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I'm learning to write balanced molecular and ionic equations. For "moderately soluble" compounds such as calcium hydroxide, how do I know whether to write the state symbol as (aq) or (s)? This is the equation I'm supposed to work with:

$$\ce{Al2(SO4)3 + 3 Ca(OH)2 -> 2 Al(OH)3 + 3 Ca(SO4)}$$

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  • $\begingroup$ Not sure of your context, but the (aq) suffix means that the substance is an aqueous solution. The (s) suffix typically means that the substance is a solid. So $\ce{Cu(s)}$ would be solid copper, but $\ce{Cu^{2+}(aq)}$ would be a solvated $\ce{Cu^{2+}}$ ion in an aqueous solution. $\endgroup$ – MaxW Oct 6 '16 at 2:23
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In principle, you cannot know without conducting the experiment. It depends on the concentration of a moderately soluble compound such as $\ce{Ca(OH)2}$ whether it is dissolved or whether there is stuff still sitting at the bottom of the vessel undissolved.

However, you are performing a reaction which is in aqueous solution. This reaction will typically only work if all the reactants are already in solution. Moreover, this is a precipitation reaction. Thus, it wouldn’t make sense to have a non-dissolved reactant to react it with a dissolved one. Given this context, you should assume both reactants are in solution.

$$\ce{Al2(SO4)3 (aq) + 3Ca(OH)2 (aq) -> 3CaSO4 v (s) + 2 Al(OH)3}$$

Note that whether or not aluminium hydroxide precipitates depends on the resulting $\mathrm{pH}$ value. If you started off with excess hydroxide, it may well stay in solution.

The exact answer for a given compound can only be known if you know both concentration and solubility product.

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