How do you tell just by looking at a chemical equation if it will react or not? For example, LiI(aq) + BaS(aq) don't react, but I don't know how to tell that just by looking at the equation. Any clear explanation is welcomed. It's not for a HW answer, just for my knowledge base. I feel silly for even having to post this.

Question

aaronq · Answer

There's a subtlety when an equation like that it's written, the aqueous symbol implies that the ions are dissolved/solvated, meaning that they're each surrounded by a clathrate ("casing") of water molecules. 
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If the species dont interact with one another and remain soluble or chemically unchanged, then no reaction took place and they remain solvated.

It's not always obvious to know if a reaction will occur because there are so many different reactions, but for now, identifying possible insoluble products (with a solubility table) will be a good start.