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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?
- 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?
- 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?
- 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?
- 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
| Property | Solution | Suspension | Colloid |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Particle size | Less than 1 nm | More than 1000 nm | 1–1000 nm |
| Visibility | Not visible | Visible to naked eye | Not visible individually |
| Separation by filtration | No | Yes | No |
| Settling | Does not settle | Settles | Does not settle |
| 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
Which of the given mixtures show the Tyndall effect? Explain the correct and incorrect options.
View Answer →
๐ NCERT Question 6 — Assertion (A): Solutions do
Assertion-reason on whether solutions show the Tyndall effect and the size of their particles.
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๐ NCERT Question 9 — Compare evaporation, crystallization and
Compare evaporation, crystallization and distillation, and say when you would prefer each.
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