Question

If a compound is slightly soluble is it aqueous solution or solid?

Answer 1

If you're asking whether to write (aq) or (s) in a chemical reaction, it really depends on how much of it dissolves, which can be realized by calculating the solubility-product constant (`K_(sp)`) of the compound's dissociation equation. A higher `K_(sp)` indicates more of the dissociated ions are present, and is thus more soluble.

Some textbooks say they write (aq) instead of (s) if the solubility is greater than `0.01g/L`, but it's merely arbitrary. Your best bet would be to refer to a solubility guidelines/chart to determine which are soluble and insoluble. (The term "insoluble" is not very practical, as all ionic compounds are soluble to some extent (calculated via `K_(sp)`), but they generally call a compound "insoluble" if the solubility guidelines say so, i.e. you would maybe write (s) in the reaction.)

If you're asking in terms of reality whether a slightly soluble compound is present as a solid or as dissolved ions, then the answer is, in essence, both.

Once a solution becomes saturated (when no more solute can dissolve in the solvent), the dissociation reaction has reached equilibrium. There is now an equal rate of the solid dissociating into ions, and ions recombining to form the solid.