Solvated solids

I have a very basic question concerning solvation. If a solid (solute) was dissolved in a liquid (solvent), is the solute still in the solid state? or does it convert into another state of matter when this happens?




Good question. Theoretically, the physical phase of the matter does not change; crystalline salt dissolved into water does not become a liquid or gas. The dissolution reaction is theoretically a kinetic change, not a thermodynamic one as the traditional solid-liquid-gas phase changes are (at least at 1atm). However, the solute really isn't a "solid" anymore, either; depending on the nature of the solute and solvent, the two materials may interact chemically, and in fact when dissolution occurs, it's because it is thermodynamically favorable for the solute to be in a dissolved state as compared to a solid.


It is often useful to describe dissolved solutes as being in this dissolved state, especially if the solvent is water, in which case it is in an "aqueous" state. You see this in chemical equations where the states of the reactants and products are of interest, such as when a solid or gas precipitates from an aqueous reaction:


$$\ce{2Na(s) + 2H2O(l) \to 2NaOH(aq) + H2(g)\uparrow}$$


Notice that the sodium hydroxide produced by addition of sodium metal into water is virtually never "pure", even if the reactants are mixed in ideal ratio. The thermodynamically-favorable product is an aqueous mixture, which is treated as a liquid even though "pure" sodium hydroxide is an off-white solid powder.


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