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:

$$\ce{Al2(SO4)3 + 3 Ca(OH)2 -&gt; 2 Al(OH)3 + 3 Ca(SO4)}$$
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.

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

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.