Separation and Purification – Comprehensive Study Notes

Pure Substances vs Mixtures

• Definitions
  • Mixture = a physical combination of ≥ 2 substances not chemically combined.
  • Pure substance = a single element or compound with no other substances mixed in.

• Everyday examples
  • “Pure” orange juice, tap water, swimming-pool water ⇒ actually mixtures (contain salts, fluorides, chlorine, etc.).
  • Brass = mixture of Cu & Zn (mixture of elements).
  • Seawater = mixture of compounds ( $ext{NaCl}$ + water).
  • Air = mixture of elements & compounds ( $ext{N}<em>2, ext{O}</em>2, ext{CO}_2,$ water vapour).

• Key ideas
  • Naturally occurring materials ≈ mixtures → purification is required before use.
  • Choice of separation method depends on physical properties (solubility, boiling point, magnetism, etc.).

Separating Solid–Liquid Mixtures

• Decision rule: is the solid soluble in the liquid?

Filtration (insoluble solid)

• Principle: filter paper acts as a sieve; liquid passes pores, solid is trapped.

• Terminology:
  • Residue = solid left on filter paper.
  • Filtrate = liquid that passes through.

• Example: removing sand from water; coffee grounds from coffee.

• Investigation 3.1: funnel + filter paper; collect filtrate in conical flask; dry residue.

• Applications: NEWater treatment (multistage micro- & ultra-filtration).

Evaporation to Dryness (soluble, heat-stable solid)

• Principle: heat until all solvent boils off.

• Apparatus: evaporating dish + tripod, wire gauze, Bunsen burner; add boiling chips to prevent bumping.

• Caution: wear goggles—hot, concentrated liquids sputter.

• Limitations:
  • Leaves behind all soluble impurities.
  • Only for substances that do not decompose on strong heating (e.g. common salt).

• Industrial example: solar ponds for harvesting sea-salt.

Crystallisation (soluble, heat-sensitive solid)

• Principle: form a hot saturated solution, cool to grow pure crystals.

• Procedure (Investigation 3.2):
  1 Dissolve impure solid (e.g. $ext{CuSO}<em>4\bullet5 ext{H}</em>2 ext{O}$).
  2 Filter off insoluble impurities.
  3 Heat filtrate until saturation (test with glass rod → crust forms).
  4 Allow to cool; crystals appear.
  5 Filter, wash with cold distilled water, dry between filter papers.

• Why preferred: excludes soluble impurities and avoids thermal decomposition (e.g. sugar → chars).

• Definitions:
  • Saturated solution = holds maximum solute at given T; no more dissolves.
  • Unsaturated = can still dissolve more solute.

Separating Mixtures of Solids

Using a Suitable Solvent

• Strategy: choose solvent in which only one solid dissolves.

• Example: NaCl + sand → water dissolves NaCl, filter, crystallise.

• Worked example: Sodium nitrate (soluble) + lead(II) sulfate (insoluble) → water as solvent, filter, crystallise.

Sublimation

• Some solids transition directly solid ⇌ gas.

• Apparatus: inverted funnel + cold surface; heat mixture; sublimed solid (e.g. iodine, ammonium chloride, dry ice) re-solidifies on funnel.

• Everyday link: freeze-drying noodles—ice sublimed under low P.

Magnetic Separation

• Magnetic elements: Fe, Co, Ni (and steels).

• Procedure: pass magnet over mixture; magnetic fractions stick.

• Use: recycling plants recover ferrous metal from municipal waste.

Separating a Solvent from Its Solution – Simple Distillation

• Goal: recover pure solvent (liquid) from solution.

• Key parts: distillation flask + side-arm, thermometer (bulb at side-arm), condenser sloping downwards, receiver.

• Water flow: in at condenser bottom, out at top → jacket always full for efficient cooling.

• Boiling chips: ensure smooth boiling.

• Temperature profile: rises to solvent b.p. (e.g. $100^{\circ}\text{C}$ for water), then plateaus until solvent exhausted.

• Result: distillate = pure solvent; residue = concentrated solute.

Separating Liquid–Liquid Mixtures

Immiscible Liquids – Separating Funnel

• Liquids form distinct density-based layers; open tap to drain lower layer.

• Example: oil/water; tetrachloromethane/water.

• Precaution: allow emulsion to stand until clear layers re-form.

Miscible Liquids – Fractional Distillation

• Fractionating column packed with glass beads → large surface for repeated condensation/evaporation cycles.

• Rule: component with lowest b.p. exits first.

• Lab example: ethanol ($78^{\circ}\text{C}$) + water ($100^{\circ}\text{C}$).
  • Temperature holds at $78^{\circ}\text{C}$ until ethanol gone, then climbs to $100^{\circ}\text{C}$; water distils.

• Industrial applications:
  • Fractionation of liquid air → $\text{N}<em>2$, $\text{O}</em>2$, Ar.
  • Petroleum refining (crude → petrol, diesel, LPG…).
  • Concentrating fermented ethanol for beverages/biofuels.

Chromatography

• Definition: separates components that travel at different rates with a mobile phase (solvent) over a stationary phase (paper, TLC plate, column packing).

Paper Chromatography (coloured samples)

• Procedure (Investigation 3.3):
  1 Draw pencil baseline 1 cm above edge.
  2 Spot sample; let dry.
  3 Suspend paper in closed tank with solvent below baseline; solvent rises by capillarity.
  4 Components separate into distinct spots → chromatogram.

• Interpretation: a pure dye → one spot; a mixture → multiple spots.

Retention Factor $R_f$

$R_f = \dfrac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$

• Constant for a given substance under fixed solvent & temperature.

• Used to identify unknowns by comparison with standards.

Analytical Example – Detecting Banned Food Dyes

• Spot food colouring X alongside dyes A–D, run chromatogram.

• Matching colour and height ⇒ identity.

• If X shows spot matching banned dye C → product unsafe.

Colourless Substances

• Locating agent (e.g. ninhydrin for amino acids) sprayed → reacts to give coloured spots.

• Essential in forensics (fingerprints), drug tests, amino-acid analysis.

Broader Uses

• Quality control in ink, pharmaceuticals, pesticides, poisons; doping tests for athletes; plant pigment studies.

Determining Purity via Melting & Boiling Points

Importance

• Detect harmful impurities; ensure efficacy of medicines; maintain electronic-grade silicon; safeguard food (melamine scandal).

Pure Solids

• Melt sharply at a fixed temperature (e.g. benzoic acid $122^{\circ}\text{C}$).

• Apparatus: melting-point tube immersed in heated paraffin-oil bath with thermometer & stirring.

Effect of Impurities

• Lower melting point and spread melting over a range.

• Greater impurity → broader/lower range (e.g. benzoic acid 118–121 °C).

Pure Liquids

• Boil at fixed temperature under given pressure (e.g. propanone $56^{\circ}\text{C}$).

• Set-up: small distillation flask in water-bath/ice-bath, condenser back to flask or receiver.

Effect of Impurities

• Raise boiling point & cause range.

• Relationship exploited for colligative-property experiments (boiling-point elevation).

Pressure Dependence

• $\uparrow P$ ⇒ $\uparrow$ boiling point; $\downarrow P$ ⇒ lower.

• On Mt Everest, water boils at $69^{\circ}\text{C}$.

Industrial & Real-World Applications

• NEWater (Singapore): ultra-filtration + reverse osmosis + UV; tests for banned substances (e.g. melamine).

• Desalination via Reverse Osmosis: apply pressure > osmotic pressure; water forced through semi-permeable membrane, salts left behind.

• Normal osmosis vs reverse osmosis illustrated:
  • Natural direction: low [solute] → high [solute].
  • Reverse osmosis: applied pressure drives opposite flow, producing pure water.

• Social-economic implications: water security, public health, industrial growth, cost of supply, international relations (imports from Malaysia).

Ethical & Practical Considerations

• Food adulteration (melamine in baby milk) highlights need for analytical testing.

• Pharmaceutical purity critical to patient safety; regulations enforce strict purity standards.

• Environmental responsibility: proper separation allows recycling (magnets in waste plants) and prevents contamination.

Concept Map Overview ("Map It")

• Mixtures (not chemically combined) → various methods of separation based on particle properties.
  • Insoluble solid + liquid → filtration.
  • Soluble solid + liquid → evaporation / crystallisation.
  • Solid mixtures → solvent extraction / sublimation / magnet.
  • Solution → simple distillation (recover solvent).
  • Immiscible liquids → separating funnel.
  • Miscible liquids → fractional distillation.
  • Components dissolved in same solvent → chromatography (chromatogram; locating agent for colourless).

• Pure substances characterised by fixed m.p./b.p.; purity checked by m.p./b.p. measurement or single-spot chromatogram.