CDS Chemistry · Matter and Its States
Colloids and Suspensions
Solutions, colloids and suspensions by particle size, the kinds of colloid named by phase and medium, and the Tyndall effect.
Why this matters
Three CDS questions: what an emulsion is made of, why soap solution looks cloudy, and which colloid has a liquid medium. The last one, from 2024 (II), has two correct options as printed.
Concept 1 of 2: Kinds of colloid
Definition
Sizes first:
- True solution: particles below 1 nm; clear; never settle.
- Colloid: 1–100 nm (or 1–1000 nm); look uniform; do not settle; scatter light.
- Suspension: larger particles that settle.
Then the kinds, by dispersed phase in dispersion medium:
- Emulsion: liquid in liquid (milk, cream) — two liquids.
- Foam: gas in liquid (shaving cream, soap lather).
- Gel: liquid in solid (jelly, cheese).
- Aerosol: liquid in gas (mist, fog) or solid in gas (smoke).
- Sol: solid in liquid (paint, mud).
| Kind | Dispersed phase | Dispersion medium | Example |
|---|---|---|---|
| Emulsion | Liquid | Liquid | MilkQ |
| Foam | Gas | Liquid | Shaving cream, soap latherQ CDS 2024 (II) asked which has a liquid medium: foam and shaving cream both do, so the question has two right options. |
| Gel | Liquid | Solid | Jelly, cheese |
| Aerosol | Liquid or solid | Gas | Mist, fog, smoke |
| Sol | Solid | Liquid | Paint |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Matter and Its States · Colloids and Suspensions
Mist is liquid in gas, not gas in liquid
An emulsion is two liquids
Concept 2 of 2: The Tyndall effect
Definition
The effect:
- Tyndall effect: scattering of light by colloid particles, making the beam visible.
- Seen in soap solution (soap micelles), milk, fog, and sunlight through a dusty room or a forest canopy.
- A true solution (salt in water) shows no Tyndall effect.
| System | Tyndall effect? |
|---|---|
| Soap solution (micelles) | Yes — micelles scatter light, so it looks cloudyQ |
| Milk | Yes |
| Salt solution | No — a true solution |
Practice this concept2 quick reps
The same idea in a real exam question:
Example 2 · Matter and Its States · Colloids and Suspensions
Cloudiness is scattering, not refraction
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Kinds of colloid5 rows
| Kind | Dispersed phase | Dispersion medium | Example |
|---|---|---|---|
| Emulsion | Liquid | Liquid | MilkQ |
| Foam | Gas | Liquid | Shaving cream, soap latherQ CDS 2024 (II) asked which has a liquid medium: foam and shaving cream both do, so the question has two right options. |
| Gel | Liquid | Solid | Jelly, cheese |
| Aerosol | Liquid or solid | Gas | Mist, fog, smoke |
| Sol | Solid | Liquid | Paint |
The Tyndall effect3 rows
| System | Tyndall effect? |
|---|---|
| Soap solution (micelles) | Yes — micelles scatter light, so it looks cloudyQ |
| Milk | Yes |
| Salt solution | No — a true solution |
Watch out for (3)
- Mist is liquid in gas, not gas in liquid→ Kinds of colloid
- An emulsion is two liquids→ Kinds of colloid
- Cloudiness is scattering, not refraction→ The Tyndall effect
Test yourself on a real paper
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