Activated charcoal is used to remove colouring matter from pure substances. It works by

It is observed that the surface of a solid (or liquid) has the tendency to attract and retain the molecules of other immiscible phase with which it is brought into contact These molecules remain only at the surface and do not go deeper into the bulk. This tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) it termed adsorption.