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💬 Think about it
- A salt solution and a glass of dilute milk both look like clear, uniform liquids.
- Is there any simple test that can tell them apart?
- There is — and all it needs is a beam of light. This is where we finally do Activity 5.1.
🔦
Activity 5.1 — Let us experiment (group activity)
In this Activity, the class splits into three groups, each makes one mixture, and a laser beam is used to tell them apart.
Materials needed
Three colourless transparent glass tumblers or 100 mL beakers, common salt, chalk powder, milk, a spatula (a small laboratory spoon), a laser pointer and a filtration set-up.
Procedure
1.
Group A
— add one spatula of common salt to 50 mL of water and stir well. Label it A.
2. Group B — add one spatula of chalk powder to 50 mL of water and stir well. Label it B.
3. Group C — add a few drops of milk to 50 mL of water and stir well. Label it C.
4. Are the particles visible in each mixture? Record your observations.
5. Direct the light of a laser pointer through each beaker and observe from the side, at right angles to the beam. Record what you see.
6. Predict what you would observe if each beaker were left undisturbed for a few minutes. To predict means to guess in advance.
7. Filter each mixture separately. Is there any residue left on the filter paper? A residue is whatever is held back by the paper.
8. From your observations, are these the same kind of mixture or different kinds?
2. Group B — add one spatula of chalk powder to 50 mL of water and stir well. Label it B.
3. Group C — add a few drops of milk to 50 mL of water and stir well. Label it C.
4. Are the particles visible in each mixture? Record your observations.
5. Direct the light of a laser pointer through each beaker and observe from the side, at right angles to the beam. Record what you see.
6. Predict what you would observe if each beaker were left undisturbed for a few minutes. To predict means to guess in advance.
7. Filter each mixture separately. Is there any residue left on the filter paper? A residue is whatever is held back by the paper.
8. From your observations, are these the same kind of mixture or different kinds?
Safety first
Never look directly into the laser beam. It can cause eye damage that cannot be reversed.
Observation
When the laser passes through
A
, the true solution, the path of the light cannot be seen at all. When it passes through
B
, the suspension, or
C
, the colloid, the whole path of the beam shows up bright and clear.
Explanation
The particles in a suspension and in a colloid are large enough that when light strikes them it is thrown off in every direction — that is, it is
scattered
. This scattering is what makes the path of the beam visible.
The scattering of light by the particles of a mixture is called the Tyndall effect , named after its discoverer, John Tyndall .
◆ Summary
- A shows no beam path — it is a true solution
- B and C show a bright beam path
- Their larger particles scatter the light
What is the Tyndall effect?
Tyndall effect
Light meets large particles
particles bigger than about 1 nm
The light is scattered
thrown off in every direction
↓
The path of the beam becomes visible
and that is the Tyndall effect
Definition
The Tyndall effect is the scattering of a beam of light by the particles of a colloid or a suspension, which makes the path of the beam visible.
Where do we see the Tyndall effect in daily life?
- When sunlight enters a completely dark room through a small hole, the dust and smoke particles floating in the air make the whole path of the light shine brightly.
- The same beautiful scene appears at night when the big floodlights of a sports stadium are switched on — the dust and tiny droplets in the air scatter the light and make it look brilliant.
What is a colloid actually made of?
In a solution
In a colloid
Solute — the smaller amount, dissolved
Dispersed phase
— the particles scattered through
Solvent — the larger amount, which dissolves
Dispersion medium
— the medium they are scattered in
- Scattering happens when light passes through a colloid — but how is a colloid itself put together?
- Every colloid has two components — the dispersed phase and the dispersion medium .
- Put in the language of solutions: the substance present in the smaller amount, the solute, is the dispersed phase ; the larger medium in which the particles are scattered, the solvent, is the dispersion medium .
🧵 Threads of Curiosity — Emulsions
- In many everyday colloids, the dispersed phase and the dispersion medium are both liquids . Such colloids are called emulsions .
- Emulsions are of two kinds — oil-in-water , such as milk, and water-in-oil , such as butter or cold cream.
- Which one it is depends simply on which component is present in the greater amount.
- Emulsions such as milk or cream are usually not stable by themselves. A third substance called an emulsifying agent is added to hold the components together — in milk, the milk proteins act as a natural emulsifier.
Pause and Ponder
🤔 Pause and Ponder — Questions 9 and 10
-
9. Clouds are made up of tiny water droplets or ice crystals floating in the air. Based on what you know about solutions, suspensions and colloids, what type of mixture do you think clouds are, and why?
View answer
AnswerClouds are a colloid — the kind we call an aerosol .Explanation
- The tiny water droplets or ice crystals are the dispersed phase .
- The air they float in is the dispersion medium .
- Their size is larger than the particles of a true solution but smaller than those of a suspension.
- That is why they do not settle out under gravity the way a suspension would — they stay stable, floating right through the air.
Droplets in air, between 1 nm and 1000 nm — that is an aerosol, and an aerosol is a colloid. -
10. Why do cities with a lot of smoke and dust in the air often look hazy?
View answer
AnswerBecause of the Tyndall effect .Explanation
- The tiny smoke and dust particles suspended in the air lie right in the path of the sunlight.
- They scatter that beam of light in every direction.
- Because of this continuous scattering, the light spreads all through the air instead of travelling straight to our eyes.
- So instead of a clear view we see a dull, cloudy haze everywhere.
More suspended particles means more scattering, and more scattering means more haze.
📋
Activity 5.9 — Let us review (Table 5.3)
In this Activity, we fill in Table 5.3 to revise the difference between solutions, suspensions and colloids in one go.
| Property | Solution | Suspension | Colloid |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Particle size | Smallest, below 1 nm | Largest, above 1000 nm | Medium, 1–1000 nm |
| Visibility | Not visible | Clearly visible | Not visible |
| Separation by filtration | Not possible | Easily separated | Not possible |
| Settling | Never settles — stable | Settles on standing — unstable | Never settles — stable |
| Tyndall effect | Not shown | Shown | Shown |
One line to remember the whole table — a solution hides its particles completely, a suspension shows them and drops them, and a colloid hides them but still scatters light.
◆ Summary
- Only the solution is homogeneous
- Only the suspension settles and filters
- Only the solution fails the Tyndall test
✅ Test Yourself
-
A beam of light passes through a liquid and its path cannot be seen. What kind of mixture is it?
View Answer
A true solution. Its particles are below 1 nm and too small to scatter light. -
In milk, what is the dispersed phase and what is the dispersion medium?
View Answer
Tiny droplets of fat or oil are the dispersed phase, and water is the dispersion medium. Milk is an oil-in-water emulsion. -
Why is an emulsifying agent added to an emulsion?
View Answer
Emulsions are not usually stable on their own, so an emulsifying agent is added to hold the two liquids together. In milk this job is done by the milk proteins. -
After whom is the Tyndall effect named?
View Answer
John Tyndall, who discovered it.
📝 Summary - What we have learnt
- The Tyndall effect is the scattering of light by particles, which makes the path of a beam visible.
- A true solution shows no Tyndall effect ; colloids and suspensions do.
- Every colloid has a dispersed phase and a dispersion medium .
- A colloid of one liquid in another is an emulsion — milk is oil-in-water, butter is water-in-oil — and an emulsifying agent keeps it stable.
Recap — Section 5.5 — Tyndall Effect
✅ Test Yourself
-
When sunlight enters a dark room through a small hole, why does the beam of light show up so clearly?
View Answer
The dust and smoke particles floating in the air scatter the light — that is the Tyndall effect making the beam's path visible. -
Why do cities with a lot of smoke and dust in the air often look hazy?
View Answer
The suspended smoke and dust particles scatter sunlight in every direction, spreading it through the air instead of letting it travel straight — so instead of a clear view, we see haze. -
What are the two parts every colloid is made of?
View Answer
The dispersed phase — the particles scattered through, like the solute — and the dispersion medium — what they are scattered in, like the solvent.
Important Definitions
- Tyndall effect
- The scattering of a beam of light by the particles of a colloid or suspension.
- Dispersed phase
- The particles scattered through a colloid, corresponding to the solute.
- Dispersion medium
- The medium in which those particles are scattered, corresponding to the solvent.
- Emulsion
- A colloid in which both the dispersed phase and the dispersion medium are liquids.
- Aerosol
- A colloid of liquid droplets or solid particles dispersed in a gas, such as a cloud.
Questions at the End of the Chapter — Related to This Section
📋 NCERT Question 2 — Which mixtures show the Tyndall effect
Identify which of four given mixtures show the Tyndall effect.
View Answer →
📋 NCERT Question 6 — Assertion and Reason on the Tyndall effect
Judge an Assertion and Reason statement about why solutions do not show the Tyndall effect.
View Answer →
