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According to “like dissolves like” rule of thumb, a homogeneous (true) solution forms between:

  • polar solvent and polar (ionic) solutes;
  • non-polar solvent and non-polar (covalent) solutes.

According to the second case, alkanes must be soluble in benzene. However, why is this the case? Why are covalent compounds soluble in non-polar solvents?

If I consider alkanes and benzene, alkanes are non-polar and benzene is non-polar as well, hence the intermolecular forces of attraction that act between both of them must be very weak van der Waals forces.

So, how are alkanes still miscible in benzene? If I have to generalise my question, then why are covalent compounds miscible in non-polar solvents?

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    $\begingroup$ Does this answer your question? Why does like dissolve like? $\endgroup$
    – Mithoron
    Apr 14 at 15:38
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    $\begingroup$ chemistry.stackexchange.com/questions/38260/… $\endgroup$
    – Mithoron
    Apr 14 at 15:38
  • $\begingroup$ @Mithoron Despite being titled, "why does like dissolve like", it looks like that question is actually asking why different doesn't dissolve in different. The comment on that post seems to suggest that question was also marked as a duplicate, incorrectly so, to a question that this question would be a dupe of. $\endgroup$ Apr 15 at 4:36

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The key is comparison/estimation of the strength of

  • intermolecular forces solvent-solute
  • intermolecular forces solute-solute and solvent-solvent.

There are three major cases (and the continous scales between them):

  • If the former interaction is strong enough, compared to the latter two, dissolving or mixing is probable.
  • If the former is not strong enough, then:
    • if forces are strongest for solvent-solvent interaction, the solvent is too polar and/or the solute is too nonpolar. Molecules of the solvent prefer to bind to each other, instead of involving the solute.
    • if forces are strongest for solute-solute interaction, the solvent is too nonpolar and/or the solute is too polar. Molecules of the solute prefer to bind to each other, instead of involving the solvent.

For polar solvents and ionic solutes, the above is applicable as well. There is the thermodynamic competition of ion solvation and ionic lattice energy.

Other factors come into play as well, like the size and shape and functional group positions of solute molecules. Big or linear molecules with good disposition of strong arrangement in the solid are harder to dissolve, as solvent molecules do not synchronize their actions.

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    $\begingroup$ So a solution is only possible for the first case right? $\endgroup$
    – Adh.
    Apr 14 at 17:00
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    $\begingroup$ As many things, it is not Yes/No state, there is wide continuous scale smoothly transitioning between the typical cases. So it is not like soluble yes/no, but rather a question "How much soluble?" $\endgroup$
    – Poutnik
    Apr 14 at 17:17

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