# Should moderately soluble compounds like calcium hydroxide be denoted (aq) or (s)?

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:

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

• 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.
– MaxW
Oct 6, 2016 at 2:23

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