Sodium Sulfate vs Sodium Sulfite I recently took a test and one of the questions was: 


  Which procedures 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 barium nitrate.
  
  II. Add crystal 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 2 and 3, however, I incorrectly answered 2 only. I have actually thought a long time about this question.

Why is III also correct, if sodium sulfate and sodium sulfite are both basic and would turn pH indicator the same color?
The last statement


  …sodium sulfate and sodium sulfite are both basic and would turn pH indicator the same color


is true only for $\ce{Na2SO4}$ as it's formed by both strong base and strong acid and won't noticeably affect pH.
$\ce{Na2SO3}$, on the other hand, undergoes hydrolysis:

$$
\begin{align}
\ce{Na2SO3 + H2O &amp;&lt;=&gt; NaOH + NaHSO3} \\
\ce{2Na+ + SO3^2- + H2O &amp;&lt;=&gt; Na+ + OH- + Na+ + HSO3-} \\
\ce{SO3^2- + H2O &amp;&lt;=&gt; HSO3- + OH-}
\end{align}
$$

resulting in elevated pH (basic solution) up to 9 (according to Wikipedia).