Write two differences between physisorption and chemisorption. OR Define the following terms with a suitable e

Question

Q. Write two differences between physisorption and chemisorption.

(OR)

Define the following terms with a suitable example of each:

(i) Associated colloids

(ii) O/W emulsion

CBSECBSE Class XII Board 2020Subjective· 2mImportance★★★★★est

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Part (a)Concept understanding — Physisorption and Chemisorption

Physisorption and Chemisorption: The Two Faces of Adsorption

Imagine a wall. If you throw a tennis ball at it, the ball might stick for a moment because of static cling or a bit of moisture — but a gentle tap knocks it off. That's one kind of sticking. Now imagine you throw a blob of superglue at the same wall. It bonds, changes the wall's surface, and you'd need to scrape it off. That's a different kind of sticking entirely.

Adsorption — the accumulation of a substance (the adsorbate) on the surface of another (the adsorbent) — happens in exactly these two ways. The first is physisorption (physical adsorption). The second is chemisorption (chemical adsorption).

Physisorption: The Gentle Touch

Physisorption is driven by van der Waals forces — the same weak, universal attractions that make gases deviate from ideal behaviour. These forces are present between any two molecules that are close enough, so physisorption is non-specific: any gas will physisorb on any solid surface, given the right conditions.

Because the forces are weak, the process is reversible. The adsorbate can be removed easily by lowering the pressure or raising the temperature — no chemical change occurs. The heat released (enthalpy of adsorption) is small, typically in the range of 20–40 kJ/mol.

Note

Physisorption is like condensation: it increases as temperature decreases and as pressure increases. At low temperatures, gas molecules have less kinetic energy and are more easily trapped by the surface.

Key characteristics at a glance:

Property	Physisorption
Force	van der Waals (weak)
Specificity	Non-specific — occurs on any surface
Reversibility	Reversible
Enthalpy change	Low (20–40 kJ/mol)
Activation energy	None — occurs instantly
Layers formed	Multilayers possible

Chemisorption: The Chemical Grip

Chemisorption involves the formation of a chemical bond (covalent or ionic) between the adsorbate and the adsorbent surface. This is a true chemical reaction — electrons are shared or transferred, and the adsorbate molecule often gets dissociated into atoms or fragments.

Because a chemical bond is formed, chemisorption is highly specific. Hydrogen will chemisorb on nickel but not on gold; oxygen will chemisorb on platinum but not on glass. The process is often irreversible — you cannot simply pump away the adsorbed gas; you may need to heat the surface to extreme temperatures or chemically react it off.

The enthalpy of chemisorption is large, typically 80–400 kJ/mol — comparable to the energy of a covalent bond.

Watch out

A common mistake: students think chemisorption is always slow. In fact, it can be fast if no activation energy is needed (e.g., oxygen on clean tungsten). But many chemisorption processes do have an activation barrier, making them slow at low temperatures — just like any chemical reaction.

Key characteristics at a glance:

Property	Chemisorption
Force	Chemical bond (covalent/ionic)
Specificity	Highly specific
Reversibility	Often irreversible
Enthalpy change	High (80–400 kJ/mol)
Activation energy	Often present — increases with temperature
Layers formed	Monolayer only (bonds saturate)

The Critical Difference: Activation Energy

This is the single most important distinction for exams.

Physisorption has zero activation energy — it happens the instant a molecule touches the surface. Chemisorption often requires an activation energy $E_a$ , just like any chemical reaction. This means:

At very low temperatures, chemisorption may be impossible because molecules lack the energy to overcome the barrier. …

Part (b)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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Concept: Types of adsorption and classification of colloidal systems.

Part (a)

Physisorption	Chemisorption
Weak van der Waals forces bind the gas	Strong chemical (covalent) bonds bind the gas

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Physisorption = weak van der Waals forces, low enthalpy, reversible; chemisorption = strong chemical bonds, high enthalpy, irreversible.
Associated colloids are micelle-forming surfactants above the CMC (soap); an O/W emulsion is oil dispersed in water (milk).

Part (a)

Adsorption is the accumulation of a substance on a surface. It differs fundamentally depending on the forces involved:

Property	Physisorption	Chemisorption
Nature of forces	Weak van der Waals forces	Strong chemical bonds (covalent/ionic)
Enthalpy of adsorption	Low, $20\text{–}40\ \text{kJ mol}^{-1}$	High, $80\text{–}240\ \text{kJ mol}^{-1}$
Temperature	Favoured at low $T$	Favoured at higher $T$ (needs activation energy)
Reversibility	Reversible	Usually irreversible
Specificity / layers	Non-specific, multilayer	Highly specific, monolayer

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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 21 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 2025Set D1 markMCQ
$Q. Which of the following adsorptions forms multimolecular layer?$
(a) Physical adsorption
(b) Chemical adsorption
(c) Both (A) and (B)
(d) None of these
›Reveal solutionSolution
Physical adsorption (physisorption) forms multimolecular layers; chemisorption forms only a monolayer.

Adsorption is classified into two types by the nature of the attractive force:

Physical adsorption (physisorption): the adsorbate is held by weak van der Waals forces. These forces are non-specific and long-range, so once the first layer of molecules is adsorbed, further layers can pile up on top. Hence physisorption gives a multimolecular (multilayer) film, especially at low temperature and high pressure. …
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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 A1 markMCQ
$Q. How many layers are involved in chemical adsorption?$
(a) One
(b) Two
(c) Many
(d) Zero
›Reveal solutionSolution

Chemical adsorption (chemisorption) is unilayer — only one layer forms.

In chemisorption the adsorbate is held to the surface by strong chemical (covalent) bonds. Because chemical bonding can only occur directly with the surface atoms, chemisorption stops after a single layer (monolayer) is formed — it cannot build up further layers.

…

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CBSE 2021Set A1 markMCQ
$Q. In physical adsorption the approximate heat energy evolved in kJ/mol is$
(a) 20-40
(b) 40-60
(c) 60-80
(d) 40-400
›Reveal solutionSolution
Physisorption is due to weak van der Waals forces, releasing a small heat of about 20-40 kJ/mol.

In physical adsorption (physisorption), the adsorbate molecules are held to the adsorbent surface by weak van der Waals forces. Because these forces are weak, only a small amount of energy is released.

Physical adsorption: enthalpy of adsorption ≈ 20-40 kJ/mol. …
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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.Write two differences between physisorption and chemisorption.

Part (a)

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