One of the most important properties of colloidal solutions is that colloidal particles possess a definite type of electrical charge. In a particular colloidal solution, all the colloidal particles carry the same type of charge, while the dispersion medium has an equal but opposite charge. Thus, the charge on colloidal particles is balanced by that of the dispersion medium and the colloidal solution as a whole is electrically neutral. Positively charged sols are TiO2,CdS,Fe2O3,SiO2 Negatively charged sols are blood, Cu,Ag, clay.