Define the following with a suitable example, of each : (a) Coagulation (b) Multimolecular colloid (c) Gel OR

Question

Q. Define the following with a suitable example, of each :

(a) Coagulation

(b) Multimolecular colloid

(OR)

(a) Out of starch and ferric hydroxide sol, which one can easily be coagulated and why ?

(b) What is observed when an emulsion is centrifuged ?

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Part (a)Concept understanding — Colloids Classification

Colloids Classification – From Intuition to Precision

Imagine you drop a handful of sand into a glass of water. The sand sinks to the bottom — that's a suspension. Now imagine you stir a drop of milk into water. The milk spreads evenly and never settles. That's a colloid. A colloid is a mixture where one substance is finely dispersed in another, but the particles are too small to settle out and too large to be a true solution.

The key question is: how do we make sense of the endless variety of colloids? The answer is classification — by what is dispersed and what it is dispersed in, and by the nature of the particles themselves.

Classification by Dispersed Phase and Dispersion Medium

Every colloid has two parts: the dispersed phase (the substance that is broken into fine particles) and the dispersion medium (the substance in which those particles are spread). Both can be solid, liquid, or gas. That gives nine possible combinations, but only eight are stable — gas-in-gas is a true solution, not a colloid.

\text{Colloid type} = \frac{\text{Dispersed phase}}{\text{Dispersion medium}}

Here is the complete table:

Dispersed Phase	Dispersion Medium	Common Name	Example
Solid	Solid	Solid sol	Coloured gemstones (e.g., ruby — gold in glass)
Solid	Liquid	Sol	Paint, ink, muddy water (fine clay)
Solid	Gas	Aerosol (solid)	Smoke, dust in air
Liquid	Solid	Gel	Jelly, cheese, butter
Liquid	Liquid	Emulsion	Milk, mayonnaise, cream
Liquid	Gas	Aerosol (liquid)	Fog, mist, cloud, hair spray
Gas	Solid	Solid foam	Pumice stone, foam rubber
Gas	Liquid	Foam	Whipped cream, froth on beer

Notice the pattern: the name often tells you the medium. "Sol" means a liquid medium, "gel" means a solid medium, "aerosol" means a gas medium, "emulsion" means both phases are liquid, and "foam" means gas dispersed in a liquid or solid.

Tip

To remember: Aerosol = particles in air (solid or liquid). Emulsion = liquid in liquid. Gel = liquid trapped in a solid network. Sol = solid particles in a liquid.

Classification by Nature of Particles

Colloids can also be grouped by how the dispersed particles interact with the dispersion medium. This is a deeper, chemical classification.

1. Lyophilic colloids (solvent-loving): The dispersed particles have a strong affinity for the dispersion medium. They form spontaneously — just shake gelatin in water and it disperses. These are reversible: if you evaporate the water, the gelatin can be redispersed. Examples: starch, gum, gelatin, proteins in water.

2. Lyophobic colloids (solvent-fearing): The particles have little or no attraction for the medium. They do not form spontaneously — you need special methods (like chemical reactions or mechanical grinding) to make them. They are irreversible: once dried, they cannot be redispersed. Examples: gold sol, silver sol, ferric hydroxide sol, sulphur sol. …

Part (b)Concept understanding — Lyophilic and Lyophobic Sols

Lyophilic and Lyophobic Sols: The First Meeting

Imagine you drop a pinch of sugar into water. It dissolves happily, forming a clear solution. Now imagine you drop a pinch of sand into water. It just sits there, or sinks — no matter how much you stir, it won't mix. That difference in "willingness" to interact with the liquid is the core idea behind lyophilic and lyophobic sols.

The words themselves tell the story: lyo = solvent, philic = loving, phobic = fearing. A lyophilic sol is one where the dispersed particles love the dispersion medium. A lyophobic sol is one where they fear it.

Lyophilic Sols: The Solvent-Loving Ones

These are colloids where the dispersed phase has a strong affinity for the dispersion medium. Think of gelatin in hot water, starch in water, or gum arabic. The particles are already "comfortable" with the solvent molecules — they form a stable, reversible system.

Key properties:

They form spontaneously — just mix the substance with the solvent, and you get a sol. No special technique needed.
They are reversible. If you evaporate the solvent, you get back the solid, and adding fresh solvent reforms the sol. The process can go back and forth.
They are stable — the particles are heavily solvated (surrounded by a thick layer of solvent molecules), which prevents them from sticking together. Coagulation is difficult; you need large amounts of an electrolyte to force them out.
The viscosity of a lyophilic sol is often higher than that of the pure solvent, because the swollen particles resist flow.

Tip

Think of lyophilic sols as "friendly" colloids. The particles and the solvent are like old friends — they mix easily, stay together, and can separate and reunite without hard feelings.

Lyophobic Sols: The Solvent-Fearing Ones

Here, the dispersed particles have little or no attraction for the dispersion medium. Examples: gold sol (tiny gold particles in water), silver iodide sol, ferric hydroxide sol. These particles would rather clump together than stay separated by the solvent.

Key properties:

They do not form spontaneously. You need special methods — chemical reactions (e.g., reduction of gold chloride), mechanical dispersion, or electrical dispersion — to force the particles into the colloidal range.
They are irreversible. Once coagulated (clumped together), you cannot get the sol back by simply adding more solvent. The process is one-way.
They are unstable — easily coagulated by small amounts of electrolytes, by heating, or even by shaking. The particles have no protective solvent layer; they rely only on mutual repulsion (usually from surface charges) to stay apart.
The viscosity of a lyophobic sol is nearly the same as that of the pure solvent, because the particles are not solvated.

Watch out

A common mistake: thinking "lyophobic" means the particles repel the solvent. They don't actively repel it — they just have no affinity for it. The solvent molecules would rather interact with each other than with the particles. That's why the particles tend to clump together: it's energetically cheaper for them to stick to each other than to stay surrounded by unfriendly solvent.

The Precise Statement

Property	Lyophilic Sol	Lyophobic Sol
Affinity for solvent	High (solvent-loving)	Low (solvent-fearing)
Formation	Spontaneous	Requires special methods
Reversibility	Reversible	Irreversible
Stability	High (hard to coagulate)	Low (easily coagulated)
Effect of electrolyte	Large amount needed	Small amount coagulates
Viscosity	Higher than solvent	Nearly same as solvent
Particle nature	Often macromolecules	Usually inorganic particles

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Coagulation = charge-neutralised precipitation of a colloid; multimolecular colloid = aggregate of small molecules (gold sol); gel = liquid dispersed in solid (silica gel).
Ferric hydroxide sol (lyophobic) coagulates more easily; centrifuging an emulsion separates the two liquids; promoters increase and poisons decrease catalyst activity.

Part (a)

Coagulation. Colloidal particles carry like charges that keep them dispersed. Coagulation (precipitation) is the aggregation and settling of these particles when their charge is neutralised — usually by adding an electrolyte, which supplies oppositely charged ions. Example: adding alum $\ce{Al2(SO4)3}$ (or an electrolyte) to muddy water neutralises the negatively charged clay particles, which then coagulate and settle, clarifying the water.
Multimolecular colloid. Here a large number of small atoms or molecules aggregate together, held by weak van der Waals forces, so the resulting particle falls in the colloidal size range (1–1000 nm). Example: a gold sol (clusters of many Au atoms) or a sulphur sol (aggregated $\ce{S8}$ molecules). …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 20 on this concept.

CBSE 2026Set A1 markMCQ
$Q. Smoke is a colloidal state in which$
(a) a gas is dispersed in solid
(b) a solid is dispersed in gas
(c) a gas is dispersed in gas
(d) a liquid is dispersed in gas
›Reveal solutionSolution

Smoke is an aerosol in which solid particles (carbon, ash) are the dispersed phase and a gas (air) is the dispersion medium.

…

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CBSE 2024Set D1 markMCQ
$Q. Which of the following is a lyophilic colloid?$
(a) Milk
(b) Gum
(c) Fog
(d) Blood
›Reveal solutionSolution

Gum is a lyophilic colloid; milk, fog and blood are lyophobic.

Lyophilic ('solvent-loving') colloids form readily and reversibly when the substance is mixed with the dispersion medium; examples are gum, starch, gelatin and proteins. Lyophobic ('solvent-hating') colloids (like metal sols) need special methods and are less stable.

…

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CBSE 2024Set ANNUAL1 markMCQ

Q. The colloidal system in which the dispersed phase is solid and the dispersion medium is gas is known as -

(a) foam

(b) gel

(d) emulsion

›Reveal solution

Colloids are classified by the physical states of their dispersed phase and dispersion medium; solid-in-gas is called an aerosol.

Colloidal systems are named by the combination of dispersed phase and dispersion medium:

Dispersed phase	Dispersion medium	Name	Example
Solid	Gas	Aerosol	Smoke
Liquid	Gas	Aerosol	Fog, mist
Solid	Liquid	Sol	Paints, mud
Liquid	Liquid	Emulsion	Milk
Gas	Liquid	Foam	Whipped cream
Solid	Solid	Solid sol	Coloured glass
Gas	Solid	Solid foam	Pumice stone

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CBSE 2023Set ANNUAL1 markMCQ
$Q. In the following the Lyophilic colloid is -$
(a) Milk
(b) Gum
(c) Blood
(d) None of these
›Reveal solutionSolution

Gum is a lyophilic (solvent-loving) colloid.

Lyophilic colloids are those in which the dispersed phase has a strong affinity (liking) for the dispersion medium. Common textbook examples are gum, gelatin, starch, proteins and enzymes dispersed in water — these disperse readily on simple mixing and are self-stabilising and reversible. …

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CBSE 2023Set ANNUAL1 mark
$Q. Proteins are ______ type of colloids.$
›Reveal solutionSolution

Proteins are macromolecular, lyophilic colloids: they have a strong affinity for the dispersion medium (water) and form colloidal solutions spontaneously and reversibly.

Lyophilic colloids ("solvent-loving") are those in which the dispersed phase has a strong attraction for (affinity towards) the dispersion medium. Proteins, being large molecules with polar and charged groups on their surface, interact strongly with water molecules, so they readily and reversibly form stable colloidal (sol) solutions simply by mixing with water - no special stabiliser is needed, and the sol re-forms even a …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following is aerosol?$
(a) Smoke
(b) Soap lather
(c) Milk
(d) Butter
›Reveal solutionSolution
An aerosol has a gas as the dispersion medium; among the options only smoke (solid-in-gas) fits.

Colloid type depends on the dispersed phase and dispersion medium:

Smoke: solid dispersed in gas -> aerosol (solid aerosol).

Soap lather: gas dispersed in liquid -> foam. …
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CBSE 2022Set ANNUAL1 markMCQ
$Q. Which of the following prepare colloidal solution?$
(a) NaCl
(b) Glucose
(c) Starch
(d) Barium Nitrate
›Reveal solutionSolution

Starch particles are large enough (1 nm - 1000 nm) to form a colloidal sol in water; NaCl, glucose and barium nitrate are small molecules/ions that form true solutions.

A colloid is a heterogeneous, two-phase system in which the size of the dispersed particles lies roughly between 1 nm and 1000 nm - too large to form a true solution but too small to settle out as a suspension. NaCl, glucose and barium nitrate all dissolve in water as individual ions or small molecules (diameter well under 1 nm), so they give true (molecular) solutions, not colloids. Starch, however, is a macromolecule (a polysaccharide of very high molar mass); when dispersed in hot water its huge molecules fall exactly in the colloidal size range, so it forms a …

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CBSE 2022Set E1 markMCQ
$Q. Which of the following is not a lyophilic colloid ?$
(a) Milk
(b) Gum
(c) Fog
(d) Blood
›Reveal solutionSolution
Fog (liquid droplets in gas, an aerosol) is not a lyophilic colloid; milk, gum and blood are lyophilic sols.

Lyophilic ("solvent-loving") colloids form readily and reversibly when the dispersed phase is mixed with the dispersion medium, and the particles are strongly solvated — e.g. gum, starch, gelatin, proteins.

Of the options:

Milk (fat/protein dispersed in water) — behaves as a lyophilic/emulsion-type colloid.

Gum (in water) — classic lyophilic colloid. …
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CBSE 2022Set ANNUAL1 markMCQ
$Q. Colloidal solution in air is called$
(a) hydrosols
(b) aerosol
(c) alcosols
(d) aquasols
›Reveal solutionSolution

Colloid names are based on the physical states of the dispersed phase and dispersion medium.

A colloid where the dispersion medium is a gas is termed an aerosol (e.g. smoke = solid particles in air, fog/mist = liquid droplets in air). Hydrosol/aquasol refer to a solid dispersed in water as the medium; alcosol refers t …

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CBSE 2021Set TERM11 mark
$Q. Smoke is a _______ type of Colloidal system.$
›Reveal solutionSolution

Smoke is a colloidal dispersion of tiny solid (carbon/soot) particles in air (a gas), which is classified as an aerosol.

Colloidal systems are classified by the physical states of the dispersed phase and dispersion medium. In smoke, tiny solid soot/carbon particles (dispersed phase, solid) are suspended in air (dispersion medium, gas). A colloidal system with solid dispersed i …

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CBSE 2020Set ANNUAL1 markMCQ
$Q. The dispersed phase and dispersion medium in smoke are respectively$
(a) Gas and liquid
(b) Liquid and gas
(c) Solid and gas
(d) Solid and liquid
›Reveal solutionSolution
Smoke is an aerosol: solid particles (soot, ash) dispersed in a gas (air).

Colloidal systems are classified by the physical state of the dispersed phase and the dispersion medium. Smoke arises from incomplete combustion, releasing tiny solid carbon/soot particles that remain suspended in air. So:

Dispersed phase = Solid (soot particles)

Dispersion medium = Gas (air)

This type of colloid (solid in gas) is called an aerosol (specifically a 'solid aerosol', as opposed to fog which is liquid-in-gas).

…
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CBSE 2020Set ANNUAL1 mark
$Q. Write dispersed phase and dispersion medium of dust, a colloidal solution.$
›Reveal solutionSolution
A colloid is classified by the physical state of its dispersed phase and its dispersion medium; dust suspended in air is a solid dispersed in a gas.

Dust is made of fine solid particles suspended in air. Matching this to the standard colloid classification:

Dispersed phase (the substance present in small quantity, in colloidal-sized particles): Solid.

Dispersion medium (the continuous phase the particles are spread through): Gas (air). …
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Q.Define the following with a suitable example, of each :

Part (a)

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