Mixtures & separating techniques

What you will learn

the difference between a pure substance and a mixture,
homogeneous (uniform) vs heterogeneous (non-uniform) mixtures, including solutions and suspensions,
which property each separating technique exploits (particle size, boiling point, magnetism, solubility),
how filtration, evaporation, distillation, chromatography, decantation and magnetic separation work,
how to calculate concentration as a percentage by mass.

Worked example 0 Real-world example: purifying a salty, sandy mixture

You are given a beaker of seawater mixed with sand. The goal is to recover pure salt and pure water.

Filter the mixture through filter paper. Sand stays in the paper (residue); salty water passes through (filtrate).
Rinse the sand with distilled water and dry it.
Distil the salty water: heat it in a flask with a condenser. Pure water boils off, condenses, and collects. Salt stays behind.
After all the water has distilled off, solid salt remains in the original flask.

Key idea: always ask — what physical property is different between the components? Here: sand is not dissolved (so filter), then salt and water have very different boiling points (so distil).

1. Pure substances, mixtures and solutions

Pure substance: one kind of particle only (e.g. pure water, pure copper, pure oxygen).
Mixture: two or more substances mixed but not chemically joined. Each keeps its own properties.
Solution: a homogeneous mixture of a solute dissolved in a solvent. Salt dissolved in water.
Suspension: a heterogeneous mixture where undissolved particles are dispersed (muddy water).

Homogeneous: uniform throughout; you cannot see separate components (air, salt water, brass). Heterogeneous: non-uniform; you can see separate regions (sand in water, oil in vinegar, granola).

Worked example 1 Classifying common mixtures

Classify: (a) air, (b) a cup of tea with a teabag, (c) bronze, (d) soil, (e) petrol.

Air — homogeneous mixture (solution of gases).
Tea with the bag in — heterogeneous (solid bag plus liquid).
Bronze — homogeneous mixture of copper and tin (an alloy).
Soil — heterogeneous (visibly different grains and organic matter).
Petrol — homogeneous mixture of many hydrocarbons.

Key idea: “you can see the separate bits” is a quick test for heterogeneity.

2. Choosing a separating technique

Every technique uses one property that differs between the components.

Technique	Property used	Typical use
Filtration	Particle size (solid vs liquid/gas)	Sand from water, tea leaves
Decantation	Settling rate (denser solid sinks)	Pouring clear water off sediment
Evaporation	Volatility of solvent	Recovering solid from solution (salt from brine)
Distillation	Boiling point	Water from salt, alcohol from wine
Chromatography	Attraction to paper vs solvent	Separating ink pigments
Magnetic	Magnetism (iron/steel)	Removing iron from recycling
Sieving	Particle size (solid from solid)	Flour from lumps
Centrifugation	Density	Separating blood cells from plasma

Worked example 2 Matching method to mixture

Pick a method for each: (a) iron filings mixed with sand, (b) red dye in water, (c) copper sulfate dissolved in water, (d) two inks on a pen mark.

(a) Magnetic separation — iron is magnetic, sand is not.
(b) Distillation — boiling points differ; water boils first, leaves dye behind.
(c) Evaporation or crystallisation — boil off water to recover copper sulfate crystals.
(d) Chromatography — different pigments travel different distances up paper.

3. Filtration

Filtration setup. Filter paper traps insoluble solid (residue); liquid (filtrate) passes through.

The mixture is poured into a funnel lined with filter paper. Solid particles too large to pass through stay in the paper (residue); the liquid (filtrate) collects in the flask below.

4. Distillation

Used when you want to recover the solvent (usually water) from a solution. The solution is heated; solvent vapour rises, passes through a cooled condenser, and liquefies into a separate flask. The solute stays behind.

5. Chromatography

A small spot of mixture (e.g. ink) is placed near the bottom of filter paper. The paper sits in a solvent. The solvent rises by capillary action, carrying different pigments different distances — those attracted more to the solvent rise further; those attracted more to the paper rise less.

Worked example 3 Identifying a mystery ink

Four known inks (A, B, C, D) and one mystery ink (X) are spotted on chromatography paper. After running, X produces two spots at the same heights as the spots from ink B.

Ink X contains the same two pigments as ink B.
The distances match — so the chemistry matches.

Therefore, the mystery ink is (or shares the pigments of) ink B.

Key idea: chromatography is a pattern-match test. Police use it on forged cheques, and drug-testers use it on athletes.

6. Concentration

A common measure of concentration is percentage by mass:

Percentage concentration

\text{concentration (\% m/m)} = \dfrac{\text{mass of solute}}{\text{mass of solution}} \times 100.

Worked example 4 Making a brine

Dissolve $20$ g of salt in $180$ g of water. What is the concentration?

Mass of solution $= 20 + 180 = 200$ g.
Concentration $= \dfrac{20}{200} \times 100 = 10\%$ .

So this is a $10\%$ salt solution by mass.

Practice: Year 7

Fluency

Tier 1: recall and identify

Define: pure substance, mixture, solution, suspension.
Classify as homogeneous or heterogeneous: (a) milk, (b) vinegar, (c) muesli, (d) salt water, (e) concrete.
What property does filtration exploit? What property does distillation exploit?
Name the technique for each: (a) separating iron nails from sawdust, (b) getting sugar from sugary water, (c) separating the inks in a felt-tip pen, (d) separating water from salt.
In filtration, what are the residue and the filtrate?
A solution has $15$ g of solute in $85$ g of solvent. What is the concentration (% m/m)?
Give one everyday example each of filtration and evaporation.
Why does distillation require both heating and cooling?
Explain the difference between evaporation and distillation.
Give an example of a heterogeneous mixture from the kitchen.

Reasoning

Tier 2: explain and reason

Explain why filtration will not separate sugar from water.
Why must chromatography use a solvent in which at least some of the pigments are soluble?
A student filters muddy river water and drinks it. Is it safe? Explain using what filtration can and cannot remove.
Explain how you would separate a mixture of sand, salt and iron filings using three techniques in order.
Why does a chromatography spot of a pure substance produce only one mark, while a spot of a mixture produces several?
A student sets up a distillation of wine (water plus ethanol). Ethanol boils at $78$ °C, water at $100$ °C. Which liquid collects first? Why?

Problem solving

Tier 3: apply to a novel context

A pharmacist must separate ground pills (insoluble) from their coating solution (dissolved sugar and colouring). Describe a three-step procedure.
Sea water is about $3.5\%$ salt by mass. How much salt is in $2.0$ kg of sea water?
A forensic scientist has a blue ink stain from a crime scene and six suspect pens. Explain how chromatography could identify (or eliminate) the source.
In industry, crude oil is separated into petrol, diesel and kerosene by fractional distillation. Using what you know about boiling points, explain why this process works.

Challenge

Reasoning

Harder reasoning

A student wants to recover pure water from sea water using only the Sun’s heat and a plastic sheet (a “solar still”). Describe the setup, which physical process is used, and why it mimics distillation without a heater.
A mixture contains salt, sand, iron filings, and sawdust. Design a step-by-step separation procedure that recovers all four components, stating the property each step exploits.
Air is a homogeneous mixture of oxygen ( $\approx 21\%$ ), nitrogen ( $\approx 78\%$ ), and minor gases. Industrial oxygen is made by cooling air to around $-200$ °C and distilling it. Explain using the particle model why this works and why it is not done at room temperature.
Blood is a mixture of plasma (liquid), red and white cells, and platelets. Explain why centrifugation — not filtration — is used to separate blood, and what property it exploits.

Answers

Answer key

Attempt the practice first. When you're ready to check, expand the answers below.

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Year 7 answers

Fluency

Tier 1: recall and identify

Pure: one kind of particle. Mixture: two or more substances not chemically joined. Solution: homogeneous mixture (solute dissolved in solvent). Suspension: heterogeneous mixture with undissolved solid particles.
(a) Heterogeneous — milk under a microscope shows fat droplets. (b) Homogeneous. (c) Heterogeneous. (d) Homogeneous. (e) Heterogeneous.
Filtration: particle size. Distillation: boiling point.
(a) Magnetic separation. (b) Evaporation or crystallisation. (c) Chromatography. (d) Distillation.
Residue is the solid trapped in the filter paper; filtrate is the liquid that passes through.
Total mass $= 15 + 85 = 100$ g. Concentration $= 15/100 \times 100 = 15\%$ .
Filtration: tea strainer, coffee filter. Evaporation: drying wet clothes, salt from salt pans.
Heating turns the solvent to vapour; cooling (condenser) turns the vapour back to liquid so it can be collected separately.
Evaporation just removes the solvent (keeps the solid behind). Distillation collects both the solid residue and the pure solvent.
Muesli, salad, fruit cocktail, a bowl of soup with chunks, oil-and-vinegar dressing.

Reasoning

Tier 2: explain and reason

Sugar is dissolved — its particles are separated into individual molecules among the water molecules, small enough to pass through the filter paper along with the water.
At least some pigments must dissolve in the solvent so they can be carried up the paper. Otherwise nothing moves and no separation occurs.
Not safe. Filtration removes undissolved solids but not dissolved chemicals, viruses, or most bacteria. Boiling or chemical treatment is still required.
(i) Add water to dissolve the salt. (ii) Use a magnet to remove iron filings. (iii) Filter to separate sand (residue) from salt solution (filtrate). (iv) Evaporate the filtrate to recover salt.
A pure substance has only one type of particle, so chromatography shows one spot. A mixture contains several pigments, each travelling a different distance — producing several spots.
Ethanol collects first because it has the lower boiling point ( $78$ °C); it vaporises and condenses before water does.

Reasoning

Tier 3: apply to a novel context

(i) Filter the mixture — insoluble pill solids stay as residue. (ii) Evaporate the filtrate — water leaves, dissolved sugar and colouring remain as solid. (iii) If pure sugar is needed, further chromatography or recrystallisation could separate the dye from the sugar.
$3.5\%$ of $2.0$ kg $= 0.035 \times 2000 = 70$ g.
Spot each suspect pen and the stain on a single chromatography sheet. Run in a solvent and compare the patterns. A suspect pen matching the stain’s distances and colours is a likely source; any pen whose pattern differs can be eliminated.
Crude oil is heated and rises through a tall column. Lighter hydrocarbons with low boiling points rise highest before condensing; heavier ones condense at lower levels. Each fraction is collected where it condenses, separating the mixture by boiling point.

Reasoning

Challenge

A solar still has a shallow tray of sea water with a sloped transparent cover. Sunlight heats the water, some evaporates, condenses on the cool cover, and runs down into a separate collection channel. The process is evaporation + condensation — the core steps of distillation — powered by solar energy instead of a flame.
(i) Magnet → remove iron filings. (ii) Add water → dissolve salt, leave sand + sawdust. (iii) Sawdust floats; pour off water with sawdust (decantation) and filter sawdust. (iv) Filter remaining mixture to separate sand (residue) from salt water (filtrate). (v) Evaporate filtrate to recover salt.
At low temperature, gas particles slow; nitrogen and oxygen condense at different points ( $-196$ °C and $-183$ °C). Fractional distillation separates them by their different boiling points. At room temperature, both are gases mixed together — no difference in state to exploit.
Blood cells are roughly the same size as plasma particles at filter scales and can clog paper, but they are denser than plasma. Centrifugation spins the sample; denser red and white cells settle to the bottom, plasma stays at the top. It exploits density rather than particle size.

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