Give information about coagulation or precipitation.

(N/A) The stability of the lyophobic sol is due to the presence of charge on colloidal particles. If the charge is removed,the particles come closer to each other to form aggregates (or coagulate) and settle down under the force of gravity.
The process of settling of colloidal particles is called coagulation or precipitation of the sol.
The coagulation of the lyophobic sols can be carried out in the following ways:
$(i)$ By electrophoresis: The colloidal particles move towards oppositely charged electrodes,get discharged,and precipitate.
$(ii)$ By mixing two oppositely charged sols: Oppositely charged sols,when mixed in almost equal proportions,neutralize their charge and get partially or completely precipitated. Mixing of hydrated ferric oxide ($+ve$ sol) and arsenious sulphide ($-ve$ sol) brings them into the precipitated form. This is called mutual coagulation.
$(iii)$ By boiling: When a sol is boiled,the adsorbed layer is disturbed due to increased collisions with the molecules of the dispersion medium. This reduces the charge on the particles and leads to settling down as a precipitate.
$(iv)$ By persistent dialysis: On prolonged dialysis,traces of the electrolyte present in the sol are removed almost completely,making the colloids unstable and leading to coagulation.
$(v)$ By addition of electrolytes: When an excess of an electrolyte is added,the colloidal particles are precipitated. The colloids interact with ions carrying a charge opposite to that present on themselves,causing neutralization. The ion responsible for this is called the coagulating ion. $A$ negative ion causes the precipitation of positively charged sols and vice versa.
According to the Hardy-Schulze rule,the greater the valence of the flocculating ion added,the greater is its power to cause precipitation. For a negative sol,the flocculating power order is: $Al^{3+} > Ba^{2+} > Na^{+}$. For a positive sol,the order is: $[Fe(CN)_{6}]^{4-} > PO_{4}^{3-} > SO_{4}^{2-} > Cl^{-}$.
The minimum concentration of an electrolyte in $millimoles$ per liter required to cause precipitation of a sol in two hours is called the coagulating value. The smaller the quantity needed,the higher the coagulating power of an ion.