Separation of Mixtures: Filtration, Crystallisation, Sublimation, Distillation and Chromatography (ICSE Class 8)
Updated 17 October 2026 · Fact-checked
To separate a mixture, find a property in which the components differ and use it: particle size (filtration), density (decantation, separating funnel), volatility (evaporation, distillation), sublimation, magnetism, solubility (crystallisation) or movement on paper (chromatography). Many mixtures need several steps.
Each method works because the components differ in one property, such as particle size, volatility, density or magnetism.
Illustration drawn for QuestionBuddy.
Understand Separation of Mixtures: Methods and Principles
Golden rule. Use a property in which the components differ.
Insoluble solid from a liquid.
- Sedimentation and decantation: heavier insoluble particles settle; the clear liquid is poured off. *Principle*: difference in density.
- Filtration: the solid (residue) stays on filter paper; the liquid (filtrate) passes. *Principle*: solid particles too large for the pores.
- Centrifugation: fast spinning throws denser particles outwards (cream from milk).
Dissolved solid from a liquid.
- Evaporation: heat until the liquid vaporises; solid remains (salt from sea water). *Principle*: liquid is volatile, solid is not.
- Crystallisation: concentrate the solution, cool, and filter pure crystals (copper sulphate). *Principle*: the solid is less soluble in the cold solution. Gives purer solid than evaporating to dryness.
Solid mixtures.
- Sublimation: when one solid sublimes (ammonium chloride, camphor, naphthalene, iodine). Heat in a china dish covered by a funnel plugged with cotton; vapour deposits on the cool funnel.
- Magnetic separation: a magnet removes iron.
Liquids.
- Separating funnel: immiscible liquids (oil and water); the denser liquid forms the lower layer. *Principle*: they do not mix and differ in density.
- Simple distillation: to collect the liquid from a solution (pure water from salt water). Vapour is cooled in a condenser; the liquid collected is the distillate.
- Fractional distillation: miscible liquids with different boiling points (alcohol and water); the lower boiling point liquid distils first.
Paper chromatography. Separates coloured components (dyes in ink). A spot of ink on filter paper is dipped in a solvent. Each dye moves a different distance, because of different solubility in the solvent and different holding by the paper.
Multi-step plans. Remove the magnetic part first, dissolve the soluble part, filter off the insoluble part, then evaporate or crystallise.
Separation of Mixtures: Methods and Principles: formulas and key facts
- Property to method
Size filtration Density decantation, separating funnel Volatility evaporation, distillation Sublimation heat and condense Magnetism magnet Solubility crystallisation Movement on paper chromatography - Choose by difference.
- Distillation terms
- Residue stays
- Filtrate passes
- Distillate is collected
- Know the three words.
- Order for a multi-step mixture
- Magnet
- Sublimation
- Dissolve
- Filter
- Evaporate or crystallise
- Simple steps first.
How to solve Separation of Mixtures: Methods and Principles questions
List the components, find how they differ, then choose and order the methods.
- 1Write the properties of each component: magnetic, sublimes, soluble, volatile, density.
- 2Choose the method that uses a difference; give the principle.
- 3Order the steps so that each is simple (magnet, sublimation, dissolving, filtering, evaporating).
- 4Name the residue, filtrate or distillate at each stage.
Separation of Mixtures: Methods and Principles in real life
Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.
Salt from sea water
Suppose salt is obtained from sea water in shallow pans in the Sun.
Name the method and the principle.
Try it first, then show the solutionSolution
- Water evaporates in the heat, leaving the salt.
- Principle: the liquid is volatile; the solid is not.
Answer: Evaporation.
Memory hook: Water leaves, salt stays.
Separating ink dyes
Imagine a spot of black ink on filter paper is dipped in water and splits into three coloured bands.
Name the process and explain the bands.
Try it first, then show the solutionSolution
- The process is paper chromatography.
- Each dye moves a different distance because of different solubility in the solvent.
Answer: Chromatography; different dyes move different distances.
Memory hook: Different dyes, different distances.
Tricks to remember Separation of Mixtures: Methods and Principles
Short memory aids. Say each one aloud once, then use it on a question the same day.
Match the difference
Size, density, volatility, sublimation, magnetism, solubility: one method each.
Use it when: Method choice.
R-F-D
Residue on paper, Filtrate through, Distillate in the receiver.
Use it when: Terms.
Magnet first
Take the magnetic part out first: it is easiest.
Use it when: Multi-step plans.
Separation of Mixtures: Methods and Principles: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Separating funnel · 1 mark · MCQBoard level
Oil and water are separated using:
(a) filtration
(b) a separating funnel
(c) sublimation
(d) a magnet
Try it first, then show the solutionSolution
- They are immiscible and the denser water forms the lower layer.
Answer: (b) a separating funnel
Example 2: Filtration · 2 marks · Short answerBoard level
State the principle of filtration and name the residue and filtrate when muddy water is filtered.
Try it first, then show the solutionSolution
- Solid particles are too large to pass through the pores of the filter paper. (1)
- Residue: the mud; filtrate: the clear water. (1)
Answer: Size of particles; residue mud, filtrate water.
Example 3: A mixture of three · 4 marks · Long answerHarder board level
Describe how you would obtain, from a mixture of iron filings, sand and common salt, each of the three in its pure state.
Try it first, then show the solutionSolution
- Remove the iron filings with a magnet. (1)
- Add water and stir so that the salt dissolves. (1)
- Filter: sand is the residue. (1)
- Evaporate (or crystallise) the filtrate to get the salt. (1)
Answer: Magnet, dissolve, filter, evaporate.
Example 4: Match the method · 4 marks · Long answerHarder board level
Choose the method and the principle for separating: (a) alcohol from water, (b) ammonium chloride from sand, (c) copper sulphate crystals from its solution, (d) cream from milk.
Try it first, then show the solutionSolution
- (a) Fractional distillation: different boiling points. (1)
- (b) Sublimation: ammonium chloride sublimes, sand does not. (1)
- (c) Crystallisation: the solid is less soluble when cold. (1)
- (d) Centrifugation: denser particles are thrown outwards. (1)
Answer: (a) fractional distillation (b) sublimation (c) crystallisation (d) centrifugation
Example 5: Reading a chromatogram · 4 marks · Case-basedHOTS / competency
A spot of ink on filter paper gives three dyes after the solvent has risen 8 cm. The red dye has moved 3 cm, the yellow 5 cm and the blue 2 cm. (a) How many dyes are in the ink? (b) Which dye is most soluble in the solvent? (c) Find the ratio of the distances moved by yellow and blue. (d) What is the principle of the method?
Try it first, then show the solutionSolution
- (a) 3 dyes. (½)
- (b) Yellow, the one that moved farthest. (1)
- (c) Yellow : blue = 5 : 2 (= 2.5 : 1). (1)
- (d) Different solubility in the solvent and different holding by the paper. (1½)
Answer: (a) 3 (b) yellow (c) 5 : 2 (d) different solubility and holding
Where students lose marks in Separation of Mixtures: Methods and Principles
The slips examiners see most often, and what to write instead.
Mistake: Naming the wrong principle
Principle means the property used.
Wrong working
"Filtration works because of boiling points."
Right working
"Filtration works because solid particles are too big for the pores."
Do this: State the difference used.
Mistake: Wrong order of steps
Dissolving before removing iron wastes time.
Wrong working
"Add water, then use the magnet."
Right working
"Use the magnet first, then dissolve."
Do this: Do the easiest physical removal first.
Mistake: Residue and filtrate swapped
Residue stays on the paper.
Wrong working
"Salt water is the residue."
Right working
"Sand is the residue; salt water is the filtrate."
Do this: Residue is the solid left behind.
Exam tips
- Name the method and the principle in every answer.
- Draw labelled diagrams for filtration and distillation.
- Use the words residue, filtrate and distillate correctly.
- For multi-step plans, number the steps.
Practice questions on Separation of Mixtures: Methods and Principles
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · Very short answerBoard level
In simple distillation of salt water, what is the liquid collected in the receiver called?
Hint
Not residue or filtrate.
Answer
The distillate.
Q2 · 2 marks · Short answerHarder board level
A copper sulphate solution is heated only until it is concentrated and then left to cool. What is seen after cooling, and why is it better than boiling the solution to dryness?
Hint
Think of purity and decomposition.
Answer
Pure blue crystals form on cooling, because the solid is less soluble in the cold solution. Boiling to dryness could decompose the solid and leave the impurities mixed with it.
Q3 · 3 marks · Short answerHarder board level
A mixture of kerosene and water is given. Which method is used, what is the lower layer and why?
Hint
Immiscible liquids differ in density.
Answer
A separating funnel is used. Water is denser, so it forms the lower layer and is run off through the tap; kerosene remains above.
Q4 · 3 marks · Short answerHarder board level
Name the process and describe the set-up for obtaining pure water from a salt solution, naming the parts: flask, condenser and receiver.
Hint
Heat, cool, collect.
Answer
Simple distillation. The solution is boiled in the flask; the vapour passes through the condenser (cold water around the tube), where it condenses; the distillate is collected in the receiver. Salt stays in the flask.
Q5 · 4 marks · Long answerHOTS / competency
Plan the separation of a mixture of iron filings, ammonium chloride, common salt and sand. Give the methods and principles in order.
Hint
Magnet, sublimation, dissolve, filter, evaporate.
Answer
1. Magnet removes iron (magnetism). 2. Heat the rest in a china dish under a funnel: ammonium chloride sublimes and collects on the funnel (sublimation). 3. Dissolve what remains in water (salt dissolves). 4. Filter: sand is the residue (particle size). 5. Evaporate or crystallise the filtrate to get salt (volatility/solubility).
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.Method for immiscible liquids?Show answer
Separating funnel.
2.Method for miscible liquids with different boiling points?Show answer
Fractional distillation.
3.Principle of crystallisation?Show answer
Solid is less soluble in the cold solution.
4.What does a condenser do?Show answer
Cools the vapour to a liquid.
5.What does chromatography separate?Show answer
Coloured components such as dyes.
Separation of Mixtures: Methods and Principles: frequently asked questions
How can you separate sand and salt?
Dissolve the salt in water, filter off the sand, and evaporate the filtrate to recover the salt.
What is the principle of distillation?
The liquid boils and turns to vapour, which is cooled and collected; solids or less volatile parts remain behind.
What is chromatography used for?
To separate coloured components, such as the dyes in ink, because they travel different distances on paper.