NDA Chemistry · Matter and Its States
Colloids and Suspensions
Between true solutions (tiny dissolved particles) and suspensions (large settling particles) sit colloids — particles big enough to scatter light (the Tyndall effect) but small enough not to settle; soap in water is a colloid of micelles.
Why this matters
Five PYQs, ranging EASY to HARD — the hardest in the chapter. The bank tests the particle-size ladder (solution → colloid → suspension), the defining properties of a colloid (heterogeneous, Tyndall effect, does not settle, particles invisible to the naked eye), and soap chemistry (micelles, cleansing action, why soap is a carboxylate not an ammonium salt). Get the 'colloids are heterogeneous, not homogeneous' fact and the micelle picture, and even the HARD lyotropic-liquid-crystal question follows.
Concept 1 of 2: True solution, colloid and suspension
Three mixtures, sorted by particle size. A true solution has particles so small they dissolve and never settle. A suspension has particles so big they settle and can be seen. A colloid is in between — invisible to the eye, never settles on standing, but big enough to scatter a light beam (the Tyndall effect).
Definition
The particle-size ladder and what distinguishes a colloid:
- True solution — particle size < 1 nm; homogeneous; transparent; no Tyndall effect; particles never settle and pass through filter paper (e.g. salt water, copper sulphate solution).
- Colloid — particle size 1–1000 nm; heterogeneous (appears uniform but is not); shows the Tyndall effect (scatters light); particles do not settle on standing but can be separated by centrifugation; particles cannot be seen by the naked eye (e.g. milk, fog, soap solution).
- Suspension — particle size > 1000 nm; heterogeneous; particles are visible to the naked eye, settle on standing, and are stopped by filter paper (e.g. muddy water, chalk in water).
- A colloid is NOT homogeneous — that is the bank's favourite false statement. Only a true solution is homogeneous.
- Tyndall effect = scattering of a light beam by colloidal particles (you see the beam, as with a torch in fog or sunlight through trees). A true solution like copper sulphate shows no Tyndall effect; milk does.
| Property | True solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | 1–1000 nm | > 1000 nm |
| Appearance | Homogeneous | Heterogeneous | Heterogeneous |
| Tyndall effect | No | Yes | Yes (if not settled) |
| Settle on standing? | No | No (needs centrifuge) | Yes Colloid particles do NOT settle on their own — but centrifugation can separate them. |
| Visible to naked eye? | No | No | Yes Colloidal particles cannot be seen by the naked eye — only suspension particles can. |
| Example | Salt water, CuSO₄ solution | Milk, fog, soap solution | Muddy water, chalk in water |
Sorted by particle size: solution < colloid < suspension. Only the true solution is homogeneous.
Practice this conceptself-check · 5 quick reps
The same idea in a real exam question:
NDA · Sep 2023 · Q114Moderate
Example 1 · Matter and Its States · Colloids and Suspensions
Which among the following is NOT true with respect to colloidal solution?
A colloid is NOT homogeneous
The statement 'a colloidal solution is homogeneous in nature' is false. Colloids are heterogeneous — they only appear uniform. Only a true solution is homogeneous.
Colloid particles are invisible to the eye
Colloidal particles cannot be seen with the naked eye (they are 1–1000 nm). Only suspension particles (> 1000 nm) are visible. Don't accept 'particles of colloids and suspensions can be seen by the naked eye'.
Concept 2 of 2: Soaps, micelles and cleansing action
A soap molecule has a water-loving head and an oil-loving tail. In water the tails huddle inward around a drop of dirt and the heads point out — a tiny ball called a micelle. The dirt is trapped inside and washes away. That is the whole cleansing action.
Definition
Soap chemistry the bank tests:
- A soap is the sodium or potassium salt of a long-chain carboxylic acid (a carboxylate) — NOT an ammonium salt.
- In water, soap molecules cluster into micelles: hydrophobic (oil-loving) tails point inward toward the dirt/oil, hydrophilic (water-loving) heads point outward toward the water.
- Cleansing action: oil and dirt are collected in the centre of the micelle; the micelle stays suspended and rinses away. Soap works by lowering the surface tension of water (and emulsifying oil), letting it wet and lift dirt.
- A soap micelle scatters light (it is colloidal), so soap solution shows the Tyndall effect.
- In hard water, soap forms an insoluble precipitate (scum) with the Ca²⁺ and Mg²⁺ ions — which is why soap lathers poorly in hard water.
- (HARD) Soap with water forms a lyotropic liquid crystal — an ordered micellar phase whose order depends on concentration (solvent), not temperature; the temperature-driven kind is thermotropic.
| Question asked | Answer |
|---|---|
| What kind of salt is a soap? | Sodium/potassium salt of a long-chain carboxylic acid A soap is a carboxylate (Na/K salt), NOT an ammonium salt — that is the bank's trap statement. |
| Where does dirt collect? | In the centre of the micelle |
| Principle of cleansing | Lowering surface tension (and emulsifying oil) |
| Does soap solution scatter light? | Yes — it is colloidal (Tyndall effect) |
| What forms in hard water? | Insoluble precipitate (scum) with Ca²⁺ and Mg²⁺ |
| Soap with water forms… | A lyotropic liquid crystal Lyotropic = order set by concentration/solvent. Thermotropic = order set by temperature. Soap micelles are lyotropic. |
Practice this conceptself-check · 5 quick reps
The same idea in a real exam question:
NDA · Apr 2021 · Q81Moderate
Example 2 · Matter and Its States · Colloids and Suspensions
Which one of the following statements about the cleansing action of soap is not true ?
Soap is a carboxylate, not an ammonium salt
Soaps are sodium or potassium salts of long-chain carboxylic acids. The statement that 'soaps are ammonium salts of long-chain carboxylic acids' is the NOT true one.
Lyotropic, not thermotropic
Soap in water forms a lyotropic liquid crystal — its order depends on concentration (the solvent). A thermotropic liquid crystal's order depends on temperature; that is the wrong choice for soap.
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)
True solution, colloid and suspension6 rows
| Property | True solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | 1–1000 nm | > 1000 nm |
| Appearance | Homogeneous | Heterogeneous | Heterogeneous |
| Tyndall effect | No | Yes | Yes (if not settled) |
| Settle on standing? | No | No (needs centrifuge) | Yes Colloid particles do NOT settle on their own — but centrifugation can separate them. |
| Visible to naked eye? | No | No | Yes Colloidal particles cannot be seen by the naked eye — only suspension particles can. |
| Example | Salt water, CuSO₄ solution | Milk, fog, soap solution | Muddy water, chalk in water |
Sorted by particle size: solution < colloid < suspension. Only the true solution is homogeneous.
Soaps, micelles and cleansing action6 rows
| Question asked | Answer |
|---|---|
| What kind of salt is a soap? | Sodium/potassium salt of a long-chain carboxylic acid A soap is a carboxylate (Na/K salt), NOT an ammonium salt — that is the bank's trap statement. |
| Where does dirt collect? | In the centre of the micelle |
| Principle of cleansing | Lowering surface tension (and emulsifying oil) |
| Does soap solution scatter light? | Yes — it is colloidal (Tyndall effect) |
| What forms in hard water? | Insoluble precipitate (scum) with Ca²⁺ and Mg²⁺ |
| Soap with water forms… | A lyotropic liquid crystal Lyotropic = order set by concentration/solvent. Thermotropic = order set by temperature. Soap micelles are lyotropic. |
Watch out for (4)
- A colloid is NOT homogeneous→ True solution, colloid and suspension
- Colloid particles are invisible to the eye→ True solution, colloid and suspension
- Soap is a carboxylate, not an ammonium salt→ Soaps, micelles and cleansing action
- Lyotropic, not thermotropic→ Soaps, micelles and cleansing action
Test yourself on Matter and Its States
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