Q.Addition of alum purifies water. Why?

Colloidal solutions display fourteen distinctive properties. A sol's colour depends on preparation method, light wavelength, particle size/shape and viewing angle (reflected vs transmitted), and need not match the bulk substance's colour. Particle size ranges 1–1000 nm. Colloids are genuinely heterogeneous (two phases, shown by dialysis/ultrafiltration/ultracentrifuging) but are considered a border-line case given how much smaller their particles are than a coarse suspension's. They pass through ordinary filter paper (filtrability) yet remain stable against gravity (non-settling). Dilute sols stay stable; concentrating them promotes coagulation, and density falls as concentration falls. Colloidal particles diffuse only sluggishly (diffusability) yet still show genuine colligative properties (b.p. elevation, f.p. depression, osmotic pressure — the last used to estimate colloidal molecular weight). Particle shape varies genuinely by system: $\text{As}_2\text{S}_3$ spherical, $\text{Fe(OH)}_3$ disc/plate-like, $\text{WO}_3$ rod-like.

Optically, colloids show the Tyndall effect: a light beam scatters visibly through a colloidal solution (particles large enough to scatter light) but stays essentially invisible through a true solution (particles too small); Faraday first observed it, Tyndall investigated it in detail. Kinetically, colloidal particles show Brownian movement — continuous random zig-zag motion from unbalanced bombardment by faster dispersion-medium molecules (Robert Brown) — which lets Avogadro's number be calculated, confirms the kinetic theory, and explains colloidal stability (particles never stay close enough, long enough, for gravity to pull them together). …