Comprehension # 1   Many lyophilic sols and few lyophobic sols when coagulated under some
Channel: Doubtnut
Date: 12/04/2020

Comprehension # 1   Many lyophilic sols and few lyophobic sols when coagulated under some special conditions changes into semi rigid mass, enclosing whole amount of liquid within itself, it is called gel and the process is called gelation, Gelatin Agar-agar, gum-Arabic can be converted into gels by cooling them under moderate concentration conditions. Hydrophobic sols like silicic acid. `Al(OH)_(3)` are prepared by double decomposition and exchange of solvent method.   Types of Gel :   (i) Elastic gel : Those gel which have elastic properties.   Ex : Gelatin, Strach, Agar-agar etc.   (ii) Non-elastic gel : Those gel which are rigid.   Ex : Silica gel.   Properties of Gel :   1. Syneresis/weeping of gel : The spontaneous liberation of liquid from a gel is called syneresis or weeping of gels. It is reverse of swelling.   Ex : Gelatin, Agar-Agar show syneresis at low concentration while sillicic acid shows it at high concentration.   2. Imbibition or swelling of gel : When gel is kept in a suitable liquid (water) it absorb large volume of liquid. The phenomenon is called imbibition or swelling of gel.   3. Thixotropic : Some gels when shaken to form a sol, on keeping changes into gel are termed as thixotropic gel and phenomenon is called thixotropy.   Ex : Gelatin and silica liquify on shaking changing into corresponding sol and sol on keeping changes back into gel.   The process of imbibing water when elastic gel are placed in water is called :