Elements, Compounds and Mixtures

Filtration

• Separates an insoluble solid from a liquid (e.g. sand from water)
• Apparatus:
  • Folded filter paper in a filter funnel
  • Funnel resting on the rim of a beaker (or held in a clamp above it)
• The mixture is poured into the funnel; the paper traps solid particles
• Residue = solid caught on the paper
• Filtrate = liquid that passes through into the beaker
• Centrifugation is an alternative for very fine particles (more common in biology than in chemistry)

Crystallisation

• Pulls a soluble solid back out of its solution as well-formed crystals
• Works because the solubility of most solids falls sharply on cooling
• Procedure:
  1. Heat the solution gently to evaporate some solvent and concentrate it
  2. To test for saturation, touch the surface with a chilled, clean glass rod. If crystals form on the rod, the solution is saturated
  3. Stop heating; cool slowly (slow cooling = larger, better-formed crystals)
  4. Filter off the crystals; wash with a small volume of cold distilled water to rinse off impurities; pat dry between filter papers
• Standard method for harvesting:
  • Hydrated copper(II) sulfate (CuSO₄·5H₂O) from CuSO₄ solution
  • Industrially: sugar, alum, potash

Simple distillation

• Separates a liquid from a dissolved solute (e.g. pure water from salt water)
• Also isolates a single pure liquid from a mixture whose other components boil much higher
• How it works:
  • The solution is heated in a round-bottomed flask
  • The volatile component vaporises and rises out of the flask
  • The vapour passes into a water-jacketed condenser and is cooled back into liquid form
  • The pure liquid is collected in a beaker; the solute stays behind in the flask

Fractional distillation

• Separates two or more miscible liquids with different boiling points
  • Example: ethanol (b.p. 78 °C) and water (b.p. 100 °C)
  • Industrial example: crude oil into petrol, kerosene, diesel etc. (see Topic 11)
• Apparatus is simple distillation plus a vertical fractionating column between the flask and the condenser
  • Column packed with glass beads (or fitted with plates)
  • Provides a large surface area for repeated condensation and re-evaporation
  • Each cycle enriches the rising vapour in the lower-boiling component
• Procedure (ethanol/water):
  • Heat the flask; thermometer at the top of the column reads 78 °C when ethanol vapour reaches it
  • Pure (or nearly pure) ethanol distils over; thermometer holds at 78 °C
  • When the ethanol is exhausted, the temperature rises towards 100 °C as water starts to come over; stop collecting, or switch flasks

Paper chromatography

• Separates dissolved substances whose solubilities in a single chosen solvent differ from one another
  • Typical targets: ink dyes, food colourings, plant pigments
• Procedure:
  1. Draw a horizontal baseline in pencil about 1–2 cm from one short edge of the chromatography paper
     • Pencil graphite is insoluble; pen ink would itself run up the paper
  2. With a fine capillary tube, place one small spot of each sample on the baseline (fresh tube per sample)
  3. Stand the paper in a covered beaker with a shallow layer of solvent
     • Solvent level must be below the baseline
  4. The solvent climbs the paper by capillary action, carrying dissolved substances with it
     • More soluble in the solvent → travels further
     • Less soluble (sticks to paper) → travels less
  5. When the solvent front is near the top, lift out the paper, mark the solvent front in pencil, and let it dry