The tendency of molecules to move toward areas of lower concentration is called

Diffusion

Explanation:

Diffusion is the tendency of molecules to rise from region of higher concentration to region lower concentration until they are even distributed. The tendency of molecules to move toward areas of lower concentration is called diffusion. Diffusion occurs base on diffusion gradient which is change in quantity of the the molecule with respect to change in variables of the medium. The rate of diffusion varies for different substances and mediums.