Provide information about the charge on colloidal particles.

Colloidal particles always carry an electrical charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either positive or negative. The presence of equal and similar charges on colloidal particles is largely responsible for providing stability to the colloidal solution because the repulsive forces between charged particles having the same charge prevent them from coalescing or aggregating when they come closer to one another. The charge on the sol particles is due to one or more reasons,such as electron capture by sol particles during electrodispersion of metals,preferential adsorption of ions from the solution,and the formation of an electrical double layer.

Positively charged Sols.	Negatively charged Sols.
$(1)$ Hydrated metallic oxides,e.g.,$Al_{2}O_{3} \cdot xH_{2}O$,$CrO_{3} \cdot xH_{2}O$ and $Fe_{2}O_{3} \cdot xH_{2}O$ etc.	$(1)$ Metals: e.g. Copper,silver,gold sols.
$(2)$ Basic dye stuffs e.g. Methylene blue sol.	$(2)$ Metallic sulphide,e.g.,$As_{2}S_{3}, Sb_{2}S_{3}, CdS$ sols.
$(3)$ Haemoglobin (Blood)	$(3)$ Acid dye stuffs e.g. eosin,congo red sols.
$(4)$ Oxides e.g. $TiO_{2}$ sol.	$(4)$ Sols of starch,gum,gelatin,clay,charcoal.