Quick questions on Solubility, aqueous solutions and like dissolves like: VCE Chemistry Unit 1

A solution is a homogeneous mixture. The solvent is the component in larger amount (often the liquid); the solute is what is dissolved. For most of Unit 1, the solvent is water and we are asking whether the solute will dissolve in it.

Water is a bent, polar molecule. Each O has two lone pairs and is partially negative ($\delta^{-}$); each H is partially positive ($\delta^{+}$).

Not all ionic compounds dissolve. Compounds with very large lattice enthalpies relative to their hydration enthalpies (silver chloride AgCl, barium sulfate $BaSO_4$, calcium carbonate $CaCO_3$) do not dissolve significantly in water; the cost of breaking the lattice exceeds the energy returned by hydration.

Polar molecules like ethanol, glucose and ammonia have $\delta^{+}$ and $\delta^{-}$ regions of their own. Water can form dipole-dipole or hydrogen-bond interactions with them, similar in strength to the water-water hydrogen bonds they replace. So they mix freely.

Non-polar substances (oils, $I_2$, hexane, $O_2$) have only dispersion forces. To dissolve in water, the strong water-water hydrogen-bond network would have to be broken and replaced with much weaker water-solute dispersion. That is energetically very unfavourable, so non-polar substances stay in their own non-polar phase.

A non-polar solvent (hexane, toluene, cyclohexane) interacts with its solutes only by dispersion. Adding a non-polar solute swaps dispersion for dispersion and dissolves; adding a polar or ionic solute would require breaking strong solute-solute forces and replacing them with much weaker dispersion, so they do not dissolve. This is why oil-and-water emulsions separate but oil and petrol mix.

A short summary you can quote in any solubility question:

At a given temperature, a fixed mass of solvent has a maximum amount of solute it will hold. That maximum is the solubility (often in g per 100 g water).

Dissolving NaCl breaks the ionic bonds in the NaCl lattice and forms new ion-dipole attractions; the O-H bonds in water are untouched.

$Na^{+}(aq)$ and $Cl^{-}(aq)$ are dissociated free ions surrounded by hydration shells, not NaCl molecules.

Some dissolutions are slightly endothermic (e.g. $NH_4NO_3$, used in instant cold packs) and still happen because the entropy of mixing outweighs the small unfavourable enthalpy.

Miscible means two liquids mix in any proportion (ethanol and water). Use "soluble" for solids.

Saturated means at the solubility limit. Concentrated just means a lot of solute relative to solvent; a small-solubility salt can have a saturated solution that is still very dilute.