Describe, explain and give examples of the specified processes of separation

The idea

A mixture contains two or more substances not chemically joined, which can be separated by physical means. Filtration separates an insoluble solid from a liquid: sand and water poured through filter paper leaves sand as residue and water as filtrate. Crystallisation recovers a dissolved solid: heat a salt solution so solvent evaporates, then cool to form crystals. Simple distillation separates a solvent from a solution, e.g. pure water from seawater: heat the solution so the solvent boils, condense the vapour and collect the distillate. Fractional distillation separates miscible liquids with different boiling points, like ethanol and water, using a fractionating column. Chromatography separates dissolved substances, like ink dyes, by their different solubilities in a solvent moving up paper.

What earns the marks

• State that a mixture is not chemically combined, so its components can be separated by physical processes without chemical reactions.
• Describe filtration as separating an insoluble solid from a liquid using filter paper and a funnel, naming the solid as the residue and the liquid as the filtrate.
• Describe crystallisation as evaporating some solvent from a solution and then cooling or leaving it so crystals of the dissolved solid form.
• Describe simple distillation as evaporating a solvent by heating, condensing the vapour, and collecting the distillate while the dissolved solute remains.
• Describe fractional distillation as separating miscible liquids by repeated evaporation and condensation in a fractionating column, using differences in boiling point.
• Describe chromatography as separating dissolved substances using a stationary phase and a moving solvent, relying on different solubilities and therefore different distances travelled.

Common mistakes and how to put them right

• Confusing filtration with evaporation.

  Put it right: Use crystallisation or distillation to recover a dissolved solid.

• Thinking distillation destroys the solute.

  Put it right: State that the distillate is the condensed solvent and the solute stays behind.

• Believing chromatography separates by colour alone.

  Put it right: Explain separation in terms of differing solubilities in the solvent.

• Writing that fractional distillation separates liquids by filtering them.

  Put it right: Describe repeated evaporation and condensation in the column.

Exam tips

• Link each named process to a specific mixture in the question rather than describing all five processes in general terms.
• Use the correct vocabulary precisely: residue, filtrate, distillate, solvent front and stationary phase all carry credit in written answers.
• When asked to explain, give the reason for separation, such as different boiling points or different solubilities, not just the practical steps.
• For a six-mark comparison, structure the answer around one process at a time and include a named example for each.