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(i) Lyophilic sols:
Colloidal sols that are formed bymixing substances such as gum, gelatin, starch, etc. with a suitable liquid(dispersion medium) are called lyophilic sols. These sols are reversible innature i.e., if two constituents of the sol are separated by any means (such asevaporation), then the sol can be prepared again by simply mixing thedispersion medium with the dispersion phase and shaking the mixture.
(ii) Lyophobic sols:
When substances such as metalsand their sulphides etc. are mixed with the dispersion medium, they do not formcolloidal sols. Their colloidal sols can be prepared only by special methods.Such sols are called lyophobic sols. These sols are irreversible in nature. Forexample: sols of metals.
Now, the stability of hydrophilicsols depends on two things- the presence of a charge and the salvation ofcolloidal particles. On the other hand, the stability of hydrophobic sols isonly because of the presence of a charge. Therefore, the latter are much lessstable than the former. If the charge of hydrophobic sols is removed (byaddition of electrolytes), then the particles present in them come closer andform aggregates, leading to precipitation.