As long as the composition remains constant
and uniform throughout, a homogeneous mixture can be treated
as a pure substance.We call this a pseudo-component.
This is a tricky question. If you look very closely or only
consider a small sample of fog,
say 1 mm3, then you could see the water doplets and the air surrounding them. You would conclude that the fog sample is not homogeneous but heterogeneous because the composition is not everywhere the same.
If you consider a medium-sized sample of fog, perhaps 1 m3, and you did not look at it with a microscope, you would conclude that
the composition is the
same at all positions
and the fog was homogeneous. This would depend on
how big your sample was from diferent positions within the 1 m3 you were considering.
To make matters even more complex, if you considered a cubic kilometer of fog above your neighborhood, you
might notice patches of dense fog with volumes of relatively clear air with few water droplets. In this case, you would conclude that fog is definitely heterogeneous.
From some perspective, the fog could be accurately described as homogeneous, but for most purposes, fog is heterogeneous.