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
Intuition
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
Try it yourself
Consider: I. Colloids and suspensions are heterogeneous. II. Colloids can be separated by centrifugation. III. Particles of colloids and suspensions can be seen by the naked eye. IV. Copper sulphate solution shows no Tyndall effect but milk does. Which are correct?
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Is a colloid homogeneous or heterogeneous?
- 2.What effect lets you tell a colloid from a true solution?
- 3.Can colloidal particles be seen by the naked eye?
- 4.Do colloidal particles settle on standing?
- 5.Does copper sulphate solution show the Tyndall effect?
From the bank · past-year question
Which among the following is NOT true with respect to colloidal solution?
[Q114 · Sep · 2023]
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
Intuition
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
Try it yourself
Which statement about the cleansing action of soap is NOT true? (a) Oil and dirt collect in the centre of the micelle (b) Soap micelles scatter light (c) Soaps are ammonium salts of long-chain carboxylic acids (d) Soap forms an insoluble precipitate with Ca²⁺/Mg²⁺ in hard water.
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Soap cleans surfaces based on which principle?
- 2.A soap is the salt of which acid?
- 3.Where is oil and dirt collected during washing?
- 4.What does soap form with Ca²⁺/Mg²⁺ ions in hard water?
- 5.Soap with water forms which type of liquid crystal?
From the bank · past-year question
Which one of the following statements about the cleansing action of soap is not true ?
[Q81 · Apr · 2021]
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
Mastery check — 3 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
Consider the following statements:
I. A colloid and a suspension are heterogeneous mixtures.
II. Colloids can be separated from the mixture by the process of centrifugation.
III. Particles of colloids and suspensions can be seen by naked eye.
IV. The aqueous solution of copper sulphate does not show Tyndall effect; however, the mixture of milk and water shows Tyndall effect.
Which of the statements given above is/are correct?
[Q78 · Apr · 2026]
Soaps clean surfaces on the principle based on
[Q74 · Sep · 2019]
Soap with water forms :
[Q105 · Apr · 2024]
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