You have to know that BaCO3 + 2HCl -> BaCl2 + CO2 +H2O
If you are given this
BaCO3 (s) + HCl (aq) ->
You know that Ba is 2+, so if it were a replacement reaction you would form BaCl and you would need 2 Cl2. So to balance the reactants BaCO3 (s) + 2 HCl (aq) ->
So now we know we will form BaCl2 and you would think we form H2CO3, but you just have to know that instead of that you make H2O and CO2. This is because H2CO3 is unstable and forms CO2 and H2O. It is just something you have to know I think.
So now you can do what Dion explained. BaCO3 is a solid. H2O is a liquid and CO2 is a gas. HCl and BaCl2 are aqueous. You have to know the solubility table for this.
BaCO3(s) + 2H+ + 2Cl- -> CO2(g) + H2O(l) + Ba + 2Cl-
The 2 Cl- cancel
You should get
BaCO3(s) + 2H+ -> CO2(g) + H2O(l) + Ba
Right?
It was a tricky one.