ü  Coagulation of colloidal solutions: Process of settling of colloidal particles into precipitate by addition of electrolytes. This can be done by different ways:
i)         By electrophoresis
ii)       By mixing two oppositely charged sols
iii)     By continuous dialysis 
iv)     By boiling
v)       By the addition of electrolyte
When an electrolyte is added to the sol, the ions carrying opposite charge to that of the sol neutralize the charge and causes precipitation. The ion of the electrolyte which causes the precipitation is called the coagulating ion or the flocculating ion. A negatively charged ion causes the precipitation of positively charged sol and vice versa
Generally, the greater the valency of the coagulating ion, the greater will be the coagulating power. This is known as Hardy – Schulze rule.
For negatively charged sol, the coagulating power of electrolytes are:
AlCl3 > BaCl2 > NaCl or Al3+> Ba2+> Na+
For positively charged, then the coagulating power of electrolytes follow the following order:  K4[Fe(CN)6] > PO43– > SO42– > Cl–.

Ø  Coagulating value: The minimum concentration of an electrolyte in millimoles per litre required for the coagulation of a sol within 2 hours is called coagulating value. The smaller the coagulating value, the higher will be the coagulation power.

Ø  Protection of colloids: Lyophilic sols are self stabilized, while lyophobic sols require some stabilizing agents. For this purpose, some lyophilic sols are added to lyophobic sols. These lyophilic sols are called protective colloids. When a lyophilic sol is added to a lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect them from electrolytes.

F Difference between Flocculation and Coagulation: When a small amount of the electrolyte is added, i.e., when the concentration of the electrolyte added is low, the process is called flocculation. It can be reversed on shaking. However, at higher concentration, coagulation takes place and the process cannot be reversed simply by shaking.


ü  Emulsions:  It is a colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids (which are otherwise immiscible). Substances like soaps which help in making the emulsions stable are called emulsifiers or emulsifying agents.
Ø  Types of Emulsions: Emulsions are of two types:
i)      Oil in water (o/w): Emulsions in which oil is the dispersed phase and water is the dispersion medium. For example, milk and vanishing cream.
ii)    Water in oil (w/o): Emulsions in which water is the dispersed phase and oil is the dispersion medium. For example, cod liver oil, butter and cream.

F Demulsification: The process of breaking an emulsion to yield constituent.



Dye test to distinguish between Emulsions