What happens when you mix NaCl and H2SO4?

Sulfuric acid and table salt. Reacting them will produce hydrogen chloride (gas) and sodium sulfate by the Mannheim process (see link below):

2NaCl(s) + H2SO4(l) --> 2HCl(g) + Na2SO4(s)

Other reactions are possible--for instance, sodium bisulfate (NaHSO4(s)) can also be expected to be produced at lower temperatures than the Mannheim process requires.

Mixing aqueous solutions of NaCl and H2SO4 will yield no net reaction because the driver for the reaction is the liberation of free HCl gas. In the presence of excess water, the HCl that would have been liberated, instead, forms hydrochloric acid and remains ionized and in solution up to a concentration of around 37% at STP.