We add alum to purify water as alum coagulates the colloidal impurities present in water, so that these impurities get settle down and remove by decantation or filtration. Thus, water gets purified by adding alum to water.

When electric potential is applied to colloidal solution, the colloidal particles move towards one or other electrode. Positively charged particles move towards the cathode while negatively charged particles move towards the anode.

The movement of colloidal particles under an applied electric potential is called electrophoresis. When electrophoresis is prevented by some means, then the dispersion medium begins to move in an electric field. This phenomenon is termed as electroosmosis.

Brownian movement may be defined as continuous zig-zag movement of colloidal particles in a colloidal sol. A state of continuous zig-zag motion of colloidal particles appears to be in view due to unbalanced bombardment of the particles of dispersed phase by molecules of dispersion medium. This Brownian movement stabilises the sol.

Positively charged ions coagulate the negatively charged sol and negatively charged ion coagulate the positively charged sol. Positively charged sol of hydrated ferric oxide is formed when $\mathrm{FeCl}_3$ is added in excess of hot water. On adding excess of NaCl to this sol, negatively charged $\mathrm{Cl}^{-}$ions coagulate the positively charged sol of hydrated ferric oxide.

Emulsifying agents stabilise the emulsion by forming an interfacial layer between suspended particles and the dispersion medium. e.g., gelatin is added to ice cream to stabilise it.