(a) Write the dispersed phase and dispersion medium of dust. (b) Why is physisorption reversible whereas chemisorption is irreversible? (c) A colloidal sol is prepared by the method shown in the figure: $KI$ solution is added dropwise into $AgNO_3$ solution taken in a test tube. What is the charge on the AgI colloidal particles formed in the test tube? How is this sol represented?

(a) Write the dispersed phase and dispersion medium of dust. (b) Why is physisorption reversible whereas chemi

Question

Q. (a) Write the dispersed phase and dispersion medium of dust.

(b) Why is physisorption reversible whereas chemisorption is irreversible?

KI

solution is added dropwise into

AgNO_3

solution taken in a test tube. What is the charge on the AgI colloidal particles formed in the test tube? How is this sol represented?

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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. …

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Dust is a solid-in-gas colloid (solid dispersed in air). Physisorption is reversible because it involves weak van der Waals forces, while chemisorption is irreversible because it forms strong chemical bonds. When KI is added to AgNO₃, the AgI sol formed has positively charged particles (due to excess Ag⁺) and is represented as AgI/Ag⁺.

The Concept Behind Each Part

Colloids are everywhere — from the dust in a sunbeam to the fog on a winter morning. The key is to identify which phase is broken into tiny particles (the dispersed phase) and which phase surrounds them (the dispersion medium). For adsorption, the difference between physisorption and chemisorption boils down to the strength of the forces involved. And for the AgI sol, the charge on the particles depends entirely on which ion is in excess during preparation — a classic case of preferential adsorption.

Let’s take each part one by one.

(a) Dispersed phase and dispersion medium of dust

Dust is simply fine solid particles floating in air. The solid particles (soil, pollen, ash) are the dispersed phase, and air is the dispersion medium.

Identify the phases: Dust consists of tiny solid particles suspended in a gas (air).
The solid particles are the dispersed phase — they are the discontinuous, distributed component.
The surrounding air is the dispersion medium — it is the continuous phase in which the particles are spread.

Note

Dust is an example of a solid-in-gas colloid (aerosol). Fog, by contrast, is a liquid-in-gas colloid.

(b) Why physisorption is reversible but chemisorption is irreversible

Adsorption is the sticking of molecules (adsorbate) onto a surface (adsorbent). The difference lies in the nature of the forces holding them.

Physisorption (physical adsorption) involves weak van der Waals forces — the same forces that make gases condense into liquids. These forces are easily overcome by a small increase in temperature or a decrease in pressure.
- Because the bonds are weak, the adsorbate can leave the surface without breaking any chemical bonds. The process is reversible.
- Example: Adsorption of nitrogen gas on charcoal at low temperature — heating releases the gas unchanged.
Chemisorption (chemical adsorption) involves the formation of actual chemical bonds (covalent or ionic) between the adsorbate and the surface atoms.
- These bonds are much stronger (bond energies ~100–400 kJ/mol). To break them, you need high temperatures, and often the adsorbate molecule is altered or decomposed.
- The process is essentially irreversible under normal conditions.
- Example: Adsorption of oxygen on platinum — the oxygen atoms bond to the metal surface and cannot be removed simply by lowering pressure.

Watch out

A common mistake is to think that all adsorption is reversible. Remember: if a chemical bond forms, you’ve changed the surface — reversing it requires breaking that bond, which is not easy.

(c) Charge on AgI colloidal particles when KI is added to AgNO₃

The figure shows KI solution being added dropwise to AgNO₃ solution. This is a precipitation reaction:

$\text{AgNO}_3 + \text{KI} \rightarrow \text{AgI} \downarrow + \text{KNO}_3$ …

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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 17 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 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 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 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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CBSE 2020Set ANNUAL1 markMCQ
$Q. Liquid in Liquid colloidal system are called$
(a) aerosols
(b) foams
(c) gels
(d) emulsion
›Reveal solutionSolution

A liquid dispersed in another liquid gives an emulsion, e.g. milk (fat globules in water) — option (d).

Colloidal systems are classified by the physical states of the dispersed phase and dispersion medium. When the dispersed phase is a liquid and the dispersion medium is also a liquid (the two being essentially immiscible with each other), the system is called an emulsion. Common examples are milk (fat dispersed in water) and cold cream (oil-in-water or water-in-oil types). Aerosols are liquid/solid dispersed in g …

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CBSE 2019Set 56/1/11 mark
$Q. Why are medicines more effective in colloidal state?$
(OR)
$What is difference between an emulsion and a gel?$
›Reveal solutionSolution

(a) Colloidal medicines have huge surface area → faster absorption, more effective.

(b) Emulsion = liquid dispersed in liquid (flows); gel = liquid held in a solid network (semi-solid).

Part (a): Why medicines are more effective in the colloidal state

Colloidal particles are 1–1000 nm in size, giving an enormous surface-area-to-volume ratio. This large surface area is easily and rapidly absorbed by body tissues and fluids, so the drug is assimilated quickly and completely, raising its bioavailability and effectiveness (e.g. colloidal medicinal preparations such as argyrol and milk of magnesia).

Part (b): Emulsion vs gel

Feature Emulsion Gel
Dispersed phase liquid liquid
Dispersion medium liquid solid
State liquid, flows freely semi-solid, holds shape
Example milk, cod-liver oil jelly, cheese, curd
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CBSE 2019Set ANNUAL1 mark
$Q. Write dispersed phase and dispersion medium of milk, a colloidal solution.$
›Reveal solutionSolution

Milk is an emulsion - tiny droplets of fat are dispersed throughout a continuous aqueous medium.

A colloidal system is described by its dispersed phase (the substance present in small particles) and its dispersion medium (the substance in which those particles are spread). In milk, minute globules of fat (a liquid) are dispersed throughout the watery fluid (also a liquid) that makes up the bulk of milk. Since both phases are liquids, milk is classifie …

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Q.(a) Write the dispersed phase and dispersion medium of dust.

The Concept Behind Each Part

(a) Dispersed phase and dispersion medium of dust

(b) Why physisorption is reversible but chemisorption is irreversible

(c) Charge on AgI colloidal particles when KI is added to AgNO₃

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