# Why does a reaction stop when turning into a solid?

I recently was looking at a question about the volume of carbon dioxide produced when reacting calcium carbonate with dilute sulfuric acid versus using nitric acid. In the answer it says that calcium sulfate is produced, which is a solid, which stops the reaction, but I don't get why. Calcium nitrate produces an aqueous solution which causes the reaction to carry on reacting.

The reaction equations are:

$$\ce{CaCO3(s) + H2SO4(aq) -> CaSO4(s) + H2O(l) + CO2(g)}$$

$$\ce{CaCO3(s) + 2HNO3(aq) -> Ca(NO3)2(aq) + H2O(l) + CO2(g)}$$