Salt from sea water, pure water from ink, ethanol from a fermented mixture: each separation uses a difference in property, whether solubility, particle size or boiling point. Pick the difference and the method follows. Then use melting and boiling points to check the product is pure.
Choose a separation method from the difference between the substances. Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid from its solution: heat to evaporate some solvent until the solution is saturated, then cool so crystals form. Simple distillation separates a solvent from a solution (for example water from salt water). Fractional distillation separates miscible liquids with different boiling points (for example ethanol and water). Dissolving in a suitable solvent separates a soluble solid from an insoluble one. A pure substance melts and boils at fixed, sharp temperatures; impurities lower the melting point and make it melt over a range, and raise the boiling point. Purity matters for food and drugs.
Name the mixture, find the property that differs, then choose.
| Mixture | Difference used | Method |
|---|---|---|
| insoluble solid in a liquid (sand and water) | particle size: solid cannot pass through the paper | filtration |
| soluble solid in solution (copper(II) sulfate solution) | solid is left as the solvent evaporates | crystallisation |
| solvent from a solution (pure water from sea water) | water boils, salt does not | simple distillation |
| miscible liquids (ethanol and water; crude oil) | different boiling points | fractional distillation |
| soluble and insoluble solids (salt and sand) | solubility in a suitable solvent | dissolve, filter, then crystallise the filtrate |
| coloured substances in solution (dyes in ink) | solubility in solvent and attraction to paper | chromatography |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which method separates sand from water?
Which method obtains pure water from sea water?
Which method separates ethanol from a mixture with water?
How is a pure substance recognised from its melting point?
The two distillations differ in one piece of apparatus: the fractionating column.
Do not heat a solution to dryness when crystallising a hydrated salt: you lose the water of crystallisation and may decompose it. Condenser water goes in at the bottom so the jacket stays full. Impurities lower the melting point (and widen the range) but raise the boiling point.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Pure aspirin melts at 136 °C. Which melting range shows an impure sample?
How is pure salt obtained from a mixture of salt and sand?
In simple distillation, why does cooling water enter the condenser at the bottom?
Hydrated copper(II) sulfate crystals are made from the solution. Why is the solution not heated to dryness?
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