Chemical methods: (1) Double decomposition: As2O3 + 3H2S -> As2S3 sol (yellow, negative). (2) Reduction: HAuCl4 + formaldehyde -> Au sol $\frac{purple}{red}$. (3) Oxidation: H2S + Br2 -> S sol. (4) Hydrolysis: FeCl3 + 3H2O -> Fe(OH)3 sol (brown, positive). Bredig's arc method: electric arc between metal electrodes under water — produces metal sols (Au, Ag, Pt). Peptisation: converting fresh precipitate to colloid by adding electrolyte (e.g., FeCl3 to Fe(OH)3 precipitate). Purification: (1) Dialysis: colloid in bag with semipermeable membrane, ions diffuse out. (2) Electrodialysis: faster, using electric field. (3) Ultrafiltration: through graded filters.