Certain sols have the property of setting to a semi-solid, jelly-like form by enclosing the entire amount of liquid within itself when they are present at high concentrations. This process is called gelation and colloidal systems with jelly-like appearance are known as gels. Some common examples of gels are: gelatin, gum Arabic, silicic acid, processed cheese, ferric hydroxide etc.
Gels possess rigid structures which are formed when the particles of dispersed phase get interlocked and create a lose network frame. The particles of dispersion medium are trapped within the loose framework. The degree of rigidity of structure varies from substance to substance. Thus, gel represents a liquid-solid system, i.e. a liquid immersed in a solid.
When the gels are allowed to stand for long time, they give out small quantity of trapped liquid which accumulates on its surface. This action of gels is known as syneresis or weeping.
Gels are divided into two classes i.e. elastic gels and non-elastic gels. The characteristic differences between the two are tabulated below:
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Elastic gels
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Non-elastic gels
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These gels change to solid mass on dehydration which can be changed back to original form by addition of water followed by warming.
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These gels change to solid mass on dehydration which cannot be changed back to original form by addition of water and warming.
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The absorb water when placed in it with simultaneous swelling of gel body. This phenomenon is called imbibitions.
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These do not imbibe.
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Some gels such as silica, gelatin, ferric phosphate, etc, liquefy on mechanical shaking and change to sols losing their semi-solid gel character. The sol on scattering changes back to the gel. This phenomenon is known as thixotropy.