(1) “A gel is a colloidal system in which a liquid is dispersed in a solid.”
(2) The lyophilic sols may be coagulated to give a semisolid jelly like mass, which encloses all the liquid present in the sol. The process of gel formation is called gelation and the colloidal system formed called gel.
(3) Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel transformation is called thixotropy.
(4) The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.
(5) Gels may shrink by loosing some liquid help them. This is known as synereises or weeping.
(6) Gels may be classified into two types
(i) Elastic gels : These are the gels which possess the property of elasticity. They readily change their shape on applying force and return to original shape when the applied force is removed. Common examples are gelatin, agar-agar, starch etc.
(ii) Non-elastic gels : These are the gels which are rigid and do not have the property of elasticity. For example, silica gel.