Upon dissolution, unstable chemical compounds of the solute with the solvent are formed (in the case of aqueous solutions, hydrates). Ions are especially susceptible to hydration. Ions attach polar water molecules, resulting in the formation of hydrated ions.
For example, copper (II) sulfate is a white crystalline salt. If dissolved in water, the solution will turn blue, since copper (II) ions have become hydrated – they are blue.
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