Shine a Torch Through Two Clear Liquids and One Gives Itself Away
Learn how a separating funnel divides immiscible liquids, how sublimation separates camphor and ammonium chloride, how solutions, suspensions and colloids differ, and what the Tyndall effect reveals.
How can you tell two clear liquids apart with a torch?
By whether the beam of light becomes visible as it passes through.
Take a glass of salt water and a glass of heavily diluted milk. Both look clear and colourless. Shine a narrow torch beam sideways through the salt water and you see nothing inside the liquid — the beam appears only where it leaves. Shine the same beam through the diluted milk and the path of the beam is plainly visible, a bright line running right through the glass.
The milk contains particles large enough to scatter light in all directions; the salt water does not. That scattering is the Tyndall effect, and it is the standard test separating a colloid from a true solution when the eye alone cannot.
This page covers the third part of the CBSE Class 9 Science chapter on mixtures and their separation, and it completes the chapter's two themes: separating mixtures, and classifying them.
Take a glass of salt water and a glass of heavily diluted milk. Both look clear and colourless. Shine a narrow torch beam sideways through the salt water and you see nothing inside the liquid — the beam appears only where it leaves. Shine the same beam through the diluted milk and the path of the beam is plainly visible, a bright line running right through the glass.
The milk contains particles large enough to scatter light in all directions; the salt water does not. That scattering is the Tyndall effect, and it is the standard test separating a colloid from a true solution when the eye alone cannot.
This page covers the third part of the CBSE Class 9 Science chapter on mixtures and their separation, and it completes the chapter's two themes: separating mixtures, and classifying them.
How does a separating funnel divide two immiscible liquids?
By letting them settle into layers and running the lower one off first.
Two immiscible liquids do not mix — they form two distinct layers, with the denser liquid at the bottom. A separating funnel exploits that density difference directly.
The method, step by step:
- Pour the mixture into the separating funnel and close the stopcock
- Let it stand undisturbed until two clear layers have formed
- Open the stopcock and run the lower layer out into a beaker
- Close the stopcock the moment the boundary reaches it
- Collect the upper layer separately
Worked example. A mixture of kerosene and water separates into water at the bottom and kerosene on top, because water is the denser of the two. Run off the water first, and the kerosene is left behind in the funnel.
Everyday evidence. Oil poured into water in a kadhai floats in a distinct layer and never mixes, however long you wait. That layer is exactly what a separating funnel collects.
Where it is used. Separating a mixture of oil and water, and removing water that has got into petrol or diesel. It is also used industrially to separate iron ore from the lighter impurities in it after crushing.
It works on density, not on boiling point. That single fact decides when to use it. Two liquids that do mix — alcohol and water, for instance — form no layers at all, and no amount of standing will produce any. Those need distillation, which uses boiling points instead, as the previous part of this chapter described.
So establish first whether the liquids are miscible. If they form layers, use a separating funnel and the job takes minutes. If they do not, a separating funnel is useless — and choosing between the two techniques is the substance of the question, not the apparatus.
Two immiscible liquids do not mix — they form two distinct layers, with the denser liquid at the bottom. A separating funnel exploits that density difference directly.
The method, step by step:
- Pour the mixture into the separating funnel and close the stopcock
- Let it stand undisturbed until two clear layers have formed
- Open the stopcock and run the lower layer out into a beaker
- Close the stopcock the moment the boundary reaches it
- Collect the upper layer separately
Worked example. A mixture of kerosene and water separates into water at the bottom and kerosene on top, because water is the denser of the two. Run off the water first, and the kerosene is left behind in the funnel.
Everyday evidence. Oil poured into water in a kadhai floats in a distinct layer and never mixes, however long you wait. That layer is exactly what a separating funnel collects.
Where it is used. Separating a mixture of oil and water, and removing water that has got into petrol or diesel. It is also used industrially to separate iron ore from the lighter impurities in it after crushing.
It works on density, not on boiling point. That single fact decides when to use it. Two liquids that do mix — alcohol and water, for instance — form no layers at all, and no amount of standing will produce any. Those need distillation, which uses boiling points instead, as the previous part of this chapter described.
So establish first whether the liquids are miscible. If they form layers, use a separating funnel and the job takes minutes. If they do not, a separating funnel is useless — and choosing between the two techniques is the substance of the question, not the apparatus.
Which substances sublime, and how does that separate them?
Sublimation is a solid changing directly into vapour on heating, without becoming a liquid, and the vapour turning straight back into solid on cooling.
Only some solids do this. The ones named in the syllabus are:
- Camphor
- Ammonium chloride
- Naphthalene
- Iodine
- Solid carbon dioxide, often called dry ice
The method for separating a mixture. Suppose common salt is mixed with ammonium chloride.
- Place the mixture in a china dish
- Invert a funnel over the dish, plugging the stem with cotton wool
- Heat gently
- The ammonium chloride sublimes, rises as vapour, and deposits as solid on the cool inner surface of the funnel
- The salt, which does not sublime, remains in the dish
Scrape the deposit off the funnel and both components are recovered separately.
Everyday evidence. A camphor tablet left in a cupboard slowly shrinks and finally disappears, leaving no puddle and no stain behind — it went straight from solid to vapour. Naphthalene balls in a wardrobe do the same, and the smell in the air is the vapour.
Sublimation is a physical change. The camphor collected on the funnel is chemically identical to the camphor that was heated. Nothing has reacted; only the state has changed and changed back.
Only one component may sublime for this to work. If a mixture contained camphor and naphthalene, both would vaporise together and both would deposit together, leaving you no better off. So the technique needs exactly one subliming component — and identifying which substances sublime is therefore half the question.
A useful contrast with evaporation. In evaporation a liquid becomes vapour and the dissolved solid is left behind. In sublimation a solid becomes vapour directly, and it is the vapour you want to collect. The two look similar in a diagram and answer opposite questions.
Only some solids do this. The ones named in the syllabus are:
- Camphor
- Ammonium chloride
- Naphthalene
- Iodine
- Solid carbon dioxide, often called dry ice
The method for separating a mixture. Suppose common salt is mixed with ammonium chloride.
- Place the mixture in a china dish
- Invert a funnel over the dish, plugging the stem with cotton wool
- Heat gently
- The ammonium chloride sublimes, rises as vapour, and deposits as solid on the cool inner surface of the funnel
- The salt, which does not sublime, remains in the dish
Scrape the deposit off the funnel and both components are recovered separately.
Everyday evidence. A camphor tablet left in a cupboard slowly shrinks and finally disappears, leaving no puddle and no stain behind — it went straight from solid to vapour. Naphthalene balls in a wardrobe do the same, and the smell in the air is the vapour.
Sublimation is a physical change. The camphor collected on the funnel is chemically identical to the camphor that was heated. Nothing has reacted; only the state has changed and changed back.
Only one component may sublime for this to work. If a mixture contained camphor and naphthalene, both would vaporise together and both would deposit together, leaving you no better off. So the technique needs exactly one subliming component — and identifying which substances sublime is therefore half the question.
A useful contrast with evaporation. In evaporation a liquid becomes vapour and the dissolved solid is left behind. In sublimation a solid becomes vapour directly, and it is the vapour you want to collect. The two look similar in a diagram and answer opposite questions.
How do a true solution, a suspension and a colloid differ?
By particle size — and every other difference follows from it.
True solution — particle size **less than nm.
- Homogeneous
- Particles cannot be seen, even under a microscope
- Does not scatter light
- Stable: particles never settle
- Cannot be separated by filtration
- Examples: salt in water, sugar in water, air
Suspension — particle size greater than nm.
- Heterogeneous
- Particles are visible to the naked eye or with a simple microscope
- Scatters light
- Unstable: particles settle on standing
- Can be separated by filtration
- Examples: chalk powder in water, muddy water, sand in water
Colloid — particle size between nm and nm.
- Appears homogeneous but is heterogeneous
- Individual particles cannot be seen
- Scatters light, showing the Tyndall effect
- Stable: does not settle on standing
- Cannot be separated by ordinary filtration; needs centrifugation
- Examples: milk, ink, smoke, fog, butter, cheese, shaving cream, blood
The two parts of a colloid are the dispersed phase — the particles, corresponding to the solute — and the dispersing medium, corresponding to the solvent.
The types of colloid, named by the states of those two parts:
- Aerosol: liquid in gas (fog, mist, clouds) or solid in gas (smoke)
- Foam: gas in liquid (shaving cream) or gas in solid (sponge)
- Emulsion: liquid in liquid (milk, face cream)
- Sol: solid in liquid (ink, milk of magnesia)
- Gel: liquid in solid (cheese, butter, jelly)
- Solid sol: solid in solid (coloured glass, gemstone)
Everyday evidence. Stir chalk powder into water and it clouds, then settles within minutes — a suspension. Stir sugar into water and nothing settles however long you wait — a solution. Milk never settles and never clears — a colloid.
A colloid is stable but heterogeneous, and that combination is the whole difficulty.** Students expect does not settle and cannot be filtered to mean homogeneous, because those are the properties of a true solution. A colloid shares both and is still classified heterogeneous, because its particles are genuinely separate — they are simply too small to settle and too small to be trapped by filter paper.
Only the suspension can be filtered. If a question says filtration separated the mixture, it was a suspension. If filtration failed, it was a colloid or a solution — and the next section decides which.
True solution — particle size **less than nm.
- Homogeneous
- Particles cannot be seen, even under a microscope
- Does not scatter light
- Stable: particles never settle
- Cannot be separated by filtration
- Examples: salt in water, sugar in water, air
Suspension — particle size greater than nm.
- Heterogeneous
- Particles are visible to the naked eye or with a simple microscope
- Scatters light
- Unstable: particles settle on standing
- Can be separated by filtration
- Examples: chalk powder in water, muddy water, sand in water
Colloid — particle size between nm and nm.
- Appears homogeneous but is heterogeneous
- Individual particles cannot be seen
- Scatters light, showing the Tyndall effect
- Stable: does not settle on standing
- Cannot be separated by ordinary filtration; needs centrifugation
- Examples: milk, ink, smoke, fog, butter, cheese, shaving cream, blood
The two parts of a colloid are the dispersed phase — the particles, corresponding to the solute — and the dispersing medium, corresponding to the solvent.
The types of colloid, named by the states of those two parts:
- Aerosol: liquid in gas (fog, mist, clouds) or solid in gas (smoke)
- Foam: gas in liquid (shaving cream) or gas in solid (sponge)
- Emulsion: liquid in liquid (milk, face cream)
- Sol: solid in liquid (ink, milk of magnesia)
- Gel: liquid in solid (cheese, butter, jelly)
- Solid sol: solid in solid (coloured glass, gemstone)
Everyday evidence. Stir chalk powder into water and it clouds, then settles within minutes — a suspension. Stir sugar into water and nothing settles however long you wait — a solution. Milk never settles and never clears — a colloid.
A colloid is stable but heterogeneous, and that combination is the whole difficulty.** Students expect does not settle and cannot be filtered to mean homogeneous, because those are the properties of a true solution. A colloid shares both and is still classified heterogeneous, because its particles are genuinely separate — they are simply too small to settle and too small to be trapped by filter paper.
Only the suspension can be filtered. If a question says filtration separated the mixture, it was a suspension. If filtration failed, it was a colloid or a solution — and the next section decides which.
What exactly is the Tyndall effect and what does it prove?
It is the scattering of a beam of light by colloidal particles, which makes the path of the beam visible.
For the effect to appear, the particles must be large enough to scatter light — comparable in size to the wavelength of light — and that is exactly the colloidal range. The particles in a true solution are far too small, so a beam passes through unseen.
The test in the laboratory. Shine a narrow beam through two clear liquids in identical glasses.
- Through salt water, the beam is invisible inside the liquid. True solution.
- Through diluted milk or a starch solution, the beam's path is clearly visible from the side. Colloid.
That is the whole test, and it settles in seconds a distinction that filtration and appearance both fail to make.
Everyday evidence, all of it the same effect.
- Sunlight streaming through a gap in the canopy of a dense forest, made visible by the mist and dust in the air
- A beam from a projector visible across a smoky hall
- The headlights of a vehicle visible as cones in fog
- Sunlight entering a dark, dusty room through a small hole, showing as a shaft of light
In each case the air is the dispersing medium and the fine water droplets or dust are the dispersed phase — a colloid, scattering the light sideways into your eye. Without the particles you would see the lamp but not the beam.
A suspension scatters light too. So the Tyndall effect on its own does not prove a mixture is a colloid; it proves the particles are big enough to scatter. But a suspension is already obviously heterogeneous and settles on standing, so in practice the test is used where it matters — on a mixture that looks like a solution and does not settle.
The practical sequence for identifying an unknown mixture:
- Does it settle on standing, or can it be filtered? Then it is a suspension.
- If not, does it show the Tyndall effect? Then it is a colloid.
- If it shows neither, it is a true solution.
Three questions in order, and they classify any mixture in the chapter — which is why the Tyndall effect earns a section of its own rather than being one line in a table.
For the effect to appear, the particles must be large enough to scatter light — comparable in size to the wavelength of light — and that is exactly the colloidal range. The particles in a true solution are far too small, so a beam passes through unseen.
The test in the laboratory. Shine a narrow beam through two clear liquids in identical glasses.
- Through salt water, the beam is invisible inside the liquid. True solution.
- Through diluted milk or a starch solution, the beam's path is clearly visible from the side. Colloid.
That is the whole test, and it settles in seconds a distinction that filtration and appearance both fail to make.
Everyday evidence, all of it the same effect.
- Sunlight streaming through a gap in the canopy of a dense forest, made visible by the mist and dust in the air
- A beam from a projector visible across a smoky hall
- The headlights of a vehicle visible as cones in fog
- Sunlight entering a dark, dusty room through a small hole, showing as a shaft of light
In each case the air is the dispersing medium and the fine water droplets or dust are the dispersed phase — a colloid, scattering the light sideways into your eye. Without the particles you would see the lamp but not the beam.
A suspension scatters light too. So the Tyndall effect on its own does not prove a mixture is a colloid; it proves the particles are big enough to scatter. But a suspension is already obviously heterogeneous and settles on standing, so in practice the test is used where it matters — on a mixture that looks like a solution and does not settle.
The practical sequence for identifying an unknown mixture:
- Does it settle on standing, or can it be filtered? Then it is a suspension.
- If not, does it show the Tyndall effect? Then it is a colloid.
- If it shows neither, it is a true solution.
Three questions in order, and they classify any mixture in the chapter — which is why the Tyndall effect earns a section of its own rather than being one line in a table.
Exam tip
Exam tip: quote the property the technique depends on
Name the technique and the property it uses. Separating funnel, because the liquids are immiscible and differ in density earns both marks.
A separating funnel needs immiscible liquids and works on density — the denser layer runs off first. It is useless for miscible liquids, which need distillation.
Sublimation needs exactly one subliming component. Learn the list: camphor, ammonium chloride, naphthalene, iodine.
In a sublimation diagram, label the inverted funnel and the cotton plug, and heat gently.
Learn the particle sizes: solution under nm, colloid to nm, suspension over nm.
Only a suspension settles and only a suspension can be filtered. A colloid does neither and is still heterogeneous.
Only a colloid and a suspension scatter light; a true solution does not.
Name the two parts of a colloid — dispersed phase and dispersing medium — and learn one example per colloid type.
For an unknown mixture, work through settle or filter, then Tyndall, then solution in that order.
And remember sublimation is a physical change — the substance collected is chemically unchanged.
A separating funnel needs immiscible liquids and works on density — the denser layer runs off first. It is useless for miscible liquids, which need distillation.
Sublimation needs exactly one subliming component. Learn the list: camphor, ammonium chloride, naphthalene, iodine.
In a sublimation diagram, label the inverted funnel and the cotton plug, and heat gently.
Learn the particle sizes: solution under nm, colloid to nm, suspension over nm.
Only a suspension settles and only a suspension can be filtered. A colloid does neither and is still heterogeneous.
Only a colloid and a suspension scatter light; a true solution does not.
Name the two parts of a colloid — dispersed phase and dispersing medium — and learn one example per colloid type.
For an unknown mixture, work through settle or filter, then Tyndall, then solution in that order.
And remember sublimation is a physical change — the substance collected is chemically unchanged.
Did you know
Why the sky is blue and a sunset is red
The same scattering that makes a torch beam visible in milk decides the colour of the sky.
Sunlight is a mixture of all the colours, and they have different wavelengths — blue light has a shorter wavelength than red. When sunlight passes through the atmosphere, the very fine particles and molecules in the air scatter the shorter wavelengths far more strongly than the longer ones.
So blue light is thrown sideways in every direction, and when you look up at a part of the sky away from the Sun, the light reaching your eye is light that was scattered towards you. That scattered light is predominantly blue, and the sky looks blue.
At sunrise and sunset the Sun is low, so its light travels a much longer path through the atmosphere before reaching you. Along that long path almost all the blue has been scattered away sideways, and what continues straight through to your eye is what remains — the reds and oranges. The Sun itself therefore looks red, and so do the clouds it lights.
The same reasoning explains a detail worth noticing. On a clear day at high altitude, where there is less air above you, the sky looks a deeper blue — less scattering has whitened it. And when the air is heavy with dust or moisture, the larger particles scatter all colours more evenly, and the sky fades towards white.
So the Tyndall effect is not a laboratory curiosity demonstrated with a torch and a glass of milk. It is the reason the sky has a colour at all — and the reason that colour changes twice a day.
Sunlight is a mixture of all the colours, and they have different wavelengths — blue light has a shorter wavelength than red. When sunlight passes through the atmosphere, the very fine particles and molecules in the air scatter the shorter wavelengths far more strongly than the longer ones.
So blue light is thrown sideways in every direction, and when you look up at a part of the sky away from the Sun, the light reaching your eye is light that was scattered towards you. That scattered light is predominantly blue, and the sky looks blue.
At sunrise and sunset the Sun is low, so its light travels a much longer path through the atmosphere before reaching you. Along that long path almost all the blue has been scattered away sideways, and what continues straight through to your eye is what remains — the reds and oranges. The Sun itself therefore looks red, and so do the clouds it lights.
The same reasoning explains a detail worth noticing. On a clear day at high altitude, where there is less air above you, the sky looks a deeper blue — less scattering has whitened it. And when the air is heavy with dust or moisture, the larger particles scatter all colours more evenly, and the sky fades towards white.
So the Tyndall effect is not a laboratory curiosity demonstrated with a torch and a glass of milk. It is the reason the sky has a colour at all — and the reason that colour changes twice a day.
Exam relevance
How do colloids and separation techniques feed into NEET and JEE Chemistry?
This page is the foundation for two Class 12 Chemistry topics that both JEE Main and NEET examine, and the classification learned here is used without further explanation in each.
Surface Chemistry in Class 12 takes the colloid section of this page and develops it in full. The dispersed phase and dispersing medium keep exactly the names used here, the colloid types are the same six, and the Tyndall effect is treated with the wavelength reasoning given in the previous section. That chapter then adds Brownian motion, electrophoresis, coagulation and the distinction between lyophilic and lyophobic colloids — all of which assume you already know what makes a colloid different from a solution and a suspension.
Separation techniques feed into Class 11 Organic Chemistry: Some Basic Principles and Techniques, where purification methods are treated formally. The separating funnel becomes solvent extraction there, and sublimation is listed among the standard purification methods for solids.
Where else it reappears. The scattering explanation connects to Class 12 Physics on the scattering of light, and the colloid concept returns in Class 12 Biology for NEET when the composition of protoplasm and of blood plasma is described.
What the questions look like. Match-the-column items pairing a colloid type with an example are the most common form, and the pairs that catch students out are foam (shaving cream) and gel (cheese and butter), because both are food items and easily swapped. Assertion-reason questions favour two statements from this page: that a colloid cannot be filtered although it is heterogeneous, and that a true solution does not show the Tyndall effect. Statement-count questions of the how many of the following are colloids kind suit this topic because there are so many familiar examples to classify.
How board and competitive emphasis differ. A board paper asks you to give three differences between a colloid and a suspension, or to describe the sublimation set-up with a labelled diagram. A competitive paper gives you a named mixture — smoke, butter, milk of magnesia — and asks which type of colloid it is, so the examples matter more than the definitions. Board papers also reward the diagram; competitive papers never ask for it.
The single trap that costs the most marks. Classifying a colloid as homogeneous because it does not settle and cannot be filtered. It is heterogeneous, and the reason is that its particles are genuinely separate — merely too small to settle or to be trapped. Fix it by the size: anything above nm is heterogeneous, however uniform it looks.
A second trap worth naming. Using a separating funnel for miscible liquids. Establish whether the liquids form layers before choosing any apparatus, because that one word settles the technique and nothing about the liquids' other properties is relevant until it has been decided.
Surface Chemistry in Class 12 takes the colloid section of this page and develops it in full. The dispersed phase and dispersing medium keep exactly the names used here, the colloid types are the same six, and the Tyndall effect is treated with the wavelength reasoning given in the previous section. That chapter then adds Brownian motion, electrophoresis, coagulation and the distinction between lyophilic and lyophobic colloids — all of which assume you already know what makes a colloid different from a solution and a suspension.
Separation techniques feed into Class 11 Organic Chemistry: Some Basic Principles and Techniques, where purification methods are treated formally. The separating funnel becomes solvent extraction there, and sublimation is listed among the standard purification methods for solids.
Where else it reappears. The scattering explanation connects to Class 12 Physics on the scattering of light, and the colloid concept returns in Class 12 Biology for NEET when the composition of protoplasm and of blood plasma is described.
What the questions look like. Match-the-column items pairing a colloid type with an example are the most common form, and the pairs that catch students out are foam (shaving cream) and gel (cheese and butter), because both are food items and easily swapped. Assertion-reason questions favour two statements from this page: that a colloid cannot be filtered although it is heterogeneous, and that a true solution does not show the Tyndall effect. Statement-count questions of the how many of the following are colloids kind suit this topic because there are so many familiar examples to classify.
How board and competitive emphasis differ. A board paper asks you to give three differences between a colloid and a suspension, or to describe the sublimation set-up with a labelled diagram. A competitive paper gives you a named mixture — smoke, butter, milk of magnesia — and asks which type of colloid it is, so the examples matter more than the definitions. Board papers also reward the diagram; competitive papers never ask for it.
The single trap that costs the most marks. Classifying a colloid as homogeneous because it does not settle and cannot be filtered. It is heterogeneous, and the reason is that its particles are genuinely separate — merely too small to settle or to be trapped. Fix it by the size: anything above nm is heterogeneous, however uniform it looks.
A second trap worth naming. Using a separating funnel for miscible liquids. Establish whether the liquids form layers before choosing any apparatus, because that one word settles the technique and nothing about the liquids' other properties is relevant until it has been decided.
Key takeaways
Funnels, sublimation, colloids and the Tyndall effect: quick revision
- A separating funnel divides immiscible liquids using their density difference — the denser layer is run off first through the stopcock.
- Used for oil and water, kerosene and water, and removing water from petrol. Useless for miscible liquids, which need distillation.
- Sublimation is a solid changing directly to vapour and back again. Camphor, ammonium chloride, naphthalene and iodine sublime.
- Method: china dish, inverted funnel with a cotton plug, gentle heating; the subliming solid deposits on the cool funnel and the other solid stays in the dish.
- It is a physical change, and only one component may sublime for the separation to work.
- Particle sizes: true solution under nm, **colloid to nm**, suspension over nm.
- True solution: homogeneous, invisible particles, no scattering, stable, not filterable. Salt or sugar in water.
- Suspension: heterogeneous, visible particles, scatters light, settles, filterable. Chalk in water, muddy water.
- Colloid: looks homogeneous but is heterogeneous, particles invisible, scatters light, does not settle, not filterable — needs centrifugation. Milk, ink, smoke, fog, butter, blood.
- A colloid has a dispersed phase and a dispersing medium.
- Types: aerosol (fog, smoke), foam (shaving cream, sponge), emulsion (milk), sol (ink), gel (cheese, butter), solid sol (coloured glass).
- The Tyndall effect is the scattering of a beam by colloidal particles, making its path visible. Salt water shows none; diluted milk shows it clearly.
- Everyday cases: sunlight through a forest canopy, a projector beam in a smoky hall, headlights in fog, a shaft of light in a dusty room.
- Identify an unknown mixture in three steps: does it settle or filter (suspension), does it show the Tyndall effect (colloid), neither (true solution).
- A suspension scatters light too, so the test matters most for a mixture that looks like a solution.
- Shorter wavelengths scatter more, which is why the sky is blue and a low Sun looks red.
Fill three glasses with salt water, diluted milk and chalky water, then classify each using only a torch and ten minutes of standing time — the three answers arrive without any apparatus at all.
- Used for oil and water, kerosene and water, and removing water from petrol. Useless for miscible liquids, which need distillation.
- Sublimation is a solid changing directly to vapour and back again. Camphor, ammonium chloride, naphthalene and iodine sublime.
- Method: china dish, inverted funnel with a cotton plug, gentle heating; the subliming solid deposits on the cool funnel and the other solid stays in the dish.
- It is a physical change, and only one component may sublime for the separation to work.
- Particle sizes: true solution under nm, **colloid to nm**, suspension over nm.
- True solution: homogeneous, invisible particles, no scattering, stable, not filterable. Salt or sugar in water.
- Suspension: heterogeneous, visible particles, scatters light, settles, filterable. Chalk in water, muddy water.
- Colloid: looks homogeneous but is heterogeneous, particles invisible, scatters light, does not settle, not filterable — needs centrifugation. Milk, ink, smoke, fog, butter, blood.
- A colloid has a dispersed phase and a dispersing medium.
- Types: aerosol (fog, smoke), foam (shaving cream, sponge), emulsion (milk), sol (ink), gel (cheese, butter), solid sol (coloured glass).
- The Tyndall effect is the scattering of a beam by colloidal particles, making its path visible. Salt water shows none; diluted milk shows it clearly.
- Everyday cases: sunlight through a forest canopy, a projector beam in a smoky hall, headlights in fog, a shaft of light in a dusty room.
- Identify an unknown mixture in three steps: does it settle or filter (suspension), does it show the Tyndall effect (colloid), neither (true solution).
- A suspension scatters light too, so the test matters most for a mixture that looks like a solution.
- Shorter wavelengths scatter more, which is why the sky is blue and a low Sun looks red.
Fill three glasses with salt water, diluted milk and chalky water, then classify each using only a torch and ten minutes of standing time — the three answers arrive without any apparatus at all.