscible solvents.

CET trap

For equal total solvent, many small portions extract more.

Shortcut

Memory clue: "little and often".

Spot the difference

Method% extracted
1 × 50 mL66.7
2 × 25 mL75
4 × 12.5 mL80

Practice questions

Easy
  1. 1What is extraction?
  2. 2What is K?
  3. 3K = 4, 50 mL ether ⇒ %?
Medium
  1. 42 × 25 mL ⇒ %?
  2. 5Why several times?
  3. 6Safety with ether?
Challenging
  1. 7One example.
  2. 8Why immiscible?
Show answers (try them yourself first)
  1. 1Moving between solvents.
  2. 2C(organic)/C(aqueous).
  3. 366.7 %.
  4. 475 %.
  5. 5Less is left.
  6. 6Flammable.
  7. 7Caffeine from tea.
  8. 8To give two separable layers.

Chromatography: principle and adsorption

From daily life

A drop of ink spreading on wet paper separates colours by distance — that is chromatography.

Simple definition

Chromatography separates a mixture by its distribution between a stationary and a mobile phase; components move at different rates.

ChromatographyPrinciple of chromatographyadsorbent Al₂O₃columnstationary phase (adsorbent) + mobile phase (solvent)stronger adsorption ⇒ moves slowerweaker adsorption ⇒ elutes firstTypes: column, TLC, paperadsorption: Al₂O₃, silica gelpartition: water-held papermixture ⇒ separated components (dyes, drugs, pesticide residues)components move at different rates

In the figure : Chromatography.

Rule 1: Adsorption chromatography — Stationary phase Al₂O₃ or silica gel; components adsorb with different strengths ⇒ different speeds.

  • Easy : Weakly adsorbed ⇒ elutes first.
  • Medium : Al₂O₃ column.
  • Exam-type : TLC: thin layer.

Rule 2: Column and TLC — Column: adsorbent in a tube, mixture on top, solvent flows ⇒ bands. TLC: thin layer on a plate, small spot.

  • Easy : Separating pigments.
  • Medium : TLC is fast, needs little sample.
  • Exam-type : Collect components separately.

Rule 3: Uses — Dyes, drug purity, pesticide residue and forensic analysis.

  • Easy : Spinach pigments.
  • Medium : Drug purity.
  • Exam-type : Ink analysis.

Reasoning: why does the weakly adsorbed one elute first?

  • Step 1: A component distributes between adsorbent and solvent.
  • Step 2: Weaker adsorption ⇒ more time in the solvent.
  • Step 3: It moves faster ⇒ comes out first.

CET level (MCQ)

  • CET 1) A stationary phase? 1) Al₂O₃ 2) solvent 3) gas 4) air — Answer: 1).
  • CET 2) A strongly adsorbed component moves? 1) first 2) slowly 3) equally 4) never — Answer: 2).

Real-life use: Pigment analysis, pharmaceuticals.

Remember

Stationary + mobile; stronger adsorption ⇒ slower.

Common mistake

Calling the solvent the stationary phase.

CET trap

In TLC keep the spot above the solvent level.

Shortcut

Memory clue: "sticky = slow, free = fast".

Spot the difference

PhaseExample
StationaryAl₂O₃, silica
Mobilesolvent

Practice questions

Easy
  1. 1What is chromatography?
  2. 2Example of a stationary phase.
  3. 3What is TLC?
Medium
  1. 4Which elutes first?
  2. 5Two uses.
  3. 6How is a column set up?
Challenging
  1. 7The mobile phase?
  2. 8Why do components separate?
Show answers (try them yourself first)
  1. 1Separation by distribution between phases.
  2. 2Al₂O₃ or silica gel.
  3. 3Thin-layer chromatography.
  4. 4The weakly adsorbed.
  5. 5Dye analysis, drug purity.
  6. 6Pack adsorbent in a tube.
  7. 7The flowing solvent.
  8. 8Different adsorption strengths.

Partition (paper) chromatography and Rf

From daily life

On filter paper, ink separates into colour rings; each colour has its own Rf.

Simple definition

Paper chromatography: water held in the paper is the stationary phase and an organic solvent is mobile. Rf = distance moved by the solute ÷ distance moved by the solvent.

RfPaper chromatography and Rfstart linesolvent front (8 cm)ABRf = distance (solute) ÷ distance (solvent)A: 2.0 ÷ 8 = 0.25B: 5.2 ÷ 8 = 0.65Rf < 1; B travels fartherRf is characteristic of a compound under fixed solvent/temperatureinvisible spots: iodine vapour or UV lamp

In the figure : Paper chromatography, Rf.

Rule 1: Partition principle — A solute partitions between water (stationary) and organic solvent (mobile); more soluble in the solvent ⇒ moves farther.

  • Easy : Separating amino acids.
  • Medium : Pigments.
  • Exam-type : Sugar types.

Rule 2: Rf calculation — Rf < 1; fixed for a compound under fixed solvent, temperature and paper.

  • Easy : A: 2.0 ÷ 8 = 0.25.
  • Medium : B: 5.2 ÷ 8 = 0.65.
  • Exam-type : Higher Rf ⇒ more soluble in the mobile phase.

Rule 3: Locating spots — Coloured spots directly; for invisible ones use ninhydrin (amino acids), iodine vapour or UV.

  • Easy : Ninhydrin ⇒ purple.
  • Medium : Iodine ⇒ brown spots.
  • Exam-type : UV lamp.

Calculation: Rf

  • Step 1: Solvent front = 8.0 cm.
  • Step 2: B spot = 5.2 cm.
  • Step 3: Rf = 5.2 ÷ 8.0 = 0.65.

CET level (MCQ)

  • CET 1) Rf is? 1) > 1 2) < 1 3) = 1 4) negative — Answer: 2).
  • CET 2) Reagent for amino acids? 1) ninhydrin 2) iodine 3) Fehling 4) Tollens — Answer: 1).

Real-life use: Drug purity, food-colour testing.

Remember

Rf = d(solute) ÷ d(solvent) < 1.

Common mistake

Expecting an Rf above 1; it cannot happen.

CET trap

Measure from the start line, not the paper edge.

Shortcut

Memory clue: "Rf = solute ÷ solvent".

Spot the difference

SpotDistanceRf
A2.0 cm0.25
B5.2 cm0.65

Practice questions

Easy
  1. 1What is Rf?
  2. 2Why is Rf never above 1?
  3. 32.0 cm, 8 cm ⇒ Rf?
Medium
  1. 4Stationary phase in paper?
  2. 5How to see invisible spots?
  3. 6What does a high Rf mean?
Challenging
  1. 7What is the solvent front?
  2. 8What does Rf depend on?
Show answers (try them yourself first)
  1. 1Solute distance ÷ solvent distance.
  2. 2A solute never outruns the solvent.
  3. 30.25.
  4. 4Water held in the paper.
  5. 5Ninhydrin, iodine, UV.
  6. 6More soluble in the solvent.
  7. 7The line the solvent reached.
  8. 8Solvent, temperature, paper.

Quick revision of the whole lesson

ConceptKey formula / fact
Filtrationinsoluble solid
Crystallisationsolubility gap
Simple distillationlarge Δbp
Fractionalsmall Δbp
Reduced pressureglycerol
Steamaniline
ExtractionK = C(org)/C(aq)
Rfd(solute)/d(solvent)
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🎯 Most important points for the exam

  • Filtration.
  • Crystallisation.
  • Simple distillation.
  • Fractional, reduced, steam.
  • Extraction.
  • Chromatography.
  • Paper chromatography, Rf.
Study mantra

understand the idea → remember the formula → solve the example → solve problems yourself.