Nelson

Colloids are classified as two types which are the lyophilic / reversible and lyophobic / irreversible. If water is the dispersion medium, then the term used are hydrophilic and hydrophobic.

Lyophilic colloids: A colloidal system which may be constructed readily by simple warming or shaking the substance in a suitable solvent is called as lyophilic colloid. Substances that form lyophilic colloids involves gelatin, starch, and rubber, proteins, gum. Lyophilic colloids are also named as reversible colloids as on evaporating the dispersion medium such as water, the substance or the residue may be again reconverted into colloidal state simply by addition of the liquid i.e. water. These solutions are reasonably stable and usually cannot be easily precipitated.

Lyophobic colloids: colloids constructed with difficulty are termed as lyophobic colloids. These colloids are constructed by substances such as As2S3 , Fe(OH)3, gold and other metals that are sparingly soluble and thus their molecules do not pass readily into colloidal state. Lyophobic colloids are called as irreversible colloids since the residue cannot readily be reconverted into sol by ordinary means. These solutions are quite readily precipitated and hence they are not stable.