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๐ฌ Think about it
Remember the laser in Activity 5.1? In salt water its path was invisible, but in chalk water and milk you could see the beam. The same thing makes sunbeams visible through trees. Why does light show up like this? Let us find out.
What is the Tyndall effect?
What Makes the Beam Visible
Light enters the mixture
a thin laser beam through the beaker
↓
The particles are big enough
a colloid or a suspension — but never a solution
↓
The light is scattered sideways
some of it reaches your eye from the side
↓
The path of the beam becomes visible — the
Tyndall effect
- It is the scattering of light by particles in a mixture.
- It makes the path of a light beam visible.
- It happens in colloids and suspensions, but not in true solutions.
- It is named after the scientist John Tyndall.
Example:
In Activity 5.1 the laser path was invisible in salt solution but visible in chalk water and milk.
Where can we see the Tyndall effect around us?
Where We See the Tyndall Effect
A dark room
dust and smoke scatter a beam from a small hole
↓
Stadium floodlights
the beams show up in the misty air
↓
A hazy city
smoke and dust scatter the sunlight
↓
Scattering needs a colloid or a suspension — never a clear solution
- When sunlight enters a dark room through a small hole, scattered by dust.
- In bright sunbeams passing through gaps in tree leaves.
- In the floodlights of a sports stadium.
What are the dispersed phase and dispersion medium?
What are the two parts of a colloid?
Colloid
Two parts spread together
1. Dispersed phase
The scattered particles
Example:
Blood cells
Fat droplets in milk
2. Dispersion medium
Carries the particles
Example:
Plasma in blood
Water in milk
Dispersed phase
Dispersion medium
The solute-like component — the dispersed particles
The component in which the dispersed phase is suspended
The part that is spread
through
the mixture
The part that does the
carrying
In blood: the blood cells and platelets
In blood: the plasma
In milk: the tiny fat droplets
In milk: the water
- A colloid has two parts: the dispersed phase and the dispersion medium.
- The dispersed phase is the solute-like scattered particles.
- The dispersion medium is the part they are spread through.
Example:
In milk, tiny fat droplets are the dispersed phase and water is the dispersion medium.
What is an emulsion?
Emulsion
Liquid droplets
the dispersed phase
in a liquid
the dispersion medium
↓
Emulsion
= a colloid where
both
parts are liquids. Milk, butter. Emulsifying agents keep it stable.
What are the two types of emulsion?
Emulsion
Both parts are liquids
1. Oil in water
Oil droplets in water
Example:
Milk
Vanishing cream
2. Water in oil
Water droplets in oil
Example:
Butter
Cold cream
Body lotion
Oil-in-water emulsion
Water-in-oil emulsion
Oil droplets dispersed in water
Water droplets dispersed in oil
Milk, vanishing cream
Butter, body lotion, cold cream
Some medicines are made this way, to disperse them in water and make them palatable
This also cuts the greasy feel of a liquid medicine
- It is a colloid where both the dispersed phase and medium are liquids.
- Oil-in-water emulsions include milk and vanishing cream.
- Water-in-oil emulsions include butter and cold cream.
- Emulsifying agents (like milk proteins) keep emulsions stable.
In this Activity, we will complete a table comparing solutions, suspensions and colloids to review what we have learnt.
Table 5.1: Properties of different types of mixtures
| S. No. | Property | Solution | Suspension | Colloid |
|---|---|---|---|---|
| 1. | Nature (homogeneous/heterogeneous) | Homogeneous | Heterogeneous | Heterogeneous |
| 2. | Particle size | Less than 1 nm | More than 1000 nm | 1–1000 nm |
| 3. | Visibility | Not visible | Visible to naked eye | Not visible individually |
| 4. | Separation by filtration | No | Yes | No |
| 5. | Settling | Does not settle | Settles | Does not settle |
| 6. | Tyndall effect | No | Yes | Yes |
โ Summary
- Compare the three mixtures.
- Note size and visibility.
- Check settling and filtration.
- Check the Tyndall effect.
Important Points
- The Tyndall effect is the scattering of light by particles.
- Colloids and suspensions show it; true solutions do not.
- A colloid has a dispersed phase spread through a dispersion medium.
๐งฌ Pause and Ponder
-
Clouds are tiny water droplets or ice crystals floating in air. What type of mixture are clouds, and why?
Clouds are colloids (or aerosols) — tiny droplets dispersed in air that stay suspended and scatter light, just like other colloids. -
Why do cities with a lot of smoke and dust in the air often look hazy?
The smoke and dust particles scatter light by the Tyndall effect, making the air look hazy and cutting down clear visibility.
โ Test Yourself
-
What is the Tyndall effect?
View Answer
The scattering of light by particles, making a light beam's path visible. -
Which mixtures show the Tyndall effect?
View Answer
Colloids and suspensions, but not true solutions. -
What is an emulsion?
View Answer
A colloid in which both the dispersed phase and the medium are liquids.
Important Definitions
- Tyndall effect — the scattering of light by particles in a colloid or suspension.
- Dispersed phase — the scattered, solute-like particles in a colloid.
- Dispersion medium — the substance in which the dispersed phase is spread.
- Emulsion — a colloid in which both the dispersed phase and dispersion medium are liquids.
๐ NCERT Question 2 — Choose the correct options, and
Choose the correct options, and explain the reason for the correct and incorrect options.
Which among the following mixtures show the Tyndall Effect? A mixture of:
(a) air and dust particles
(b) copper sulfate and water
(c) starch and water
(d) acetone and water
(i) a and b (ii) b and d (iii) a and c (iv) c and d
View Answer →
Which among the following mixtures show the Tyndall Effect? A mixture of:
(a) air and dust particles
(b) copper sulfate and water
(c) starch and water
(d) acetone and water
(i) a and b (ii) b and d (iii) a and c (iv) c and d
๐ NCERT Question 6 — Assertion (A): Solutions do not
Assertion (A): Solutions do not exhibit the Tyndall effect.
Reason (R): The particles in solutions are larger than 100 nm, so they cannot scatter light.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
View Answer →
Reason (R): The particles in solutions are larger than 100 nm, so they cannot scatter light.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
๐ NCERT Question 9 — Compare evaporation, crystallization and distillation.
Compare evaporation, crystallization and distillation. In which situation, would you prefer each of these over the others?
View Answer →
