Colloids — Revision Notes

⚡ 30-Second Revision

Colloids: — Heterogeneous systems, particle size $1,\text{nm}$ to $1000,\text{nm}$.
Dispersed Phase (DP): — Substance distributed.
Dispersion Medium (DM): — Continuous phase.
Types by State: — Sol (S/L), Aerosol (S/G, L/G), Emulsion (L/L), Foam (G/L), Gel (L/S).
Types by Interaction:

* Lyophilic: Solvent-loving, stable, reversible (starch, gum). * Lyophobic: Solvent-hating, less stable, irreversible (metal sols).

Types by Particle:

* Multimolecular: Aggregates of small molecules (sulfur sol). * Macromolecular: Large molecules (starch, proteins). * Associated (Micelles): Aggregates of amphiphilic molecules above CMC ($T_k$).

Preparation: — Dispersion (mechanical, Bredig's Arc, peptization), Condensation (chemical methods).
Purification: — Dialysis, Electrodialysis, Ultrafiltration.
Properties:

* Tyndall Effect: Light scattering, visible path. * Brownian Movement: Random zigzag motion, stability. * Electrophoresis: Movement in E-field (charge determination). * Coagulation: Settling of particles. * Hardy-Schulze Rule: Coagulating power $propto$ Valency of active ion. * Protection: Lyophilic colloids protect lyophobic ones (Gold Number).

Emulsions: — L/L dispersions, stabilized by emulsifiers (O/W, W/O).
Gels: — L/S dispersions, jelly-like.

2-Minute Revision

Colloids are intermediate mixtures with particle sizes between $1,\text{nm}$ and $1000,\text{nm}$. They consist of a dispersed phase in a dispersion medium. Key classifications include lyophilic (stable, reversible, e.

g., starch) and lyophobic (less stable, irreversible, e.g., metal sols) based on interaction with the medium. Based on particle type, they can be multimolecular (aggregates of small molecules, e.g., sulfur), macromolecular (large single molecules, e.

g., proteins), or associated (micelles formed by surfactants above Critical Micelle Concentration, CMC). Colloids exhibit unique properties like the Tyndall effect (light scattering), Brownian movement (random motion preventing settling), and electrophoresis (movement in an electric field due to particle charge).

Their stability is often due to this charge. Coagulation, the settling of particles, is induced by electrolytes, with the efficiency governed by the Hardy-Schulze rule: higher valency of the oppositely charged ion means greater coagulating power.

Preparation methods include dispersion (peptization, Bredig's arc) and condensation (chemical reactions), while purification uses dialysis or ultrafiltration. Emulsions (liquid-liquid colloids) and gels (liquid-solid colloids) are important sub-types.

5-Minute Revision

Colloids are heterogeneous systems with dispersed particles ranging from $1,\text{nm}$ to $1000,\text{nm}$, bridging the gap between true solutions and suspensions. They are composed of a dispersed phase and a dispersion medium.

Classification is crucial: by physical state (e.g., sol, aerosol, emulsion, foam, gel), by interaction (lyophilic or 'solvent-loving' like starch, which are stable and reversible; and lyophobic or 'solvent-hating' like metal sols, which are less stable and irreversible), and by particle type (multimolecular like sulfur sol, macromolecular like proteins, and associated colloids or micelles formed by surfactants above CMC).

Preparation methods include dispersion techniques like mechanical grinding, Bredig's Arc method for metals, and peptization (converting precipitate to sol). Condensation methods involve aggregating smaller units through chemical reactions (e.g., oxidation, reduction). Purification is essential to remove impurities (crystalloids) and is achieved by dialysis, electrodialysis, or ultrafiltration.

Colloids exhibit distinct properties: the Tyndall effect (scattering of light, making the beam visible), Brownian movement (random zigzag motion due to molecular bombardment, preventing settling), and electrophoresis (movement of charged particles in an electric field, used to determine charge).

Colloidal particles carry a charge, which contributes significantly to their stability. Coagulation is the process of settling of colloidal particles upon neutralization of their charge, often by adding electrolytes.

The Hardy-Schulze rule states that the coagulating power of an electrolyte is directly proportional to the valency of the active ion (the ion with opposite charge to the sol). For example, for a negative sol, $Al^{3+} > Ba^{2+} > Na^+$.

Lyophilic colloids can act as protective colloids for lyophobic ones. Emulsions are liquid-liquid colloids (e.g., milk, butter) stabilized by emulsifiers, while gels are liquid-solid colloids (e.g.

, jelly). Understanding these concepts and their real-world applications is key for NEET.

Prelims Revision Notes

Colloids: Quick Facts for NEET

1. Definition & Size:

* Heterogeneous system. Dispersed Phase (DP) in Dispersion Medium (DM). * Particle size: $1,\text{nm}$ to $1000,\text{nm}$. * True solutions: $<1,\text{nm}$. Suspensions: $>1000,\text{nm}$.

2. Classification:

* By Physical State: * Solid in Liquid: Sol (Paint, ink) * Liquid in Gas: Liquid Aerosol (Fog, mist) * Solid in Gas: Solid Aerosol (Smoke, dust) * Liquid in Liquid: Emulsion (Milk, butter) * Gas in Liquid: Foam (Whipped cream, soap lather) * Gas in Solid: Solid Foam (Pumice stone) * Liquid in Solid: Gel (Jelly, cheese) * Solid in Solid: Solid Sol (Colored glass, gemstones) * By Interaction (DP & DM): * Lyophilic (Solvent-loving): Stable, reversible, high affinity.

Formed by organic substances (starch, gum, proteins). Not easily coagulated. * Lyophobic (Solvent-hating): Less stable, irreversible, low affinity. Formed by inorganic substances (metal sols, sulfides).

Easily coagulated. * By Particle Type: * Multimolecular: Aggregation of many small atoms/molecules (Gold sol, Sulfur sol). * Macromolecular: Large molecules of colloidal size (Starch, proteins, nylon).

* Associated (Micelles): Formed by amphiphilic molecules (soaps, detergents) above CMC and Kraft temperature ($T_k$).

3. Preparation Methods:

* Dispersion: Breaking larger particles. * Mechanical: Colloidal mill. * Electrical (Bredig's Arc): For metals (Au, Ag, Pt). * Peptization: Precipitate $ightarrow$ Sol by peptizing agent (electrolyte). * Condensation: Aggregating smaller particles. * Chemical: Oxidation ($H_2S + SO_2 \rightarrow S$), Reduction (AuCl$_3 \rightarrow$ Au), Hydrolysis ($FeCl_3 \rightarrow Fe(OH)_3$), Double Decomposition ($As_2O_3 + H_2S \rightarrow As_2S_3$).

4. Purification Methods:

* Dialysis: Removes crystalloids (impurities) through semi-permeable membrane. * Electrodialysis: Faster dialysis using electric field. * Ultrafiltration: Separates colloids from solutes using ultrafilters under pressure.

5. Properties:

* Colligative: Small magnitude (fewer particles). * Tyndall Effect: Scattering of light, visible path. Conditions: particle size $approx lambda$ of light, $Delta$ refractive index. * Brownian Movement: Random zigzag motion.

Due to unbalanced bombardment. Prevents settling, provides stability. * Charge: All particles in a sol have same charge (positive or negative). Due to selective adsorption of ions. Stability due to repulsion.

* Electrophoresis: Movement of charged particles in electric field. Cations to cathode, anions to anode. * Electro-osmosis: Movement of dispersion medium when particles are fixed. * Coagulation (Flocculation): Settling of particles by neutralizing charge.

* Hardy-Schulze Rule: Coagulating power $propto$ Valency of active ion (opposite charge). E.g., for negative sol: $Al^{3+} > Ba^{2+} > Na^+$. * Protection: Lyophilic colloids protect lyophobic sols.

Gold Number: Min. mg of protective colloid to prevent coagulation of $10,\text{mL}$ standard gold sol by $1,\text{mL}$ of $10$ NaCl solution. Lower gold number = higher protective power.

6. Emulsions: Liquid-liquid colloids. O/W (oil in water, e.g., milk) or W/O (water in oil, e.g., butter). Stabilized by emulsifiers.

7. Applications: Medicines, food, paints, water purification (alum), rubber, photographic films, smoke precipitation (Cottrell precipitator), delta formation.

Vyyuha Quick Recall

To remember the order of coagulating power for a negative sol (Hardy-Schulze Rule): NaBaAl. Think: 'Na Ba Al' (Sodium, Barium, Aluminium) - increasing valency, increasing power. For a positive sol: ClSOPO. Think: 'Cl SO PO' (Chloride, Sulfate, Phosphate) - increasing valency, increasing power.