Purification and Separation Methods

Every separation question in 0620 is secretly the same question: what property differs between the substances, and which technique exploits it? Paper 6 asks it with diagrams (thermometer position, condenser water direction), while Papers 3 and 4 ask you to name and sequence the techniques. The mark scheme rewards the matching, not the apparatus list.

The vocabulary first

A solute dissolves in a solvent to make a solution. A saturated solution contains as much dissolved solute as possible at that temperature. Residue stays in the filter paper; filtrate passes through. These six words are marking points in their own right. “The liquid that comes through” is not “filtrate”.

The methods and what each separates

Method	Separates	Property exploited
Filtration	insoluble solid from a liquid	solubility
Crystallisation	dissolved solid from its solution	solubility falls as temperature falls
Simple distillation	solvent from a solution	boiling point (solvent boils, solute doesn’t)
Fractional distillation	miscible liquids	different boiling points
Separating funnel	immiscible liquids	density; liquids form layers

Filtration. Pour the mixture through filter paper in a funnel. Sand and salt solution: sand is the residue, salt solution the filtrate. Filtration is also step two of every insoluble-salt preparation in acids, bases and salts.

Crystallisation. Heat the solution to evaporate solvent until the point of crystallisation. Test by dipping a glass rod; crystals forming on it mean the solution is saturated. Then stop heating and leave to cool: crystals form as solubility falls. Filter them off and dry between filter papers. The phrase “do not evaporate to dryness” carries a mark, with the reason: heating to dryness decomposes hydrated salts and loses the water of crystallisation.

Simple distillation. Boil the solution; the solvent vaporises, passes the thermometer at the side-arm, and condenses in a condenser whose cooling water enters at the bottom (counter-current, so the coldest water meets the coolest vapour last). Collect the distillate. This recovers the water from salt water; crystallisation recovers the salt. Read the question for which product is wanted.

Fractional distillation. For miscible liquids: ethanol and water from fermentation is the syllabus example. Add a fractionating column between flask and condenser. The liquid with the lower boiling point (ethanol, 78°C) reaches the top first; the thermometer holds at 78°C while ethanol distils. The same principle at industrial scale separates petroleum into fractions, covered under fuels.

Separating funnel. Immiscible liquids, such as oil and water, form two layers. Open the tap, run off the lower (denser) layer, close the tap before the boundary passes.

Worked exam question

A student is given a mixture of sand and sodium chloride. Describe how to obtain pure, dry crystals of sodium chloride from the mixture. [5]

Model answer, mark by mark:

• M1: add water and stir. The sodium chloride dissolves; the sand does not.
• M2: filter; the sand is left as the residue, the salt solution passes through as the filtrate.
• M3: heat the filtrate to evaporate water until the point of crystallisation (crystals form on a glass rod dipped in).
• M4: leave to cool so crystals form; do not evaporate to dryness.
• M5: filter off the crystals and dry them between filter papers (or in a warm oven).

Five steps, five marks, and the sequence matters: dissolve before filtering, saturate before cooling. This exact question (sometimes dressed up with copper(II) sulfate) has appeared in well over a decade of past papers.

The mistakes that cost marks

1. Evaporating to dryness. The single most-flagged error in this topic. Heat to the point of crystallisation, cool, then filter, and say why.
2. Thermometer in the liquid. In any distillation diagram the bulb sits at the side-arm, level with the condenser opening, measuring the vapour temperature.
3. Condenser water flowing downhill. Cooling water enters at the bottom and exits at the top. Arrows drawn the other way lose the mark.
4. Wrong method for the pair. Fractional distillation for oil and water (immiscible: funnel), or filtration for dissolved salt (soluble: crystallisation). Always name the property difference first, then the method follows.
5. Residue and filtrate swapped. Residue stays on the paper. Mixing them up turns a correct method into a wrong answer.

How examiners want it phrased

Student wording	Mark-scheme wording
”Boil off all the water"	"Evaporate to the point of crystallisation, then allow to cool and crystallise"
"Strain the mixture"	"Filter; the insoluble sand remains as the residue and the solution passes through as the filtrate"
"The ethanol comes out first"	"Ethanol has the lower boiling point, so it reaches the top of the fractionating column first, at 78°C"
"Let the oil and water settle"	"The immiscible liquids form two layers in a separating funnel; run off the lower, denser layer through the tap”

Separation sequences are core ground for Paper 6, where diagrams replace the lab bench, and they anchor the whole of experimental techniques and chemical analysis. If the five-mark sand-and-salt answer above didn’t come out in order, that is a thirty-minute fix in a free trial lesson. Method questions are the most coachable marks in the paper.

Test yourself

Try these three without re-reading the methods. Answers stay hidden until you click.

Q1 (2 marks). Name the method used to separate ethanol from water, and state the property difference it exploits.

Show answer

• fractional distillation (with a fractionating column) [1] • the miscible liquids have different boiling points: ethanol boils at 78°C, water at 100°C [1]

Q2 (2 marks). Define the terms residue and filtrate.

Show answer

• residue: the (insoluble) solid that remains in the filter paper [1] • filtrate: the liquid that passes through the filter paper [1]

Q3 (3 marks). Describe how to obtain a sample of pure water from seawater.

Show answer

• use simple distillation: heat the seawater so the water boils/vaporises [1] • the vapour passes into a condenser and is cooled and condensed [1] • collect the distillate (pure water); the salt remains in the flask [1] (the thermometer bulb sits at the side-arm, reading 100°C while water distils)