Question

what is the primary cause of diffusion

Answer 1

Difference in concentration.

Answer 2

Nature likes balance in most physical processes, so diffusion occurs to balance the difference.
Just like having a hot cup of tea in cold weather. The cup cools down to balance with the cold temperature outside.

Answer 3

yeah,primeralph is right,it is called diffusion gradient.

Answer 4

eh, i would say brownian motion.

Answer 5

@aaronq Are you kidding?

Answer 6

a concentration gradient is the driving force of mass transfer (e.g. diffusion)

Answer 7

well no, diffusion is transport phenomenon that stems from the random movement of molecules, so the primary cause for diffusion is brownian motion.