Match the method to the physical difference
Pick the method from the physical property that differs. An insoluble solid in a liquid differs in particle size — filter it. A soluble solid in solution needs crystallisation, as solubility falls on cooling. Two miscible liquids of different boiling point need fractional distillation. Coloured solutes dissolved in a solvent need paper chromatography. In a "suggest why" question, justify by that difference, not just name the method.
Crystallisation: evaporate, cool, filter, dry
Four marked steps: (1) heat gently until the solution is saturated (a dipped cool glass rod grows crystals); (2) stop heating and cool, so crystals form as solubility falls; (3) filter the crystals from the leftover solution; (4) dry with filter paper at room temperature — never "evaporate to dryness" or a hot oven, which powders crystals and drives water of crystallisation from hydrated salts. Saturated means no more solute dissolves at that temperature.
Distillation and chromatography
Simple distillation recovers a pure solvent from a solution: it vaporises, a water-cooled condenser returns it as the distillate, and the dissolved solid stays behind. Fractional distillation splits two miscible liquids of different boiling point using a fractionating column, whose temperature gradient lets the lower-boiling one distil first. Chromatography separates dissolved solutes by solubility and adsorption: a pure substance gives one spot, a mixture several.
Drawn from real examiner reports.
Crystallisation dried to a powder
The most-penalised crystallisation error is a final step written as "evaporate to dryness" or "heat in an oven". Boiling off all the water leaves a powder, and a hot oven drives water of crystallisation out of a hydrated salt. The credited step is to pat the crystals with filter paper and dry at room temperature, having stopped at saturation, not boiled to dryness.
W22 P31 Q6(f); W22 P32; W22 P42 Q4(d)(ii); W22 P41 Q4(e)(i)
Chromatography baseline and solvent errors
Three procedure errors recur. A pen baseline lets its dyes travel up the paper as false spots — use pencil (insoluble graphite). Solvent above the baseline dissolves the spots into the pool so they wash away — keep the solvent below it. Not marking the solvent front the instant the paper is removed lets it evaporate, losing the distances any value needs.
Filtration cannot recover a dissolved solid
Filtration only separates an insoluble solid from a liquid — its particles are too large for the filter-paper pores. A dissolved (soluble) solid passes straight through with the liquid, leaving nothing on the paper. To recover a dissolved solid you must crystallise or evaporate: for salt in water, filtration recovers nothing; crystallisation returns the solid.
Two volatile liquids need fractional
Simple distillation works only when one component is non-volatile — a dissolved solid stays behind as the solvent distils. If both are volatile liquids (ethanol, water), both vaporise and condense together, giving an impure distillate. Two miscible volatile liquids need fractional distillation, whose column re-evaporates the vapour so each comes over near its boiling point.
Insoluble salt is made by precipitation
Asked how to make an insoluble salt, candidates often name crystallisation — the method for a soluble salt. An insoluble salt is made by precipitation: mix two soluble solutions whose ions combine to give the insoluble salt, which drops out as a solid and is filtered off, washed and dried. Crystallisation only recovers a salt that stays dissolved.
W22 P42 Q2(d)(i)
Residue and filtrate get swapped
Under pressure candidates swap the two filtration products. The residue is the insoluble solid trapped on the filter paper; the filtrate is the liquid that passes through it. Remember: the filtrate goes through the filter. Naming the collected liquid the "residue", or the trapped solid the "filtrate", loses easy marks in practical-technique questions.
Saturated solution is temperature-specific
Two errors surround "saturated". Some link it only to organic chemistry (a saturated hydrocarbon with single bonds); here it means a solution holding the most solute it can dissolve. That maximum is temperature-specific: a hot saturated solution holds far more than a cold one, so cooling it deposits crystals. State saturation at a given temperature.
W22 P42 Q2(c)(ii)
Give each step with its key mark word
"Describe the method" questions give one mark per step. Crystallisation: evaporate to saturation, cool, filter, dry with filter paper (not to dryness). Chromatography: pencil baseline, solvent below it, mark the solvent front. A missing key word loses that step's mark.
Label distillation apparatus correctly
Two diagram marks are routinely dropped. Put the thermometer bulb at the side-arm, level with the outlet, so it reads the vapour temperature — not down in the liquid. Run the condenser water counter-current (in at the bottom, out at the top) to keep the jacket full.
MCQ: name the property that differs
For "which method" MCQs, first name the property that differs — particle size, boiling point, or solubility — then pick the method that exploits it: filtration for particle size, distillation for boiling point, crystallisation or chromatography for solubility.
Separation is a physical process that divides a mixture into its components without forming new substances. The method chosen depends on the physical properties that differ between the components.
Filtration: separates an insoluble solid from a liquid. The solid is trapped in the filter paper (the residue); the liquid passes through as the filtrate. Works because the insoluble solid particles are too large to pass through the tiny pores in the filter paper.
Crystallisation: obtains a soluble solid from its solution. Relies on the fact that solubility decreases as temperature falls — a hot saturated solution deposits crystals on cooling.
Distillation: separates a liquid from either dissolved solids (simple distillation) or from another liquid with a different boiling point (fractional distillation). Relies on different boiling points.
Paper chromatography (ascending): separates a mixture of solutes dissolved in a solvent. Relies on the different solubilities of the components in the mobile-phase solvent and their different adsorption to the stationary phase (paper). A component that is more soluble in the solvent travels further up the paper.
A mixture contains sand, dissolved potassium nitrate and water. Describe, in order, how you would obtain both pure dry crystals of potassium nitrate and a separate sample of pure water, naming each technique.
Draw and label a diagram showing the correct setup for ascending paper chromatography, including the chromatography paper, solvent, and the positions of the pencil baseline and sample spots. [4]
