Why is this method only suitable for making soluble salts?:?

Question

Method:You add the base/carbonate/metal to the acid until no more will dissolve and you have some base left over (called an excess). You filter the mixture to remove the excess base then evaporate the water in the filtrate to leave the salt behind.

Answer 1

I assume that you would start with a base or carbonate that have low water solubility, otherwise adding excess of either one will result in their presence in the filtered solution. To answer your question: it's all about equilibrium. Adding acid will shift the equilibrium to the right, because you make soluble salt, so the initial substrate will continue dissolving until you've added all the acid. Now, if you add acid to an insoluble base/carbonate, it will only react on the surface and protect the rest of the solid from reacting with the acid. Plus being insoluble, you would only get an equilibrium amount of salt in the solution, with the unreacted substrates and insoluble salt left on the filter

Answer 2

If the salt that you are trying to make is insoluble, it will stay where it is formed - on the surface of the metal, carbonate or base - because it can't float away, and it will stop any more acid getting to what is beneath. End of reaction!