What is peptisation ? Give an example.

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Q. What is peptisation ? Give an example.

Tamil Nadu DgeTamil Nadu HSC (DGE) Board 2017Subjective· 3mImportance★★★★★

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Peptisation converts a bulk precipitate into a stable colloidal sol by using a small amount of electrolyte to impart a common surface charge to the particles, so they repel each other instead of aggregating.

Definition: Peptisation is the process by which a freshly precipitated (fresh, not aged) precipitate is redispersed into a stable colloidal sol on treatment with a suitable dispersion medium containing a small quantity of an electrolyte, known as the peptising agent.

Mechanism: The precipitate preferentially adsorbs one particular type of ion (common to the electrolyte and the precipitate) from the peptising electrolyte on its surface. This selective adsorption gives every particle of the precipitate the same electric charge. The resulting electrostatic repulsion between similarly charged particles overcomes the tendency of the particles to aggregate under van der Waals forces, so the bulk precipitate breaks up into colloidal-sized particles that remain suspended as a stable sol.

Example: Freshly precipitated ferric hydroxide, $Fe(OH)_3$ , when washed free of excess ions and then shaken with water containing a small amount of $FeCl_3$ , adsorbs $Fe^{3+}$ ions on its surface. The positively charged particles repel one another and disperse to form a stable reddish-brown positively charged $Fe(OH)_3$ sol.

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

CBSE 2023Set ANNUAL1 markMCQ
$Q. Which one is correctly matched ?$
(a) Foam - mist
(b) Emulsion - smoke
(c) Sol - whipped cream
(d) Gel - butter
›Reveal solutionSolution

Butter is a semi-solid colloidal system (a gel, liquid-in-solid type), the correctly matched pair among the options; the other three pairings mismatch the colloid type with its example.

Going through each option using the standard dispersed-phase/dispersion-medium classification: (a) 'Foam - mist' is wrong — foam is gas dispersed in liquid (e.g. whipped cream, soap lather) or gas in solid (e.g. rubber, pumice), while mist is actually an example of an aerosol (liquid dispersed in gas).

(b) 'Emulsion - smoke' is wrong — an emulsion is liquid dispersed in liquid (e.g. milk, cream), while smoke is an aerosol (solid dispersed in gas).

(c) 'Sol - whipped cream' is wrong — a sol is solid dispersed in liquid (e.g. paints, cell fluids), while whipped cream is a foam (gas dispersed in liquid). …

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CBSE 2022Set ANNUAL1 markMCQ
$Q. Fog is colloidal solution of :$
(a) liquid in gas
(b) solid in gas
(c) gas in liquid
(d) gas in gas
›Reveal solutionSolution

Fog is an aerosol formed by fine water-droplet dispersion (liquid) in air (gas), so it is classified as a liquid-in-gas colloidal system, the same class as mist and clouds.

Colloids are classified by the physical state of the dispersed phase and the dispersion medium. In fog, the dispersed phase is minute liquid water droplets and the dispersion medium is the surrounding air (a gas); this liquid-dispersed-in-gas combination is the aerosol type called a 'liquid aerosol,' of which fog, mist, and cloud are common examples. Smo …

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CBSE 2018Set ANNUAL1 markMCQ
$Q. Cheese is an example of :$
(a) aerosol
(b) foam
(c) solid sol
(d) gel
›Reveal solutionSolution

Colloid classification by dispersed-phase/dispersion-medium: cheese has liquid (whey/moisture, fat) dispersed throughout a solid protein (casein) network, giving it a semi-solid, jelly-like consistency — the classic hallmark of a gel.

An aerosol (a) is a liquid or solid dispersed in a gas (e.g. fog, smoke) — not applicable to cheese. A foam (b) is a gas dispersed in a liquid or solid (e.g. whipped cream, bread) — cheese is not gas-dispersed. A 'solid sol' (c) is a solid dispersed in a solid (e.g. coloured glass, some alloys) — cheese's dispersed phase is liquid, not solid. C …

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CBSE 2017Set ANNUAL1 markMCQ
$Q. An example for lyophilic colloid is :$
(a) colloidal solutions of metal
(b) sulphur in water
(c) gelatin
(d) $Fe(OH)_3$ colloid
›Reveal solutionSolution

Gelatin is a lyophilic colloid because it dissolves directly in water to form a stable, reversible colloidal sol due to strong particle-solvent affinity.

Lyophilic colloids ('solvent-loving') are formed when the dispersed phase has a strong affinity for the dispersion medium; such sols form spontaneously just by mixing the substance with the solvent, are generally reversible (the sol can be converted back to the solid by evaporation and reconstituted again by adding solvent), and are relatively stable, needing no special stabiliser. Common examples are gelatin, starch, gum, and proteins in water.

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Q.What is peptisation ? Give an example.

Concept understanding — Colloids — Classification and Preparation

Q.What is peptisation ? Give an example.

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