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I understand that the definition of a heterogeneous mixture is a mixture in which 2 or more phases are present, i.e. you can see the mixture is not uniform. I know a homogeneous mixture is a mixture in which only one visible phase is present, but why are suspensions and colloids considered heterogeneous, for you only see one phase as the definition states? I also know that homogeneous mixtures are usually referred to as solutions, but if only solutions are homogeneous then why do we define homogeneous as anything with 1 phase; isn't this misleading? Is there a more accurate definition of a homogeneous and heterogeneous mixtures?

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  • $\begingroup$ You're taking "see" a bit too literately. For example milk has suspended fat globules which you can't "see" with your naked eye. $\endgroup$
    – MaxW
    Sep 4 '18 at 19:57
  • $\begingroup$ A solution of table salt (or any other low Mw solute) in water is surely homogeneous, because all inhomogeneieties have a similar (on a log scale) size and diffusion coefficient as the solvent molecules. For a polymer solution, you have to stretch this definition quite a bit already. For a colloidal solution even more. $\endgroup$
    – Karl
    Sep 4 '18 at 20:27
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I know a homogeneous mixture is a mixture in which only one visible phase is present, but why are suspensions and colloids considered heterogeneous, for you only see one phase as the definition states?

The question is about what is going on at the molecular level. In a homogeneous mixture, the components are mixed all the way to the particular level. One way to see this with your eyes is that these mixtures are clear, not turbid, and have no phase separation. Grape juice is a solution, while milk is not (it is turbid) and neither is a mixture of oil and water (it often has two colors, and if you mix it vigorously, it turns turbid).

I also know that homogeneous mixtures are usually referred to as solutions, but if only solutions are homogeneous then why do we define homogeneous as anything with 1 phase; isn't this misleading?

Solutions are not the only homogeneous mixtures, they also exist in the solid state, like certain alloys. In the gas phase (if the sample is small enough and you wait a bit), all mixtures are homogenous. So a solution is a subset of possible homogenous mixtures.

Is there a more accurate definition of a homogeneous and heterogeneous mixtures?

If you can take smaller and smaller samples (larger than a couple thousand particles, though) and you always get the same composition, it is homogenous, otherwise heterogeneous.

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