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While studying mixtures and solutions, I read that glass is a mixture and not a solution.

Then I searched over the Internet about it, but I got the same answer that it is a homogenous mixture.

But nowhere did I find any meaningful answer about what the difference is in a homogeneous mixture and solution.

People talked about particle size. But if we zoom enough even salt in water is not completely homogeneous.

So, I got some questions here:

  1. Is there any difference in a homogenous mixture and solution?

  2. Is there any clear cut boundary of what size particle are considered a solution or mixture?

  3. Why is brass or any other alloy considered a solution while glass is not?

  4. Last, but not least - Why is air not a solution despite oxygen, nitrogen, etc. molecules being so small?

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    $\begingroup$ Much of this is semantics. Certainly in a glass (or the liquid it is formed from) there are free energy effects from enthalpies and entropies of mixing that will influence whether you end up with a one-phase or two-phase (or more) chunk of glass. $\endgroup$
    – Jon Custer
    Commented Sep 5, 2023 at 17:00
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    $\begingroup$ Solution is a homogeneous mixture. $\endgroup$
    – Proscionexium
    Commented Sep 5, 2023 at 17:00
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    $\begingroup$ I wouldn't think it odd for someone to call air a solution. Maybe a bit nonstandard as people don't usual, but it certainly does fit the definition. $\endgroup$
    – Hearth
    Commented Sep 6, 2023 at 3:54
  • $\begingroup$ @Hearth - indeed, air is pretty much an ideal solution, with only an entropy of mixing term to deal with. $\endgroup$
    – Jon Custer
    Commented Sep 6, 2023 at 13:56

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[OP] Is there any difference in homogeneous mixture and solution?

According to an intro-college textbook, there is no difference: "A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout."

If you look at the definition from IUPAC (the professional organization of chemists), the definition of solution is a bit vague:

[IUPAC] A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. [...]

So it does not mention the requirement that the phase be homogeneous.

[OP] Why air is not a solution despite oxygen, nitrogen etc molecules being so small.

Gas mixtures fit the definition of a solution as homogeneous mixture. However, gases are always completely miscible with each other, so at equilibrium they would always be a homogeneous mixture. Maybe the term solution is not used for the gas phase because there is no reason to distinguish, or because there would be no solute-solvent interactions.

[OP] Why glass is mixture and not solution?

Without a source for the statement, and a specification of what types of glasses it applies to, there is no good answer. If someone claimed that glass is a heterogeneous mixture instead of a homogeneous mixture, there would be ways to test that.

It is possible that glass is so different from other materials that it makes sense to classify it with its own term. Here is one source to study the weirdness further.

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    $\begingroup$ I just want to thank you for showing clearly where the quotes are from. I wish this was more widespread in SE! $\endgroup$
    – JiK
    Commented Sep 6, 2023 at 9:48
  • $\begingroup$ I feel like there's more to a "solution" than the convenience of the idea. For example, would volume changes matter when referring to a solution vs a mixture? If we mix sand with water, the volume is the sum of the two separately. When we dissolve salt in water the volume is less than the sum of the two. I don't know about glass too much, but it seems to me like the only volumes lost are gasses while heating. $\endgroup$
    – user8035
    Commented Sep 6, 2023 at 13:16
  • $\begingroup$ But, when considering a solution of water and alcohol, I can see the convenience argument when deciding which is the solvent of the other. $\endgroup$
    – user8035
    Commented Sep 6, 2023 at 13:21

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