When we make chemical equations, it is no problem for me to understand what is on the reactant side, but how products are formed is difficult to understand. For example:
POCl3 + 3H2O ==> H3PO4 (aq) + 3 HCl (aq)
Why isn't POH or something else produced? Is there any criteria for this?
In order to determine what products are formed from particular reactants it is more complicated than simply mixing up different combinations of the elements. You have to know the general class and reactivity of each reactant to determine what products they will produce all together. You also need to know the different ions and bonding characteristics for each element to know what makes general sense. For example, POH does not exist and you can know that because OH is called the hydroxide anion and it always has a -1 charge. That would mean that P would have to have a +1 charge to balance it out to make a neutral compound. But P can never exist as a +1 cation, so this chemical (POH) cannot exist.
The best way to determine the general reactivity of a reactant is to know what class of compound it falls into. Many compounds are acids (produce protons) or bases (absorb protons). Also, many compounds are oxidants (lose electrons) or reductants (gain electrons). In the case where different ionic salt solutions are mixed, you need to know which kinds of compounds stay in aqueous solution (nitrate salts usually dissolve) versus salts that precipitate from water (AgCl for example). Once you learn these basic concepts, predicting chemical reaction products becomes more reasonable.