Class 12 Chemistry Notes Chapter 1 (Colloids) – Lab Manual (English) Book

Alright class, let's dive into the fascinating world of Colloids. This topic, stemming from Chapter 1 of your Lab Manual and overlapping significantly with the 'Surface Chemistry' chapter in your main textbook, is crucial not just for your board exams but also for various government competitive exams. We'll focus on the key concepts, definitions, and properties relevant from an examination perspective.

Chapter 1: Colloids - Detailed Notes for Exam Preparation

1. Introduction:

• Definition: A colloid is a heterogeneous system in which one substance of microscopically dispersed insoluble particles (dispersed phase) is suspended throughout another substance (dispersion medium).
• Particle Size: The size of the dispersed phase particles ranges from 1 nm to 1000 nm. This size is intermediate between true solutions (< 1 nm) and suspensions (> 1000 nm).
• Heterogeneous Nature: Although colloids often appear homogeneous to the naked eye, they are fundamentally heterogeneous.

2. Classification of Colloids:

(a) Based on the Physical State of Dispersed Phase (DP) and Dispersion Medium (DM):

(b) Based on Nature of Interaction between DP and DM:

• Lyophilic Colloids (Solvent-loving):

  • Strong affinity between DP and DM.
  • Easily formed by direct mixing (e.g., dissolving starch, gelatin, gum, proteins in water).
  • Highly stable and not easily coagulated.
  • Reversible in nature (if DM is evaporated, sol can be reformed by adding DM).
  • Viscosity is much higher than the DM.
  • Surface tension is generally lower than the DM.
  • Particles are extensively solvated (hydrated if water is DM).
  • Examples: Starch sol, gum arabic sol, gelatin sol, egg albumin sol. (Relates to Lab Manual Experiment: Preparation of Lyophilic Sol)

• Lyophobic Colloids (Solvent-hating):

  • Little or no affinity between DP and DM.
  • Require special methods for preparation.
  • Inherently unstable, require stabilizing agents for preservation.
  • Irreversible in nature.
  • Viscosity is nearly the same as the DM.
  • Surface tension is nearly the same as the DM.
  • Particles are not significantly solvated.
  • Easily coagulated by adding small amounts of electrolytes, heating, or shaking.
  • Examples: Metal sols (Au, Ag), metal sulfide sols (As₂S₃), metal hydroxide sols (Fe(OH)₃, Al(OH)₃). (Relates to Lab Manual Experiment: Preparation of Lyophobic Sol like Fe(OH)₃ or As₂S₃)

(c) Based on Type of Particles of the Dispersed Phase:

• Multimolecular Colloids:

  • Formed by aggregation of a large number of small molecules (diameter < 1 nm).
  • Held together by weak van der Waals forces.
  • Examples: Sulphur sol (S₈ molecules), Gold sol.

• Macromolecular Colloids:

  • Dispersed particles are large molecules (macromolecules) like proteins, starch, cellulose, synthetic polymers (nylon, polythene).
  • These molecules have sizes in the colloidal range.
  • Often resemble lyophilic sols in properties (stable, high viscosity).
  • Examples: Starch sol, protein sol, cellulose sol.

• Associated Colloids (Micelles):

  • Substances which behave as normal, strong electrolytes at low concentrations but exhibit colloidal behaviour at higher concentrations due to the formation of aggregates (micelles).
  • The formation of micelles takes place only above a particular temperature called Kraft Temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC).
  • These molecules have both lyophobic (hydrophobic/water-repelling) and lyophilic (hydrophilic/water-attracting) parts.
  • Examples: Soaps (Sodium stearate C₁₇H₃₅COONa), synthetic detergents.

3. Preparation of Colloids:

• Lyophilic Sols: Simple mixing/warming of substance with dispersion medium (e.g., Starch + hot water).
• Lyophobic Sols: Require special methods:
  • Dispersion Methods: Breaking down larger particles.
    • Bredig's Arc Method: For metal sols (Au, Ag, Pt). Involves striking an electric arc between metal electrodes immersed in the dispersion medium (kept cool). Intense heat vaporizes metal, which then condenses into colloidal particles.
    • Peptization: Converting a fresh precipitate into colloidal particles by shaking it with the dispersion medium in the presence of a small amount of electrolyte (peptizing agent). The electrolyte usually has an ion common to the precipitate, which gets adsorbed onto the surface, giving it a charge and causing repulsion. (e.g., adding FeCl₃ solution to Fe(OH)₃ precipitate to form Fe(OH)₃ sol).
  • Condensation Methods: Aggregating smaller particles (atoms/molecules).
    • Chemical Methods:
      • Oxidation: H₂S (aq) + SO₂ → 3S (sol) + 2H₂O
      • Reduction: AuCl₃ (aq) + HCHO + H₂O → Au (sol) + HCOOH + HCl
      • Hydrolysis: FeCl₃ (aq) + 3H₂O (boiling) → Fe(OH)₃ (sol) + 3HCl (Common lab prep)
      • Double Decomposition: As₂O₃ (aq) + 3H₂S (aq) → As₂S₃ (sol) + 3H₂O (Common lab prep)

4. Purification of Colloidal Solutions:

• Colloidal solutions prepared often contain excess electrolytes which can destabilize them. Purification involves reducing these impurities to a minimum.
• Dialysis:
  • Removal of dissolved substances (crystalloids) from a colloidal solution by means of diffusion through a suitable membrane (parchment paper, cellophane).
  • The membrane allows ions/small molecules to pass through but retains colloidal particles.
  • The bag containing the colloid is suspended in fresh flowing water. (Relates to Lab Manual Experiment: Purification by Dialysis)
• Electrodialysis:
  • Dialysis carried out under the influence of an electric field.
  • Faster than simple dialysis as ions migrate quickly towards oppositely charged electrodes.
• Ultrafiltration:
  • Separation of colloidal particles from the solvent and soluble solutes by filtration through special filter papers called ultrafilter papers (pores made smaller by impregnating with collodion solution).

5. Properties of Colloidal Solutions:

• Colligative Properties: Exhibit colligative properties (osmotic pressure, elevation in boiling point, etc.) but the values are generally lower than true solutions of same concentration due to the larger size and smaller number of particles.

• Tyndall Effect:

  • Scattering of light by colloidal particles when a beam of light is passed through the colloidal solution. The path of light becomes visible.
  • Conditions: (i) Diameter of dispersed particles is not much smaller than the wavelength of light used. (ii) Refractive indices of DP and DM differ significantly.
  • Used to distinguish colloids from true solutions. Observed in ultramicroscope.

• Brownian Movement:

  • Continuous, random, zig-zag motion of colloidal particles.
  • Cause: Unbalanced bombardment of the dispersed particles by the molecules of the dispersion medium.
  • Significance: Counters gravity, provides stability to the sol by preventing settling.

• Charge on Colloidal Particles:

  • Colloidal particles always carry an electric charge (either positive or negative). The dispersion medium has an equal and opposite charge, making the system electrically neutral overall.
  • Origin of Charge: Selective adsorption of common ions from the medium, dissociation of surface molecules, electron capture (Bredig's arc).
  • Positively charged sols: Metal hydroxides [Fe(OH)₃, Al(OH)₃], basic dyes (methylene blue), hydrated metallic oxides (TiO₂), haemoglobin.
  • Negatively charged sols: Metal sols (Au, Ag, Cu), metal sulphides (As₂S₃, CdS), starch sol, gum sol, clay, charcoal, acid dyes (eosin, congo red).

• Electrophoresis (Cataphoresis):

  • Movement of colloidal particles towards the oppositely charged electrode under the influence of an applied electric field.
  • Used to determine the sign of charge on colloidal particles and also for coagulation.

• Coagulation or Precipitation (Flocculation):

  • The process of settling down of colloidal particles, i.e., precipitation of the sol.
  • Methods:
    • By adding an electrolyte: Oppositely charged ions from the electrolyte neutralize the charge on colloidal particles, causing aggregation and settling.
    • By electrophoresis: Particles move to opposite electrode, get discharged and precipitate.
    • By mixing two oppositely charged sols.
    • By boiling: Increased collisions and desorption of charge layer.
    • By persistent dialysis: Removal of stabilizing electrolytes.
  • Hardy-Schulze Rule: Governs coagulation by electrolytes.
    • The effective ion for coagulation is the one carrying charge opposite to that of the colloidal particles (counter-ion).
    • Greater the valency (charge) of the coagulating ion, the greater is its coagulating power.
    • Example: For negative sol (As₂S₃), coagulating power: Al³⁺ > Ba²⁺ > Na⁺. For positive sol (Fe(OH)₃), coagulating power: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻.
  • Coagulating Value: Minimum concentration of an electrolyte (in millimoles per litre) required to cause precipitation of a sol in a given time (e.g., 2 hours). Smaller the coagulating value, higher the coagulating power.

• Protection of Colloids:

  • Lyophobic sols are unstable and easily coagulated.
  • Lyophilic sols are much more stable. They can be used to protect lyophobic sols from coagulation by electrolytes.
  • The lyophilic colloid forms a protective layer around the lyophobic particles.
  • Example: Gelatin (lyophilic) is added to gold sol (lyophobic) to protect it.
  • Gold Number: A measure of the protective power of a lyophilic colloid. Defined as the minimum weight (in milligrams) of the protective colloid required to just prevent the coagulation of 10 mL of a standard gold sol when 1 mL of 10% NaCl solution is added to it. Smaller the gold number, greater the protective power.

6. Emulsions:

• Colloidal systems in which both the dispersed phase and dispersion medium are liquids (immiscible or partially miscible).
• Types:
  • Oil in Water (O/W): Oil is DP, Water is DM. (e.g., Milk, vanishing cream).
  • Water in Oil (W/O): Water is DP, Oil is DM. (e.g., Butter, cold cream).
• Emulsifying Agent (Emulsifier): A third component added to stabilize the emulsion. Forms an interfacial film between DP and DM. (e.g., Soaps, detergents, proteins, gums for O/W; heavy metal salts of fatty acids, long chain alcohols for W/O). (Relates to Lab Manual Experiment: Role of Emulsifying Agents).
• Demulsification: Process of breaking an emulsion into its constituent liquids (by heating, freezing, centrifuging, adding electrolytes).

7. Applications of Colloids:

• Medicines: Colloidal medicines are easily assimilated (e.g., Argyrol - silver sol eye lotion, Milk of Magnesia - antacid).
• Food: Milk, butter, ice cream, fruit juices.
• Industry: Paints, inks, rubber, lubricants, cement.
• Purification of Water: Alum (Al³⁺ ions) coagulates negatively charged clay/mud particles.
• Sewage Disposal: Coagulation of dirt particles.
• Smoke Precipitation (Cottrell Precipitator): Smoke (carbon particles in air) passed through charged plates precipitates the carbon.
• Artificial Rain: Spraying oppositely charged colloidal dust/sand onto clouds.
• Formation of Delta: River water (colloidal clay) meets sea water (electrolytes), causing coagulation and deposition.
• Cleansing Action of Soap: Micelle formation emulsifies grease/dirt.
• Leather Tanning: Animal hides (positively charged protein) soaked in tanning solution (negatively charged tannin) causes mutual coagulation.

Multiple Choice Questions (MCQs):

1. Which of the following has particle size in the range of 1 nm to 1000 nm?
   (a) True Solutions
   (b) Colloidal Solutions
   (c) Suspensions
   (d) None of the above

2. Milk is an example of:
   (a) Sol
   (b) Gel
   (c) Emulsion (O/W type)
   (d) Aerosol

3. The phenomenon of scattering of light by colloidal particles is known as:
   (a) Brownian movement
   (b) Tyndall effect
   (c) Electrophoresis
   (d) Dialysis

4. The zig-zag random motion of colloidal particles is called:
   (a) Tyndall effect
   (b) Electrophoresis
   (c) Brownian movement
   (d) Coagulation

5. Which method is used for the purification of colloids where ions are removed faster under an electric field?
   (a) Dialysis
   (b) Ultrafiltration
   (c) Electrodialysis
   (d) Peptization

6. According to the Hardy-Schulze rule, the coagulating power of ions for a negatively charged As₂S₃ sol follows the order:
   (a) Na⁺ > Ba²⁺ > Al³⁺
   (b) Al³⁺ > Ba²⁺ > Na⁺
   (c) Cl⁻ > SO₄²⁻ > PO₄³⁻
   (d) PO₄³⁻ > SO₄²⁻ > Cl⁻

7. Which of the following is a lyophobic colloid?
   (a) Starch sol
   (b) Gelatin sol
   (c) Gold sol
   (d) Gum arabic sol

8. The process of converting a fresh precipitate into a colloidal sol by shaking it with the dispersion medium containing a small amount of electrolyte is called:
   (a) Coagulation
   (b) Peptization
   (c) Dialysis
   (d) Emulsification

9. Associated colloids (micelles) are formed:
   (a) At all concentrations and temperatures
   (b) Only above Kraft temperature and below CMC
   (c) Only below Kraft temperature and above CMC
   (d) Only above Kraft temperature and above CMC

10. Gold number is a measure of the:
    (a) Amount of gold in a sol
    (b) Stability of a gold sol
    (c) Protective power of a lyophilic colloid
    (d) Particle size of gold sol

Answer Key for MCQs:

1. (b)
2. (c)
3. (b)
4. (c)
5. (c)
6. (b)
7. (c)
8. (b)
9. (d)
10. (c)

Study these notes thoroughly. Pay special attention to the definitions, classifications (especially lyophilic vs lyophobic), properties like Tyndall effect, Brownian movement, charge & coagulation (Hardy-Schulze rule), and the preparation/purification methods linked to your lab work. Understanding these concepts well will be very beneficial for your exams. Good luck!