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, i.e. 10⁻⁷ to 10⁻⁴ cm) is suspended throughout a dispersion medium.
• The path of a light beam becomes visible inside a colloid — this is the Tyndall effect, caused by scattering of light by colloidal particles (absent in true solutions).
• Colloidal particles undergo Brownian movement (zig-zag motion from molecular collisions), which prevents them from settling under gravity.
• Lyophilic colloids (e.g. gum, starch) are solvent-loving and reversible; lyophobic colloids (e.g. metals like gold sol) are solvent-hating and need stabilisers.
• Addition of an electrolyte with opposite charge causes coagulation, governed by the Schulze–Hardy rule: coagulating power ∝ (charge on the oppositely charged ion)⁶.
• Emulsions, sols, gels, foams and aerosols are the major colloidal types — classification is based on the physical states of the dispersed phase and dispersion medium.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Components and Size Range

Every colloid has two components. The dispersed phase is the substance present as tiny particles, while the dispersion medium is the medium in which these particles are distributed. The particle diameter of 1–1000 nm (10⁻⁷–10⁻⁴ cm) is the diagnostic range — larger than true solutions but smaller than suspensions, so the particles cannot be filtered by ordinary filter paper but can be retained by ultra-filters (e.g. cellophane, collodion).

Classification by Physical State

Dispersed phase	Dispersion medium	Name	Example
Solid	Solid	Solid sol	Coloured gemstones, alloys
Solid	Liquid	Sol	Starch in water, gold sol
Solid	Gas	Aerosol	Smoke, dust
Liquid	Solid	Gel	Cheese, jelly
Liquid	Liquid	Emulsion	Milk, cod-liver oil
Liquid	Gas	Aerosol	Fog, mist, cloud
Gas	Solid	Solid foam	Pumice, foam rubber
Gas	Liquid	Foam	Whipped cream, soap lather

Lyophilic vs Lyophobic

Lyophilic colloids (solvent-loving, e.g. gum arabic, starch, protein, gelatin) are directly formed by simply dissolving in the medium; they are reversible and inherently stable because of strong solvation. Lyophobic colloids (solvent-hating, e.g. metals, metal sulphides, metal hydroxides) require special methods such as chemical reduction, Bredig’s arc method or hydrolysis; they are irreversible and need trace stabilisers or electrolytes to remain dispersed.

Stability and Coagulation

All colloidal particles carry a fixed charge (positive sols: Fe(OH)₃, Al(OH)₃; negative sols: metal sulphides, starch, gum). Mutual repulsion between like charges keeps the system stable. Adding an electrolyte with oppositely charged ions neutralises this charge and causes coagulation/flocculation. The Schulze–Hardy rule states that the coagulating power of an ion is proportional to the sixth power of its valency, so Al³⁺ is far more effective than Na⁺ for a negatively charged sol.

Emulsions and Micelles

An emulsion is a colloidal dispersion of two immiscible liquids stabilised by an emulsifier (surfactant). Oil-in-water (O/W, e.g. milk) and water-in-oil (W/O, e.g. butter) are the two types. Micelles are associated colloids formed when soap molecules (RCOO⁻Na⁺) aggregate above the Kraft temperature and at the Critical Micelle Concentration (CMC); the hydrophobic tails cluster inward while the hydrophilic –COO⁻ heads point outward, giving soap its cleansing action.

CUET Pattern

CUET UG Chemistry typically asks 1–2 questions from this topic as MCQs. Expect one direct fact-based question (Tyndall effect, Bredig’s arc, CMC, or emulsion type identification) and one application-based question linking the Schulze–Hardy rule to a numerical coagulating power comparison.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Optical and Kinetic Properties

The Tyndall effect arises because colloidal particles are large enough to scatter visible light (wavelength 400–700 nm) but small enough to remain suspended. This is the laboratory test used to distinguish a colloid from a true solution — both look clear, but only the colloid shows a visible light cone (the Faraday–Tyndall cone).

Brownian movement, observed by Robert Brown in 1827, results from unbalanced collisions of dispersion-medium molecules with colloidal particles. It is the kinetic basis of colloidal stability against sedimentation (Stokes’ law settling is countered by this random motion) and is also the experimental evidence for the kinetic theory of matter.

Electrical Properties — Electrophoresis and Zeta Potential

When an electric field is applied, colloidal particles migrate toward the electrode of opposite charge — this is electrophoresis. It proves that colloidal particles carry charge. The zeta potential (potential at the slipping plane of the double layer) quantifies stability: a value above ±30 mV gives a stable colloid; below that, coagulation becomes likely. The Hardy–Schulze rule quantifies the valency dependence: for an As₂S₃ (negative) sol, the coagulating power follows the order Al³⁺ > Ba²⁺ > Na⁺.

Purification Methods

• Dialysis — separation of ions from colloids using a semipermeable membrane.
• Ultrafiltration — filtration through collodion or cellophane membranes.
• Electro-dialysis — dialysis accelerated by an applied electric field.

Coagulating Value and Gold Number

Coagulating value (or flocculation value) is the minimum concentration (in millimoles per litre) of an electrolyte required to coagulate a given colloid — a lower coagulating value means higher coagulating power. The Gold number, devised by Zsigmondy, is the milligrams of a protective colloid needed to prevent coagulation of 10 mL of a standard gold sol by 1 mL of 10% NaCl; lower Gold number = greater protective power. Gelatin has Gold number ≈ 0.005 (excellent protector), while gum arabic has ≈ 0.15.

Practice Prompts

1. A reddish-brown sol is prepared by Bredig’s arc method using gold electrodes in water. Predict the sign of the charge on the sol particles. Justify using an electrolyte that would be most effective at coagulating it.
2. Sodium stearate forms micelles only above 298 K and at a CMC of about 10⁻³ mol L⁻¹. Explain, with a diagram in words, why micelle formation suddenly begins at the CMC and why the Kraft temperature is the lower limit.

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