Example Problem with Complete Solution

2A-3 : Think About Fog 2 pts
Fog is very large number of liquid water droplets suspended in air. Can fog be considered to be a pure substance? Do you think fog is heterogeneous or homogeneous? Explain your answers.
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.