ScienceCBSE Class 9 · Exploring Mixtures and their Separation

Separation of Mixtures, Colloids and the Tyndall Effect (Class 9)

Updated 17 October 2026 · Fact-checked

A mixture is separated by physical methods that use a difference in a property: crystallisation for a pure solid from solution, distillation for miscible liquids whose boiling points differ (fractional distillation if they differ by less than about 25 °C), paper chromatography for dissolved dyes, a separating funnel for immiscible liquids, sublimation for a sublimable solid, and centrifugation or coagulation for fine suspended particles. A solution has particles under 1 nm and shows no Tyndall effect; a colloid has 1 to 1000 nm particles and scatters light; a suspension has particles over 1000 nm that settle and are filtered off.

Only particles bigger than 1 nm scatter light (Tyndall effect), and only suspension particles, over 1000 nm, settle on standing.

Illustration drawn for QuestionBuddy.

Understand Separation Methods, Suspensions, Colloids and the Tyndall Effect

Mixtures. A mixture has two or more substances that are not chemically combined, so physical methods can separate them. A homogeneous mixture has the same composition throughout (sugar solution, vinegar, soda, air, alloys such as brass). A heterogeneous mixture varies from place to place (sand in water, oil and water, smoke). Gas mixtures are homogeneous; most solid-solid mixtures are heterogeneous. Alloys are homogeneous mixtures of metals and cannot be separated by simple methods like filtration or decantation.

Choosing a method. The method uses the property in which the components differ. Crystallisation uses changes in solubility with temperature. Distillation uses boiling points: the lower-boiling liquid vaporises first, and is condensed and collected as the distillate. It separates two miscible liquids with boiling points differing by about 25 °C or more (acetone 56 °C and water 100 °C), or recovers a solvent from a solution (pure water from salt water). In the set-up, the thermometer is at the side arm, and cold water enters the condenser at the lower end and leaves at the upper end so that the jacket stays full of cold water. If the boiling points differ by less than 25 °C, fractional distillation with a fractionating column is used; refineries separate crude petroleum into petroleum gas, petrol, kerosene, diesel, lubricating oil and bitumen. Evaporation occurs at the surface at any temperature; boiling occurs throughout the liquid at a fixed temperature.

Paper chromatography. Dissolved substances move up paper at different rates, because each sticks to the paper and dissolves in the solvent to a different extent. Draw the start line in pencil (ink would itself separate), place a small spot on it, and keep the water level below the spot. A pure substance gives one spot; a mixture gives several. It is used for dyes in inks and food colours, pigments in leaves, and checking purity.

Heterogeneous mixtures. Immiscible liquids (such as mustard oil and water) are separated in a separating funnel based on density: the denser layer is run off first through the stopcock, which is closed just before the boundary. Sublimation separates a solid that changes directly to vapour on heating from one that does not (camphor, naphthalene, ammonium chloride, iodine); an inverted funnel with a cotton plug collects the deposit on its cool wall. A suspension has solid particles (over 1000 nm) that are visible, settle on standing and are removed by filtration. Very fine particles are removed by centrifugation (heavier particles are thrown to the bottom of the tube; used to separate cream from milk and blood into plasma and cells) or by coagulation (alum makes fine particles clump and settle; the clear water is then decanted or filtered).

Colloids and the Tyndall effect. A colloid has particles of 1 to 1000 nm: too small to see or settle, but large enough to scatter light. Examples: milk, blood, tomato sauce, fog, smoke, butter. The Tyndall effect is the scattering of a beam of light by particles, which makes its path visible; it is seen in colloids and suspensions but not in true solutions. A colloid has a dispersed phase (the particles) and a dispersion medium. Emulsions are colloids of two liquids, kept stable by emulsifying agents such as soap.

Solution, colloid and suspension. Solution: homogeneous, particles under 1 nm, invisible, passes through filter paper, does not settle, no Tyndall effect. Colloid: appears uniform, 1 to 1000 nm, invisible, passes through filter paper, does not settle, shows Tyndall effect. Suspension: heterogeneous, over 1000 nm, visible, leaves a residue on filter paper, settles, shows the Tyndall effect.

Separation Methods, Suspensions, Colloids and the Tyndall Effect: formulas and key facts

Particle size ranges: $S o l u t i o n < 1 nm ∣ co l l o i d 1 t o 1000 nm ∣ s u s p e n s i o n > 1000 nm$
Distillation rule: Boiling points differ by about 25 °C or more: simple distillation | less than 25 °C: fractional distillation
Method for each mixture: Pure solid from solution crystallisation
miscible liquids distillation
dyes chromatography
immiscible liquids separating funnel
sublimable solid sublimation
Fine particles: Centrifugation (spin heavier particles down) | coagulation with alum (clump, settle, decant or filter)
Percentage recovery: Mass or volume recovered ÷ mass or volume present × 100

Pure solid from solution	crystallisation
miscible liquids	distillation
dyes	chromatography
immiscible liquids	separating funnel
sublimable solid	sublimation

How to solve Separation Methods, Suspensions, Colloids and the Tyndall Effect questions

Use this method for 'which method and why' and recovery questions.

1Classify the mixture: homogeneous or heterogeneous; solid, liquid or gas parts.
2Find the property in which the components differ (boiling point, density, solubility, sublimation, particle size, colour spread).
3Choose the method that uses that property and name it exactly.
4Describe the key step and apparatus (for example 'cold water enters the condenser at the lower end').
5For recovery numericals, calculate the amount present in the mixture and divide the recovered amount by it.
6Check against a second property (for example the Tyndall test or filtration) when classifying.

Separation Methods, Suspensions, Colloids and the Tyndall Effect 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.

Clearing muddy water with alum

Suppose a family in a village keeps a bucket of muddy pond water for a day, but the water stays cloudy even after settling, so a neighbour suggests adding a small piece of alum.

Explain why the water stays cloudy and how alum helps, naming the steps of the process.

Try it first, then show the solutionSolution

The cloudiness is due to very fine particles of clay, which stay suspended and may even pass through filter paper.
Alum is a coagulant: stirring it in makes the fine particles clump together into heavier particles.
The heavier clumps settle at the bottom (sedimentation), and the clear water is then poured off (decantation) or filtered.

Answer: Coagulation with alum, then sedimentation and decantation or filtration.

Memory hook: Alum makes the small ones stick together and sink.

Is it a solution or a colloid?

Suppose a student shines a torch from the side through a glass of clear salt solution, through a glass of diluted milk, and through a glass of chalk powder stirred in water.

State what she will see in each glass and name the type of mixture.

Try it first, then show the solutionSolution

Salt solution: the beam is not visible inside the liquid because particles are under 1 nm and do not scatter light; it is a true solution.
Diluted milk: the beam is clearly visible (Tyndall effect); it is a colloid.
Chalk in water: the beam is visible and the chalk settles with time; it is a suspension.

Answer: Invisible beam in the solution; visible beam in milk (colloid) and in chalk water (suspension).

Memory hook: Tyndall means the light road shows.

Tricks to remember Separation Methods, Suspensions, Colloids and the Tyndall Effect

Short memory aids. Say each one aloud once, then use it on a question the same day.

Property to method
Boiling point: distillation. Density: separating funnel. Sublimes: sublimation. Dye speeds: chromatography. Solubility with temperature: crystallisation. Fine heavy particles: centrifuge.
Use it when: Any 'choose a method' question.
Cold in at the bottom
Cold water goes in at the lower end of the condenser, so the jacket is always full and the vapour cools fully.
Use it when: Distillation diagram and explanation.
Pencil, not pen
The start line on chromatography paper is drawn in pencil because ink would itself separate into colours.
Use it when: Chromatography procedure.
1 and 1000
Under 1 nm solution; 1 to 1000 nm colloid; over 1000 nm suspension.
Use it when: Classification by particle size.

Separation Methods, Suspensions, Colloids and the Tyndall Effect: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Match the method · 4 marks · Short answerBoard level

Name the best method to separate each: (a) ammonium chloride from common salt, (b) mustard oil from water, (c) the dyes in black ink, (d) acetone from water (boiling points 56 °C and 100 °C).

Try it first, then show the solutionSolution

(a) Sublimation, because ammonium chloride changes directly to vapour on heating and salt does not. (1)
(b) Separating funnel, because the two liquids are immiscible and have different densities. (1)
(c) Paper chromatography, because the dyes move up the paper at different rates. (1)
(d) Simple distillation, because the boiling points differ by 100 − 56 = 44 °C, which is more than about 25 °C. (1)

Answer: (a) sublimation; (b) separating funnel; (c) chromatography; (d) simple distillation.

Example 2: Solution, colloid or suspension · 3 marks · Short answerBoard level

State, with a reason, whether each is a solution, colloid or suspension: (a) salt in water, (b) milk, (c) sand in water. Which of the three show the Tyndall effect?

Try it first, then show the solutionSolution

(a) Solution: homogeneous, particles under 1 nm, no Tyndall effect. (1)
(b) Colloid: particles 1 to 1000 nm, appear uniform but scatter light. (1)
(c) Suspension: particles over 1000 nm, visible and settling. Tyndall effect is shown by (b) and (c). (1)

Answer: (a) solution; (b) colloid; (c) suspension; milk and sand in water show Tyndall effect.

Example 3: Distilling an acetone-water mixture · 5 marks · Long answerHarder board level

250 mL of a mixture contains 40% v/v acetone (boiling point 56 °C) and the rest water (boiling point 100 °C). The mixture is distilled.
(a) Find the volume of acetone present. (1)
(b) Which liquid is collected first, and at about what thermometer reading? (1)
(c) The student collects 92 mL of acetone. Find the percentage recovery. (1)
(d) Where should cold water enter the condenser, and why? (1)
(e) Why is simple distillation suitable here? (1)

Try it first, then show the solutionSolution

(a) 40 ÷ 100 × 250 = 100 mL (the rest, 150 mL, is water). (1)
(b) Acetone, the lower-boiling liquid, at about 56 °C. (1)
(c) Recovery = 92 ÷ 100 × 100 = 92%. (1)
(d) At the lower end and leaving at the upper end, so the condenser jacket stays full of cold water and the vapour condenses fully. (1)
(e) The boiling points differ by 100 − 56 = 44 °C, more than about 25 °C, so a fractionating column is not needed. (1)

Answer: (a) 100 mL; (b) acetone at about 56 °C; (c) 92%; (d) lower end; (e) 44 °C difference.

Example 4: Identify three liquids from their behaviour · 5 marks · Long answerHarder board level

A student tests three samples. P is clear, passes through filter paper, shows no beam in the torch test. Q looks milky, passes through filter paper, does not settle in a day and shows a clear beam. R is cloudy, leaves a residue on filter paper, settles in an hour and shows a beam.
(a) Classify P, Q and R. (1)
(b) The particles in Q are about 200 nm across and in R about 5000 nm. Show that these sizes agree with your classification. (1)
(c) State which of Q's particles could be seen with the naked eye (the eye cannot separate points closer than 0.1 mm = 100 000 nm). (1)
(d) Name a method to clear R and one to separate the components of Q (say, cream from milk). (2)

Try it first, then show the solutionSolution

(a) P is a true solution; Q is a colloid; R is a suspension. (1)
(b) 200 nm lies between 1 and 1000 nm, so Q is a colloid; 5000 nm is above 1000 nm, so R is a suspension. (1)
(c) 200 nm is far smaller than 100 000 nm, so the particles in Q cannot be seen by the naked eye (even 5000 nm is only 1/20 of 100 000 nm). (1)
(d) R: filtration, or decantation after settling (alum if very fine). Q: centrifugation, which spins the heavier particles out to separate cream from milk. (2)

Answer: (a) solution, colloid, suspension; (b) sizes fit; (c) not visible; (d) filtration or alum for R, centrifugation for Q.

Example 5: A three-component mixture · 4 marks · Case-basedHOTS / competency

A 60 g mixture contains 10 g camphor, 20 g sand and 30 g common salt.
(a) Give the percentage composition by mass. (1)
(b) Write a plan to separate all three in the right order, naming each method. (1)
(c) After the separation the student recovers 9.5 g camphor, 19.6 g sand and 29.2 g salt. Find the total loss and the percentage loss. (1)
(d) Name the step in which most loss of camphor may occur and one precaution. (1)

Try it first, then show the solutionSolution

(a) Camphor 10 ÷ 60 × 100 = 16.7%; sand 20 ÷ 60 × 100 = 33.3%; salt 30 ÷ 60 × 100 = 50%. (1)
(b) First sublimation: heat the mixture in a china dish under an inverted funnel with a cotton plug, camphor collects on the funnel. Then add water to the residue and stir; salt dissolves. Filter to remove the sand. Finally evaporate or crystallise the filtrate to get salt. (1)
(c) Recovered = 9.5 + 19.6 + 29.2 = 58.3 g. Loss = 60 − 58.3 = 1.7 g; percentage loss = 1.7 ÷ 60 × 100 = 2.8%. (1)
(d) In sublimation, camphor vapour can escape; the cotton plug in the funnel stops the vapour from leaving. (1)

Answer: (a) 16.7%, 33.3%, 50%; (b) sublimation, dissolve, filter, evaporate; (c) 1.7 g and 2.8%; (d) sublimation, use the cotton plug.

Where students lose marks in Separation Methods, Suspensions, Colloids and the Tyndall Effect

The slips examiners see most often, and what to write instead.

Mistake: Naming the wrong method because the property is not identified.
Students pick a familiar method such as filtration for everything.
Wrong working
Separate acetone and water by filtration.
Right working
Distillation, because the liquids are miscible and differ in boiling point.
Do this: State the property first, then the method.
Usually costs: 1 mark each.
Mistake: Drawing the chromatography start line with a pen.
Ink dissolves and separates itself.
Wrong working
Start line drawn with a pen.
Right working
Draw the start line in pencil.
Do this: Pencil line, spot above water level.
Usually costs: 1 mark.
Mistake: Saying a colloid settles like a suspension.
Both are heterogeneous and both show the Tyndall effect.
Wrong working
Milk will separate into layers on standing.
Right working
A colloid is stable: its particles (1 to 1000 nm) do not settle.
Do this: Settling means suspension.
Usually costs: 1 mark.
Mistake: Putting cold water into the top of the condenser.
Water is poured in from the end that is easiest to reach.
Wrong working
Water enters at the upper end of the condenser.
Right working
Water enters at the lower end and leaves at the upper end so the jacket stays full.
Do this: Cold in at the bottom.
Usually costs: 1 mark in a diagram.
Mistake: Using evaporation for a mixture of two miscible liquids.
Evaporation loses both liquids.
Wrong working
Evaporate the mixture to separate alcohol from water.
Right working
Use distillation and condense the vapour.
Do this: Evaporation does not collect the liquid.
Usually costs: 1 mark.

Exam tips

In a 'which method' answer name the property (boiling point, density, sublimation) and the method.
Label the distillation diagram: flask, thermometer, condenser with water in at the bottom and out at the top, receiver.
Give particle size ranges and one test (Tyndall, filtration, settling) in a solution-colloid-suspension table.
For a sequence of separations, order the steps so each one is possible (sublimation before dissolving in water).
For recovery questions, show amount present, amount recovered and then the percentage.

Practice questions on Separation Methods, Suspensions, Colloids and the Tyndall Effect

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Write two differences between a true solution and a suspension.
Hint
Think of particle size and settling.
Answer
A solution is homogeneous with particles under 1 nm that do not settle and pass through filter paper (1 mark); a suspension is heterogeneous with particles over 1000 nm that are visible, settle on standing and leave a residue on filter paper (1 mark). Also acceptable: a suspension shows the Tyndall effect and a true solution does not.
Q2 · 3 marks · Short answerHarder board level
(a) In distillation, why is the thermometer placed at the side arm of the flask? (b) Why is a fractionating column needed when two liquids have boiling points 70 °C and 80 °C? (c) Give one use of fractional distillation.
Hint
The bulb should read the vapour that is going to the condenser.
Answer
(a) The bulb at the side arm records the temperature of the vapour that is about to enter the condenser, which is the boiling point of the liquid being collected (1 mark). (b) The boiling points differ by only 10 °C, which is less than about 25 °C, so simple distillation would collect a mixture; the column separates the vapours (1 mark). (c) Separating crude petroleum into petrol, kerosene, diesel and other fractions (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): In paper chromatography, a pure substance gives a single spot. Reason (R): All the components of a mixture move up the paper at the same rate. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
If every component moved at the same rate, would a mixture be separated?
Answer
(c). A is true: one substance gives one spot. R is false: the components of a mixture move at different rates, which is why chromatography separates them.
Q4 · 3 marks · Short answerHarder board level
The water of a village pond is muddy with very fine clay that passes through filter paper. Describe, in order, how you would obtain clear water, naming each process and the role of alum.
Hint
Coagulation, sedimentation, decantation.
Answer
Add a small piece of alum and stir; alum is the coagulant, making the fine particles clump together (1 mark). Leave undisturbed so the heavier clumps settle at the bottom (sedimentation) (1 mark). Carefully pour off the clear water (decantation) or filter it to remove any remaining clumps (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
A student shines a torch from the side through (i) salt solution, (ii) diluted milk and (iii) chalk stirred in water. (a) In which will the beam path be visible? (b) Which will leave a residue on filter paper? (c) Which will form a layer of sediment after an hour? (d) Explain why the beam of a car's headlight is visible in fog.
Hint
Compare particle sizes of the three.
Answer
(a) (ii) and (iii), because their particles scatter light (Tyndall effect) (1 mark). (b) (iii), the suspension (1 mark). (c) (iii), because its particles are larger than 1000 nm and settle (1 mark). (d) Fog is a colloid of tiny water droplets in air; the droplets scatter the beam, making its path visible (1 mark).

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.What is the size range of colloidal particles?Show answer
1 to 1000 nm.
2.Which method separates two immiscible liquids?Show answer
A separating funnel.
3.What is the Tyndall effect?Show answer
The scattering of a beam of light by particles, which makes its path visible.
4.Name two solids that sublime.Show answer
Camphor and ammonium chloride (also naphthalene, iodine).
5.What does alum do to muddy water?Show answer
It makes fine suspended particles clump together so they settle.
6.When is fractional distillation used?Show answer
When the boiling points differ by less than about 25 °C.

Separation Methods, Suspensions, Colloids and the Tyndall Effect: frequently asked questions

Is milk a solution?

No. Milk is a colloid, with particles of 1 to 1000 nm that scatter light, so it shows the Tyndall effect.

Why can we not see the path of a torch beam through salt water?

The dissolved particles are smaller than 1 nm and too small to scatter visible light, so the beam passes through unseen.

What is the difference between evaporation and boiling?

Evaporation happens at the surface at any temperature; boiling happens throughout the liquid at the boiling point.

Why is camphor placed under a funnel with a cotton plug?

The vapour deposits on the cool wall of the funnel, and the cotton plug stops vapour from escaping.

Can we separate an alloy by filtration?

No. An alloy is a homogeneous mixture of metals, so its metals are uniformly mixed and cannot be separated by filtration.