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.

Some medicines are more effective in colloidal form because they have large surface area so easily assimilated in the body.

Animal hide is colloidal in nature and has positively charged particles. When it is soaked in tanin (negatively charged), a colloid, it results in mutual coagulation and gets harden. Thus, leather get hardened after tanning.

In Cottrell precipitator, smoke particles (charged) are passed through a chamber containing plates with charge opposite to the smoke particles, smoke particles lose their charge on the plates and get precipitated.