Hardening of leather in tanning industry is based on (A) Electrophoresis (B) Electro-osmosis (C) Mutual coagul

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Q. Hardening of leather in tanning industry is based on (A) Electrophoresis (B) Electro-osmosis (C) Mutual coagulation (D) Tyndall effect

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Concept understanding — Coagulation of Colloids

Coagulation of Colloids

Imagine you have a glass of muddy water. The tiny clay particles don't settle — they stay suspended, making the water look cloudy. That's a colloid. Now, if you add a pinch of salt, something dramatic happens: the particles clump together into heavier aggregates and settle down. The water clears. That process is coagulation (or flocculation).

Why do colloidal particles stay suspended in the first place?

Colloidal particles are tiny (1–1000 nm). But size alone doesn't explain stability. Two forces are at play:

Electrostatic repulsion: All particles in a given colloid carry the same charge (all positive or all negative). Like charges repel, so they stay apart.
Brownian motion: Random thermal collisions keep them moving, preventing gravity from pulling them down.

The key is that every colloidal particle is surrounded by an electrical double layer — a layer of counter-ions that neutralises its surface charge partially. The net effect is a repulsive barrier that prevents particles from coming close enough to stick.

What breaks this stability?

Add an electrolyte (a salt that dissociates into ions). The ions from the electrolyte neutralise the charge on the colloidal particles. Once the charge is gone, the repulsive barrier collapses. Particles can now approach each other, van der Waals attractive forces take over, and they clump into larger aggregates that settle under gravity.

Important

Coagulation is the destabilisation of a colloid by neutralising the charge on its particles, causing them to aggregate and settle.

The Hardy-Schulze Rule

This rule tells you which electrolyte will be most effective. It has two parts:

The ion that causes coagulation is the one with charge opposite to that of the colloidal particle. If the colloid is negatively charged (e.g., arsenic sulphide sol), it is the cation of the added electrolyte that does the job. If the colloid is positively charged (e.g., ferric hydroxide sol), it is the anion.
The higher the charge (valency) of the coagulating ion, the greater its coagulating power. This is the central insight. A trivalent ion ( $\text{Al}^{3+}$ ) is far more effective than a divalent ion ( $\text{Ca}^{2+}$ ), which in turn is more effective than a monovalent ion ( $\text{Na}^+$ ).

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This is known as the Hardy-Schulze rule. The "to the sixth power" part is an empirical observation — it means a trivalent ion is roughly $3^6 = 729$ times more effective than a monovalent one.

A concrete example

Consider a negatively charged sol of arsenic sulphide ( $\text{As}_2\text{S}_3$ ). To coagulate it, you need cations. Compare three electrolytes:

Electrolyte	Cation	Valency	Relative coagulating power
NaCl	$\text{Na}^+$	1	Lowest
CaCl $_2$	$\text{Ca}^{2+}$	2	Intermediate
AlCl $_3$	$\text{Al}^{3+}$	3	Highest

A tiny amount of AlCl $_3$ will coagulate the sol, while you'd need a much larger amount of NaCl to achieve the same effect.

Tip

The coagulation value (or flocculation value) is the minimum concentration of an electrolyte required to cause coagulation. It is inversely related to coagulating power — lower coagulation value means a more effective electrolyte.

Why does valency matter so much? …

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The hardening of leather (tanning) is a mutual coagulation process where positively charged collagen fibres and negatively charged tannin particles neutralise each other, causing precipitation and cross-linking.

The Concept: Why Tanning Works

Tanning is the chemical process of converting raw animal hide (skin) into durable leather. Raw hide is mostly collagen — a protein that, when wet, is soft, putrefies easily, and has no structural strength. The problem is: how do you make it tough, water-resistant, and stable?

The answer lies in colloidal chemistry. Collagen fibres in water form a positively charged colloidal sol (due to the amino groups in the protein). Tannins (plant-derived polyphenols) form a negatively charged colloidal sol in water. When you bring these two oppositely charged colloids together, they neutralise each other's charges, lose their stability, and coagulate — the tannin particles precipitate onto and between the collagen fibres, forming strong cross-links. This is the essence of mutual coagulation.

Watch out

A common mistake is to think tanning is electrophoresis (movement under electric field) or electro-osmosis (movement of liquid through a membrane under field). Those are separation phenomena, not hardening phenomena. The Tyndall effect is just light scattering by colloids — irrelevant to the chemistry of leather.

Step-by-Step Reasoning

Identify the nature of the raw material.

Raw hide is a hydrophilic colloid (lyophilic) — it loves water. The collagen fibres are positively charged in aqueous suspension because the amino groups ( $-\text{NH}_2$ ) get protonated to $-\text{NH}_3^+$ at the pH of the tanning bath.
Identify the nature of the tanning agent.

Tannins are hydrophobic colloids (lyophobic) — they dislike water and are stabilised by a negative surface charge (from phenolic $-\text{OH}$ groups that lose $H^+$ in water, giving $-\text{O}^-$ ).
What happens when they mix?

The positive collagen sol and the negative tannin sol attract each other electrostatically. The charges cancel out, the protective hydration layers break down, and both colloids coagulate — they precipitate together. This is mutual coagulation (also called mutual precipitation or sensitisation). …

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

CBSE 2025Set D1 markMCQ
$Q. Which of the following electrolytes is the least effective in causing coagulation of colloidal solution of ferric hydroxide?$
(a) KBr
(b) K2SO4
(c) K2CrO4
(d) K3[Fe(CN)6]
›Reveal solutionSolution
Fe(OH)3 sol is positive; coagulating power of anions rises with charge (Hardy-Schulze), so the singly-charged Br- (KBr) is least effective.

Colloidal ferric hydroxide, Fe(OH)3, is a positively charged sol. It is coagulated by the negative ions (anions) of the added electrolyte.

By the Hardy-Schulze rule, the coagulating power of an ion increases sharply with the magnitude of its charge. Comparing the anions:

Br- : charge 1 …
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CBSE 2024Set ANNUAL1 mark
$Q. What is Hardy-Schulze rule?$
›Reveal solutionSolution

A higher-charged, oppositely-charged ion is a far more effective coagulant for a colloidal sol than a singly-charged one.

The Hardy-Schulze rule states that when an electrolyte is added to coagulate (precipitate) a colloidal sol, only the ion carrying a charge OPPOSITE to that of the colloidal particles is effective at causing coagulation, and its coagulating (precipitating) power increases sharply with the MAGNITUDE (valency) of its charge.

For example, a negatively-charged As2S3 sol is coagulated by cations, and the coagulating power follows the order:

Al³⁺ (trivalent) > Ba²⁺ (divalent) > Na⁺ (monovalent)

…

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CBSE 2023Set ANNUAL1 markMCQ
$Q. The process of setting of colloidal particles is called$
(a) electrophoresis
(b) peptization
(c) coagulation
(d) dialysis
›Reveal solutionSolution
Coagulation is the destabilisation and settling of colloidal particles, distinct from electrophoresis, peptization and dialysis.

Coagulation (flocculation): aggregation of colloidal particles into larger particles that settle down, often caused by adding an electrolyte that neutralises the particles' charge.

Electrophoresis: movement of charged colloidal particles under an electric field (does not cause settling by itself). …
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CBSE 2022Set HE2181 mark
$Q. Fill in the blank: Coagulation is opposite of ______.$
›Reveal solutionSolution
Coagulation destroys a sol by aggregating its particles into a precipitate; peptization does the opposite, redispersing a fresh precipitate into a sol.

Coagulation (flocculation): the process by which the dispersed phase particles of a colloidal sol aggregate into larger particles and settle out (precipitate) as a bulk phase, usually caused by adding an electrolyte whose ions neutralise the charge stabilising the colloidal particles (per the Hardy-Schulze rule, higher-charge counter-ions coagulate more effectively). …
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CBSE 2021Set ANNUAL1 mark
$Q. Bleeding stops by rubbing alum, why?$
›Reveal solutionSolution

Blood is a colloidal sol; alum's trivalent Al³⁺ ion coagulates it (Hardy–Schulze rule), forming a clot that stops bleeding.

Blood behaves like a colloidal solution (sol) in which the dispersed protein/corpuscle particles carry a net negative surface charge, which keeps them mutually repelled and dispersed (this is what keeps blood flowing rather than clotting spontaneously).

Alum, potassium aluminium sulphate $\text{K}_2\text{SO}_4\cdot\text{Al}_2(\text{SO}_4)_3\cdot24\text{H}_2\text{O}$ , ionises in water to release the Al³⁺ ion — a small, highly charged (trivalent) cation.

By the Hardy–Schulze rule, the coagulating power of an ion for a colloid of opposite charge increases sharply with its charge (roughly as the 6th power of the charge). So Al³⁺ is a far more effective coagulant for the negatively charged blood sol than a monovalent ion would be.

…

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CBSE 2021Set TERM11 mark
$Q. Clotting of blood is due to _______ process.$
›Reveal solutionSolution

Blood is a colloidal solution, and clotting is an example of coagulation of a colloid, aided by electrolytes present in the blood.

Blood is a colloidal system, and a wound exposes it to electrolytes present in the tissue fluid/at the wound site. These electrolytes neutralise the charge that normally keeps the colloidal blood particles (including platelets/proteins) dispersed and stable, causing them to aggregate and settl …

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CBSE 2020Set 56/1/11 markMCQ
$Q. Hardening of leather in tanning industry is based on (A) Electrophoresis (B) Electro-osmosis (C) Mutual coagulation (D) Tyndall effect$
›Reveal solutionSolution
The hardening of leather (tanning) is a mutual coagulation process where positively charged collagen fibres and negatively charged tannin particles neutralise each other, causing precipitation and cross-linking.

The Concept: Why Tanning Works

Tanning is the chemical process of converting raw animal hide (skin) into durable leather. Raw hide is mostly collagen — a protein that, when wet, is soft, putrefies easily, and has no structural strength. The problem is: how do you make it tough, water-resistant, and stable?

The answer lies in colloidal chemistry. Collagen fibres in water form a positively charged colloidal sol (due to the amino groups in the protein). Tannins (plant-derived polyphenols) form a negatively charged colloidal sol in water. When you bring these two oppositely charged colloids together, they neutralise each other's charges, lose their stability, and coagulate — the tannin particles precipitate onto and between the collagen fibres, forming strong cross-links. This is the essence of mutual coagulation.

Watch out

A common mistake is to think tanning is electrophoresis (movement under electric field) or electro-osmosis (movement of liquid through a membrane under field). Those are separation phenomena, not hardening phenomena. The Tyndall effect is just light scattering by colloids — irrelevant to the chemistry of leather.

Step-by-Step Reasoning

Identify the nature of the raw material.

Raw hide is a hydrophilic colloid (lyophilic) — it loves water. The collagen fibres are positively charged in aqueous suspension because the amino groups ( $-\text{NH}_2$ ) get protonated to $-\text{NH}_3^+$ at the pH of the tanning bath.

Identify the nature of the tanning agent.

Tannins are hydrophobic colloids (lyophobic) — they dislike water and are stabilised by a negative surface charge (from phenolic $-\text{OH}$ groups that lose $H^+$ in water, giving $-\text{O}^-$ ).

What happens when they mix?

The positive collagen sol and the negative tannin sol attract each other electrostatically. The charges cancel out, the protective hydration layers break down, and both colloids coagulate — they precipitate together. This is mutual coagulation (also called mutual precipitation or sensitisation). …
Start your 14-day free trial to see the full solution →
CBSE 2020Set HE8221 mark
$Q. Fill in the blank: Coagulation is the just reverse of ______.$
›Reveal solutionSolution

Coagulation (destabilising and precipitating a sol) and peptization (redispersing a fresh precipitate into a sol using an electrolyte) are exact opposites of each other.

Coagulation (or flocculation) is the process by which the dispersed particles of a colloidal sol are made to aggregate and settle out as a precipitate — usually by adding an electrolyte whose ions neutralise the charge stabilising the colloidal particles.

…

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CBSE 2020Set ANNUAL1 mark
$Q. What is Coagulation?$
›Reveal solutionSolution

Coagulation is the aggregation and settling of colloidal particles caused by neutralising the electric charge that normally keeps them apart.

Colloidal particles in a sol all carry the same kind of charge (either all positive or all negative, acquired by selective adsorption of ions from the dispersion medium). This like charge causes mutual repulsion, which — along with Brownian movement — keeps the particles permanently suspended and prevents them from settling.

When an electrolyte is added to the sol, the oppositely charged ions of the electrolyte are attracted to and neutralise the charge on the colloidal particles. Once uncharged, the particles are no longer repelled from one another; they collide, aggregate into larger particles, and eventually settle out of the dispersion medium as a precipitate (or flocculate).

This phenomenon — precipitation of a colloidal sol by the addition of a suitable electrolyte — is called coagulation or flocculation.

…

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Q.Hardening of leather in tanning industry is based on (A) Electrophoresis (B) Electro-osmosis (C) Mutual coagulation (D) Tyndall effect

The Concept: Why Tanning Works

Step-by-Step Reasoning

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