Emulsions are colloidal solutions in which both the dispersed phase and dispersion medium are liquids. However, the two liquids are immiscible, as miscible liquids will form true solution.
Emulsion are of two kinds:
The liquids forming emulsion i.e. oil and water will separate out on keeping as they are immiscible. Therefore, an emulsifying agent or emulsifier is added to stabilize the emulsion. Soap is a common emulsifier. The preparation of emulsion in the presence of an emulsifier is called emulsification.
Finely powdered ore is treated with an oil emulsion and the mixture is vigorously agitated by compressed air when the ore particles are carried to the surface and removed.
Gels are the type of colloids in which the dispersed phase is a liquid and the dispersion medium is a solid. Cheese, jelly, boot polish are common examples of gel. Most of the commonly used gels are hydrophilic colloidal solution in which a dilute solution, under suitable conditions set as elastic semi solid masses. For example, 5% aqueous solution of gelatin in water on cooling forms the jelly block.
Gels may shrink on keeping by loosing some of the liquid held by them. This is known as syneresis or resetting on standing.
Gels are divided in two categories: elastic gels and non elastic gels. Elastic gels are reversible. When partly dehydrated on loosing water, they change back into the original form on addition of water. The non elastic gels are not reversible.
Gels are useful in many ways. Silica, cheese, jelly, boot polish, curd are commonly used gels. Solidified alcohol fuel is a gel of alcohol in calcium acetate.