A. Chemical Methods (Lyophobic Colloids)

1. Reduction (Gold Sol): $AuCl_{3}$ + HCHO (formaldehyde) → Colloidal Au sol (reddish color) + HCl $AuCl_{3}$ + $SnCl_{2}$ → Au sol (Faraday's purple of Cassius) + $SnCl_{4}$

2. Hydrolysis (Fe(OH)_{3} Sol): $FeCl_{3}$ + $3H_{2}O$ → Fe(OH)_{3} (colloidal, red-brown) + 3HCl Condition: $FeCl_{3}$ added dropwise to boiling water. Excess $FeCl_{3}$ adsorbs $Fe^{3+}$ on particles → positive charge.

3. Double Decomposition ($As_{2}S_{3}$ Sol): $As_{2}O_{3}$ + $3H_{2}S$ → $As_{2}S_{3}$ (colloidal, yellow) + $3H_{2}O$ $S^{2-}$ ions adsorb on particles → negative charge.

B. Physical Methods

4. Bredig's Arc Method: Electric arc between metal electrodes (Pt, Au, Ag) submerged in cold water → metal vapor condenses to colloidal particles. Key: Cold water ensures rapid quenching → small (colloidal) particle size.

5. Peptization: Adding electrolyte (peptizing agent) to freshly prepared precipitate → colloidal dispersion. Example: $FeCl_{3}$ peptizes fresh Fe(OH)_{3} precipitate ($Fe^{3+}$ adsorbs on particles → charge → dispersal). Note: Only freshly prepared precipitates can be peptized; old precipitates cannot.

C. Purification Methods

6. Dialysis: Semi-permeable membrane bag in pure solvent. Small ions/molecules diffuse out; colloidal particles retained.

7. Electrodialysis: Dialysis with applied electric field. Accelerates removal of ionic impurities.

8. Ultrafiltration: Fine-pore membranes (pore size 1–100 nm) under pressure. Retains colloidal particles; small molecules pass through.