Which procedure(s) will allow a student to differentiate between solid sodium sulfate and solid sodium sulfite? I. Make solutions of each and look for a precipitate when added to 0.10 M Ba(NO3)2. 
II. Add crystals of each to 0.10 M HCl and watch for bubbles. 
III. Make solutions of each and test with a pH indicator.

The answer is II and III only. I understand how III works, but I don't see how sulfite and sulfate can be differentiated by adding HCl; don't both get reduced to SO2 bubbles?
Sodium sulfite and hydrochloric acid partially react to $\ce{SO2}$ which is gaseous and thus will form bubbles if not dissolved immediately.

When sodium sulfate is added to hydrochloric acid it will dissolve. The solution then contains sulphate and hydrogensulphate ions. No gaseous product will be formed.