Colloidal

Colloidal

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

A colloid is a heterogeneous mixture where the dispersed phase (particle size 1–1000 nm) is suspended uniformly in a dispersion medium. The system looks homogeneous to the eye but is non-homogeneous at the microscopic level.

• Tyndall effect: scattering of light by colloidal particles; absent in true solutions because solute particles (<1 nm) are too small to scatter visible light.
• Hardy-Schulze rule: coagulating power of an ion is proportional to the sixth power of its charge (valency); the ion must carry charge opposite to that on the colloidal particle.
• Gold number: lower value = greater protective power of the lyophilic colloid against coagulation.

JEE Main tests: classification tables, Tyndall effect reasoning, Hardy-Schulze valency order, Bredig’s arc (metals only), and Cottrell precipitator applications. One MCQ (~3–4 marks) appears almost every shift.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification by Physical State

The 8 colloidal types arise from pairing the physical state of dispersed phase (solid/liquid/gas) with that of the dispersion medium.

Dispersed Phase	Dispersion Medium	Type	Example
Solid	Solid	Solid sol	Coloured gemstones, glass
Solid	Liquid	Sol	Starch in water, gold sol
Solid	Gas	Solid aerosol	Smoke, dust
Liquid	Solid	Gel	Cheese, jellies
Liquid	Liquid	Emulsion	Milk (O/W), butter (W/O)
Liquid	Gas	Liquid aerosol	Mist, fog, cloud
Gas	Solid	Solid foam	Pumice, foam rubber
Gas	Liquid	Foam	Whipped cream, soap lather

Lyophilic vs Lyophobic Sols

Lyophilic sols (e.g., gum, starch, gelatin, proteins) are reversible — the solvent can be evaporated and the colloid re-formed by adding solvent back. They are self-stabilized and viscous.

Lyophobic sols (e.g., metals, metal hydroxides, metal sulfides) are irreversible, require stabilizers, and are easily coagulated by traces of electrolyte.

Key Properties

• Tyndall effect: visible light scattering; the path of a light beam becomes visible when passed through a colloidal sol.
• Brownian movement: zig-zag random motion of particles caused by uneven molecular bombardment from the medium; prevents sedimentation.
• Electrophoresis: charged colloidal particles migrate towards the oppositely charged electrode; the direction of migration reveals the sign of charge on the particles.
• Coagulation/Flocculation: aggregation of colloidal particles into a precipitate; induced by electrolytes, boiling, or mixing two oppositely charged sols.

Trap: Tyndall effect is not shown by true solutions, even if they are coloured — particle size must be comparable to the wavelength of light (~10⁻⁷ m).

Hardy-Schulze Rule (Numerical Order)

For a negatively charged sol like As₂S₃, the coagulating power order is:

$$\text{Al}^{3+} > \text{Ba}^{2+} > \text{Na}^{+}$$

So AlCl₃ needs a far lower concentration than NaCl to coagulate the same sol. Coagulating power ∝ (charge)⁶; conversely, coagulation value (minimum mol L⁻¹ of electrolyte needed) is inversely proportional to coagulating power.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Preparation of Colloids

• Chemical methods: double decomposition (H₃AsO₃ + H₂S → As₂S₃ sol), reduction (AuCl₃ + tannic acid → gold sol), hydrolysis (FeCl₃ + H₂O → Fe(OH)₃ sol at boiling point).
• Bredig’s arc method: an electric arc is struck between two metal electrodes (Au, Ag, Pt, Cu) immersed in ice-cold water containing a trace of KOH. Vapourised metal condenses as a colloidal sol. Applicable only to metals.
• Peptization: conversion of a freshly prepared precipitate into a colloidal sol by adding a small amount of suitable electrolyte (e.g., Fe(OH)₃ + FeCl₃ → Fe(OH)₃ sol).
• Mechanical dispersion: colloidal mill shears coarse suspension into colloidal size.

Purification

• Dialysis: electrolyte ions diffuse out through a semipermeable membrane (parchment, cellophane) leaving purified colloid behind.
• Electrodialysis: an electric field speeds up ion removal.
• Ultrafiltration: filtration through a graduated filter paper (collodion membrane) impregnated with gelatin or collodion; the sol passes while larger particles are retained.

Emulsions and Micelles

Emulsions are classified as O/W (oil dispersed in water; e.g., milk, vanishing cream — stabilized by hydrophilic emulsifiers like proteins, gums) or W/O (water dispersed in oil; e.g., butter, cold cream — stabilized by hydrophobic emulsifiers like long-chain alcohols, heavy metal salts of fatty acids).

Soap action: soap molecules form micelles above the Kraft temperature, with hydrophobic tails inside the oil droplet and hydrophilic heads outside, allowing grease to be washed away.

Worked Comparison: Coagulating Power

For a positively charged Fe(OH)₃ sol, the relevant ion is the anion:

Electrolyte	Coagulating ion	Charge
Na₃PO₄	PO₄³⁻	3
Na₂SO₄	SO₄²⁻	2
NaCl	Cl⁻	1

Coagulating power follows the order PO₄³⁻ > SO₄²⁻ > Cl⁻, opposite to the cation order seen for a negative sol.

Common Mistakes

• Particle size boundary: 1 nm is the JEE discriminator between true solution and colloid; 1000 nm separates colloid from suspension.
• Hardy-Schulze sign: always pick the ion whose charge is opposite to the sol’s charge, not the same sign.
• Bredig’s arc: works only for metals under water/ice; cannot make sulfide or oxide sols.
• Gold number direction: smaller gold number means stronger protective action; do not invert this.

Applications

• Cottrell precipitator: removes smoke/dust from industrial gases using electrophoresis (charged dust particles attracted to oppositely charged electrodes).
• Water purification: alum coagulates colloidal clay/impurities, settling them out.
• Medicines: colloidal silver (argyrol) for antiseptic action; colloidal gold for arthritis treatment.
• Tanning: chrome salts form lyophilic colloids that bind leather fibres.

Practice Prompts

1. An As₂S₃ sol is coagulated most efficiently by which electrolyte — AlCl₃, BaCl₂, or NaCl? Justify using the Hardy-Schulze rule and the sign of charge on the sol.
2. A protective colloid with gold number 0.05 is mixed with another having gold number 25. Which is the better protector, and what does this mean for the minimum mass needed to stabilise 10 mL of standard gold sol against 1 mL of 10% NaCl?

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