SC2 · Methods of separating and purifying substances · Topic 2 — States of matter and mixtures | GCSE Edexcel Separate Sciences

Revise the key ideas

A chemically pure substance contains a single element or compound, with a fixed composition. Everyday “pure” can mean natural or without additives and need not mean chemically pure.
An element contains only one type of atom. A compound contains two or more elements chemically combined in fixed proportions; a mixture has substances together without chemical bonding between its components.
A molecule contains covalently bonded atoms: O₂ is an element and CO₂ a compound. Not all compounds consist of separate molecules; sodium chloride has an ionic lattice.
Mixtures such as air or salt water can have variable proportions. Their components can be separated using physical methods based on different properties.
A pure substance has a sharp melting point under given conditions. Mixtures commonly melt over a range; impurities often lower the melting point. Compare measured data with a reliable reference.
Pure substances have characteristic boiling points at a specified pressure. Mixtures often boil over a range; a single measurement alone cannot establish purity with certainty.

A solute is dissolved in a solvent to form a solution. Water is a common solvent; the dissolved substance can be solid, liquid or gas.
Dissolving distributes solute particles through the solvent. It is not necessarily a chemical reaction and does not always break molecules apart.
Solubility depends on the substance, solvent and temperature. A substance insoluble in water might dissolve in another solvent.
Choose filtration for an insoluble solid and liquid, crystallisation for a dissolved solid, and simple distillation to collect solvent from a solution.
Use fractional distillation to separate liquids that mix together (miscible liquids) and have different boiling points. Use chromatography to separate dissolved substances that move at different rates through a stationary phase such as paper.
To separate sand and salt, add water to dissolve salt, filter off sand, then crystallise salt from the filtrate. No single filtration can remove dissolved salt.

In filtration, filter paper in a funnel retains insoluble solid as the residue; liquid and dissolved substances pass through as the filtrate.
Ordinary filter paper retains insoluble solid, not dissolved salt.
Particles of a dissolved solute are too small to be trapped by ordinary filter paper. Filtering salt water will not produce pure water.
To obtain crystals, gently heat a solution to evaporate some solvent until it is near saturation. Then allow it to cool so crystals form.
Filter the cooled crystals, wash with a little suitable cold solvent if appropriate, and dry between filter papers. Extensive washing can dissolve the product.
Keep crystals; avoid heating hydrated salts to dryness.
Do not boil every solution completely dry: this can cause spitting or damage hydrated crystals. Use suitable controlled heating and eye protection.
Evaporation obtains a dissolved solid but loses the solvent. If the solvent is needed, collect it using distillation instead.

In simple distillation, heat a solution so its solvent evaporates. Cool the vapour in a condenser and collect the liquid, called the distillate. Dissolved salts that do not evaporate (non-volatile salts) stay in the flask.
A thermometer should measure vapour temperature near the side arm. Cooling water enters the lower condenser inlet and leaves the upper outlet, keeping the jacket full.
Schematic: condenser water flows from lower inlet to upper outlet; system is not sealed.
Keep the apparatus open to the atmosphere: never heat a sealed distillation system. Use an appropriate heat source; avoid naked flames with flammable solvents.
Fractional distillation uses a fractionating column. As vapour rises, repeated evaporation and condensation mean that more of it is the liquid with the lower boiling point. This helps separate the liquids.
A column improves separation of miscible liquids.
Different liquid fractions are collected over suitable temperature ranges. Distillation changes state; it does not turn one substance chemically into another.
The ink core practical uses distillation to recover solvent and chromatography to investigate dissolved dyes. The processes answer different questions about the mixture.

Draw a pencil baseline near the bottom of the paper and add small spots of sample and known references. Pencil graphite does not dissolve into the solvent as ordinary ink may.
Place the paper in a little solvent with the baseline above the solvent level. Cover the container to reduce evaporation and allow solvent to rise up the paper.
The solvent moves up the paper, so it is the mobile phase. The stationary phase stays in place in the paper. Substances separate because they dissolve differently in the solvent and are attracted differently to the stationary phase.
Remove the paper before solvent reaches the top and immediately mark the solvent front in pencil. The resulting separated pattern is a chromatogram.
Rf = distance travelled by component ÷ distance travelled by solvent front. Measure both from the baseline; component distance is to the centre of its spot. Rf has no unit.
If a spot moves 3 cm and the solvent front 6 cm, Rf = 3 ÷ 6 = 0.5. A valid spot on this chromatogram has Rf between 0 and 1.
Distances are measured from the baseline to spot centre and solvent front; schematic, not a measuring scale.
Several spots indicate a mixture. One spot is consistent with purity under those conditions, but overlapping spots or insoluble components can limit the conclusion.
Compare known and unknown spots using the same solvent and conditions. Matching positions or Rf values support identity but are not absolute proof; changing conditions can change Rf.

Potable water is safe to drink; it need not be chemically pure. Dissolved minerals can remain in drinking water.
Fresh water treatment can involve sedimentation to settle suspended solids, filtration through sand and gravel, and chlorination to kill harmful microorganisms.
Potable water can still contain dissolved minerals.
Sedimentation and ordinary filtration do not reliably remove dissolved salts. Disinfection targets microorganisms rather than removing all dissolved chemicals.
Sea water can be desalinated by distillation: evaporate water, condense the vapour and collect water while salts remain. This requires substantial energy.
Water for chemical analysis should not contain dissolved salts because they can contaminate samples or interfere with tests. Distilled or appropriately deionised water is used.
Select treatment based on the contaminant: insoluble particles, microorganisms and dissolved salts require different approaches.