Tyndall effect: scattering of light by colloidal particles (cone of light visible). True solutions don't show this. Brownian motion: random zig-zag movement of particles due to unbalanced molecular bombardment. Proves kinetic nature of matter. Electrophoresis: movement of colloidal particles under electric field. Reveals charge on particles. Positive colloids: Fe(OH)3, Al(OH)3, basic dyes. Negative colloids: As2S3, Au, Ag, clay, starch, acidic dyes. Charge arises from: selective adsorption of ions, dissociation of surface groups, or friction. Helmholtz double layer: fixed layer of ions on particle + diffuse layer in solution.