The colloidal system constituting the liquid as the dispersed phase and the solid as the dispersion medium is known as gel. There are some sols that have a high concentration of dispersed solid and change spontaneously into semi solid form on cooling. These are known as gels and the process is known as gelatin.
For example: gelatin dissolved in warm water forming a colloidal solution which when cooled sets to a jelly. Other example includes gum Arabic, agar, silicic acid, ferric hydroxide, cheese etc.
Gels are formed by the interlocking of the particles of solid dispersion medium in the form of a loose frame work inside which liquid is contained.
When the gel is allowed to stand for a long time it shrinks and loses the entire liquid held by it. This shrinking of gel is termed as syneresis or weeping.
Classification of gels:
Gels are classified into elastic and non elastic gels.
Elastic gels:
These are those gels which posses the property of elasticity i.e. they change to solid mass on dehydration which can again be converted into gel by addition of water followed by heating and cooling. When they are placed in contact with water they absorb water and swell. This property is known as imbibitions. Examples of Elastic gels are gelatin, agar, starch etc.
Non-elastic gels:
These are those gels which do not possess the property of elasticity i.e. they change to solid mass on dehydration which becomes rigid and cannot be converted into the original form by heating with water. They do not show the phenomenon of imbibitions. Best example of these gels is silicic acid.
