What are lyophilic and lyophobic sols ? Give are example of each type. Why are hydrophobic sols easily coagulated ? 

Explanation

Lyophilic sols : Colloidal sols directly formed by mixing substances in a suitable dispersion medium are called lyophilic sols. These are quite stable. These are also known as reversible sols because the dispersion medium can be separated from the colloid by means of physical methods like evaporation. e.g., gum, gelatin, starch, rubber etc.
Lyophobic sols (Liquid hating) Such sols are not formed by simply mixing the substance with the dispersion medium. These are not stable, i.e., these sols are coagulated easily when a small amount of electrolyte is added to them or they are heated or shake. These sols need stabilizing agent for their preservation, e.g., gold sol,`As_2O_3, Fe(OH)_(3)` etc.
Reason for coagulation of hydrophobic sols : These sols are precipitated due to the removal or neutralization of charge on their particles, i.e., they come nearer, form aggregate and settle down, (in other words, coagulation or precipitation occurs.) This is done by addition of a small amount of electrolyte .