Purification: Crystallization, Distillation & Sublimation Purification by Phase Changes — Why and When In organic chemistry, a product often comes mixed with unreacted reagents, solvents, and by-products. Physical purification methods work by changing phase (solid ⇄ liquid ⇄ vapour) so that the desired substance behaves differently from impurities. You will choose: crystallization (different solubilities), distillation (different boiling points), or sublimation (solid directly to vapour). Mastering these lets you quickly match a compound to the correct lab setup — a frequent NEET task. Before vs after purification: cloudy mixture to clear pure solid/liquid — the goal of phase-change methods. Big picture of purification methods. In this lesson: crystallization, distillation variants, and sublimation. Crystallization Desired solute is soluble in hot solvent, but sparingly soluble in cold; impurities remain dissolved. Solid organic compounds Benzoic acid from colored impurities Hot-saturated solution, slow cooling, crystals + mother liquor Simple distillation Components have wide b.p. gap (≈ ≥25 K). Lower-b.p. liquid vaporizes first. Liquid + non-volatile impurity or liquids with large b.p. difference Benzene (bp 353 K) from toluene (bp 384 K) only if one is dominant Round-bottom flask, condenser, thermometer at still head Fractional distillation Close b.p. components (<25 K) separated via multiple vapor–condensation cycles in a column. Liquid–liquid mixtures with close b.p. Ethanol–water mixture Fractionating column (glass beads/plates), temperature gradient Vacuum (reduced-pressure) distillation Lower external pressure lowers the boiling temperature — avoids thermal decomposition. High-b.p. or heat-sensitive liquids Glycerol (decomposes at 290 C ) distills at ≈ 180 C under 12 mm Hg Aspirator/vacuum pump, pressure gauge, short-path condenser Steam distillation For water-immiscible, volatile substances: total vapour pressure is sum of individual vapour pressures; boils below either pure b.p. Aniline, essential oils, o-nitrophenol Aniline (bp 457 K) co-distills with steam at <373 K Steam generator, two-flask setup, receiver with separator Sublimation Solid directly forms vapour on heating; non-subliming impurities remain. Sublimable solids Naphthalene, camphor, NH4Cl, iodine, anthracene Porcelain dish with inverted funnel (cold finger on top) Method Principle Suitable for Typical example Setup cue Choosing by phase-change difference Purification method In pharma, final purification often relies on crystallization to meet strict purity specs. After synthesizing an API (e.g., paracetamol), repeated recrystallization removes colored by-products and trace catalysts to ensure safety and efficacy. remember Crystallization (Recrystallization) Why it works: a good solvent dissolves the solute well when hot but poorly when cold. Make a hot saturated solution of the impure solid, filter hot (to remove insoluble dust), then cool slowly. Pure crystals form and the impurities mostly stay in the mother liquor (the remaining solution). Collect, wash with cold solvent, and dry. A pure solid shows a sharp, constant melting point and remains unchanged after multiple recrystallizations. 2026-05-26T17:05:11.480Z Recrystallization workflow: dissolve hot, filter hot, cool to crystallize, collect and dry crystals; impurities stay in mother liquor. Four-panel vector diagram of recrystallization on white background. Panel 1: impure solid dissolving in hot solvent; Panel 2: hot filtration; Panel 3: slow cooling with crystals growing; Panel 4: vacuum filtration and drying. Label 'mother liquor'. Arrows in red; neutral textbook style. gpt-image-2 The solution left after crystals separate during crystallization; usually retains most soluble impurities. Mother liquor Activated carbon used in hot solution to adsorb colored impurities before crystallization; removed by hot filtration. Decolorizing charcoal Choosing a solvent: the solute should be highly soluble hot, sparingly soluble cold; impurities should be either very soluble at all temperatures or completely insoluble. Test on a small sample first. tip Simple Distillation Principle: when a mixture is heated, the component with significantly lower boiling point (typically ≥25 K lower) vaporizes first and condenses in the condenser. This also purifies a liquid from non-volatile solids. Place the thermometer bulb at the level of the still head so it records the vapour temperature of the distilling component accurately. gpt-image-2 2026-05-26T17:05:11.472Z Clean vector diagram of simple distillation apparatus. Show RBF with boiling chips, thermometer bulb at the still head, Liebig condenser with water in/out arrows, receiving flask. Label parts clearly; red arrows for vapour flow, blue for coolant. Simple distillation setup: heat source, round-bottom flask, still head with thermometer bulb at vapour path, condenser, receiver. Use boiling chips/porous pot to prevent bumping; keep the receiver cool for volatile products. If the b.p. gap is small, switch to fractional distillation. neet-alert Simple distillation can separate liquids whose boiling points are very close. For close boiling points (<25 K difference), vapour composition stays similar to the liquid; you need a fractionating column (fractional distillation) to enrich the lower-b.p. component. Fractional Distillation When boiling points are close, a fractionating column (packed with glass beads or trays) creates repeated mini-cycles of vaporization and condensation. Each cycle enriches the vapour in the more volatile component, like climbing stairs toward higher purity. The more theoretical plates in the column, the better the separation. This is used for ethanol–water mixtures and, at industrial scale, for crude oil refining. Fractional distillation: the column provides multiple condensation–evaporation steps (theoretical plates) and a temperature gradient. Column efficiency (plates) Number of theoretical plates N increases with column length L and decreases with height equivalent to a theoretical plate (HETP). Higher N means better separation. neet-alert Azeotropes cannot be separated into pure components by ordinary fractional distillation (e.g., ethanol–water 95.6% v/v at 351 K). That exception is covered under solutions and colligative properties. 2026-05-26T17:05:11.632Z Tall vertical refinery column cutaway, temperature decreasing from bottom to top. Show trays and side draw-offs labeled with fraction names and b.p. ranges. Clean vector, neutral palette, red arrows for vapour rise, blue for condensate fall. Petroleum refinery fractionating tower with labeled outlets: LPG, petrol (gasoline), kerosene, diesel, lubricating oils, bitumen residue. gpt-image-2 LPG < 40 C3–C4 Fuel for cooking/heating Petrol (gasoline) 40–180 ≈ C5–C12 Motor fuel (SI engines), solvents Kerosene 150–250 ≈ C10–C16 Jet fuel, lamps, heating Diesel 250–350 ≈ C15–C18 Diesel engines Lubricating oils > 300 Heavier hydrocarbons Lubricants, greases Residue (bitumen/asphalt) Residue Very heavy hydrocarbons Road surfacing, roofing Petroleum refining fractions (typical ranges) Fraction Approx. b.p. range (°C) Composition (C-range) Typical uses Refinery cut Refinery order (from top to bottom): LPG → Petrol → Kerosene → Diesel → Lubes → Bitumen. Think: "Lovely People Keep Driving Luxurious Bikes." Distillation under Reduced Pressure (Vacuum Distillation) Lowering external pressure lowers the temperature at which a liquid’s vapour pressure equals the external pressure. Many high-boiling or heat-sensitive liquids then distill without decomposing. Example: glycerol (propane-1,2,3-triol; common name glycerine; often written as HO–CH2–CH(OH)–CH2–OH) decomposes around 290 C at 1 atm, but distills cleanly near 180 C at ≈12 mm Hg. Industries use vacuum distillation for lubricating oils and vitamins. Clausius–Clapeyron (qualitative form) As external pressure p decreases, the boiling temperature T decreases. This relation explains vacuum distillation. gpt-image-2 Vacuum distillation: system connected to a vacuum source with pressure gauge; shorter path condenser for high-b.p. liquids. Schematic of vacuum distillation apparatus: round-bottom flask with heating mantle, short-path head, condenser to receiver, vacuum adapter connected to pump/aspirator with gauge. Label reduced pressure and flow direction. Vector style. 2026-05-26T17:05:12.671Z Steam Distillation If a volatile organic liquid is immiscible with water, when both are heated together the total vapour pressure equals the sum of their individual vapour pressures. The mixture boils when this sum reaches the external pressure, often below 373 K, allowing safe distillation without decomposition. Applications: aniline (benzenamine; bp 457 K) and essential oils (rose, eucalyptus, sandalwood), and o-nitrophenol (intramolecular H-bonded, more steam-volatile than its para isomer). The distillate is a two-phase mixture collected and then separated. Steam distillation criterion Boiling occurs when this sum equals the external pressure (≈1 atm in the lab). Steam distillation apparatus: steam generator feeds into flask with immiscible organic; condenser leads to receiver; a separatory funnel recovers organic layer. Three-part vector schematic: left steam generator, center boiling flask with organic + water, right condenser to receiver. Show two layers forming in the receiver. Label steam inlet, thermometer, and separator. 2026-05-26T17:05:13.391Z gpt-image-2 Keep a steady steam flow. If organic is solid at room temperature, collect warm distillate and extract quickly to prevent crystallization in the condenser. tip The distillate often needs further drying, extraction, or redistillation. High purity usually needs multiple steps. Steam distillation (or any single step) always gives a perfectly pure product. Sublimation Some solids directly form vapours on gentle heating, bypassing the liquid state. Sublimation purifies such solids from non-subliming impurities. Place the impure solid in a dish, cover with an inverted funnel or cold finger, and cool the top so vapours deposit as crystals. Examples: naphthalene, camphor, ammonium chloride, iodine, anthracene. Handle iodine in a fume hood: its purple vapours are irritating. Sublimation setup: impure solid in dish, inverted funnel as cold surface; crystals deposit on the cooler area. Side-view vector of a porcelain dish with impure solid heated, inverted funnel above with cooled top (ice on watch glass). Show vapour rising and solid crystals depositing inside the funnel. Label heat source and deposit. 2026-05-26T17:05:13.466Z gpt-image-2 Solid Common sublimable solids Compound Notes/Use Safety cue Naphthalene Mothballs; purifies easily by sublimation Avoid inhalation Camphor Fragrant solid; sublimable Ventilation needed Ammonium chloride (NH4Cl) White fumes; classic demo Irritant; avoid inhalation Iodine Purple vapour, disinfectant source Corrosive vapour; use fume hood Anthracene PAH; purifies well by sublimation Gloves; avoid dust High-Yield Matching: Compound → Method Glycerol (heat-sensitive, decomposes at atmospheric b.p.) → Vacuum distillation. Aniline (water-immiscible, volatile under steam) → Steam distillation. Naphthalene, camphor, NH4Cl, iodine → Sublimation. Ethanol–water mixture (close b.p.) → Fractional distillation. A liquid + dissolved non-volatile solid (e.g., salt in water, or toluene with trace dye) → Simple distillation (or evaporation + crystallization depending on goal). Pure solid from soluble colored impurities (e.g., benzoic acid) → Crystallization. Quick picks for NEET neet-alert Tricky exception: Azeotropes (e.g., ethanol–water) limit purity by fractional distillation; ordinary columns cannot break the azeotrope. Fractional distillation works for any mixture. It fails for azeotropes that boil at a constant composition; special methods (azeotropic or extractive distillation) are needed. Cross-links (next chapter methods) Extraction and chromatography are covered next. The core performance measures are included here for continuity. Larger K D favours transfer to the organic phase. Partition coefficient (extraction) A dimensionless ratio to compare movement on a chromatographic plate. Chromatography R f Always label apparatus: heat source, thermometer position, condenser water in/out. In NEET diagrams, these cue the method being tested. tip Paper Chromatography Water molecules trapped in the cellulose fibers ( Liquid ) Solvent ( Liquid ) Differential Partition Thin Layer Chromatography ( TLC ) Thin layer of SiO 2 or Al 2 O 3 on plate ( Solid ) Liquid solvent ( Liquid ) Differential Adsorption Column Chromatography Bulk adsorbent ( SiO 2 or Al 2 O 3 ) in a glass tube ( Solid ) Eluant solvent ( Liquid ) Differential Adsorption Gas-Liquid Chromatography ( GLC ) Non-volatile liquid film on inert support ( Liquid ) Carrier gas such as He or N 2 ( Gas ) Differential Partition Adsorption Chromatography (General) Specific Solid adsorbent surface Liquid or Gas Selective Adsorption on the surface of stationary phase Partition Chromatography (General) Liquid film supported on an inert matrix Liquid or Gas Selective Partition between two phases based on solubility Method Organic Chemistry Purification Techniques NEET Analytical Chemistry Stationary Phase Mobile Phase Separation Principle Chromatography Techniques Paper Partitions while Thin Columns Adsorb. COMPARISON Understanding the basis of separation (Adsorption vs Partition). Kjeldahl Method ( N ) Digestion with conc. H 2 SO 4 to (NH 4 ) 2 SO 4 followed by NH 3 liberation using NaOH and titration against standard acid. %N = 1.4 N V w where N, V are normality and volume of acid used to neutralize NH 3 . Dumas Method ( N ) Organic compound heated with CuO in CO 2 atmosphere to produce N 2 gas, collected over KOH solution. %N = 28 22400 V STP w 100 where V STP is volume of N 2 at STP . Carius Method ( Cl ) Heating with fuming HNO 3 and AgNO 3 in a sealed tube to form AgCl precipitate. %Cl = 35.5 143.5 m AgCl m 100 Liebig Method ( C ) Complete combustion of compound with CuO ; CO 2 produced is absorbed in a KOH bulb. %C = 12 44 m CO 2 m 100 Liebig Method ( H ) Complete combustion of compound with CuO ; H 2 O produced is absorbed in anhydrous CaCl 2 tube. %H = 2 18 m H 2 O m 100 Carius Method ( S ) Oxidation with fuming HNO 3 to H 2 SO 4 , then precipitated as BaSO 4 using BaCl 2 . %S = 32 233 m BaSO 4 m 100 Carius Method ( Br ) Heating with fuming HNO 3 and AgNO 3 in a Carius tube to form AgBr precipitate. %Br = 80 188 m AgBr m 100 Carius Method ( I ) Heating with fuming HNO 3 and AgNO 3 to form AgI precipitate. %I = 127 235 m AgI m 100 Phosphorus Estimation Oxidation to H 3 PO 4 and precipitation as MgNH 4 PO 4 , which is ignited to Mg 2 P 2 O 7 . %P = 62 222 m Mg 2 P 2 O 7 m 100 Oxygen Estimation (Aluise Method) Pyrolysis in N 2 stream; O 2 converted to CO over red hot coke, then CO to CO 2 via I 2 O 5 . %O = 32 88 m CO 2 m 100 or by difference: 100 - % of other elements. Organic Chemistry Quantitative Analysis NEET High Yield Formulas Element Detected Principle/Reaction Formula Quantitative Estimation Methods Method Liebig burns C-H, Kjeldahl distills N (but not for Azo/Nitro), Dumas traps the gas, and Carius weighs the halide salt. LEXICON Formulas for percentage composition calculations.