Are all homogeneous mixtures just solutions?

Or can there be a homogeneous mixture that is not a solution? 




Homogeneous and heterogeneous classifications are scale-dependent. What we mean when we say a mixture is homogeneous is that there are no visible phase differences on the scale of interest.


For example, milk (when homogenized) and lotions are both examples of [colloidal suspensions](https://en.wikipedia.org/wiki/Colloid). There are no phase differences at the human scale, but neither one is a solution. On a more microscopic scale, you could call the suspended micelles phase boundaries, and in that case might *not* call the mixture homogeneous.


"Solution" always implies mixing on the molecular level (single-phase). 


To summarize:



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> Solutions are always homogeneous mixtures, but homogeneous mixtures are not always solutions.
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 14

According to IUPAC gold book: 



> 
> A solution is a liquid or solid phase containing more than one substance...
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So, the concept "solution" is reserved to solid and liquid phases.
In gas phase: Air is an example of homogeneous mixture of nitrogen, oxygen, carbon dioxide and other gases present in air. It's obvious that air is not a solution. 


 10

For the liquid phase, I think it is probably true that all homogeneous mixtures are solutions.


But the situation is more complicated for solid phases. In a diamond crystal, if the $\ce{^{13}C}$ atoms are homogeneously distributed in the crystal, could you really say that the diamond is a "solution" of $\ce{^{13}C}$ in a $\ce{^{12}C}$ "solvent"? I don't think so. $n$-doped or $p$-doped silicon is another example. In contrast, other homogeneous solids, like [brass](http://chemistry.about.com/od/alloys/f/Is-Brass-A-Solution.htm), are correctly regarded as solid solutions.


No one usually regards gas phases as "solutions", so mixtures of gases are probably not solutions.


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