Why is sulfuric acid used in crystallization I read that:


  Take a cupful of water in a beaker and add a few drops of dilute
  sulfuric acid. Heat the water. When it starts boiling add copper
  sulfate powder slowly while stirring continuously.
  Continue adding copper sulfate powder till no more powder can be
  dissolved. Filter the solution. Allow it to cool. Do not disturb the
  solution when it is cooling. Look at the solution after some time. Can
  you see the crystals of copper sulfate? If not, wait for some more
  time.


So why is sulfuric acid required in this experiment?
The sulfuric acid will dissolve any residual copper metal or any copper oxides that exist as impurities in the copper sulfate, converting those impurities into copper sulfate.

$$\ce{H2SO4(aq) + Cu(s) -&gt; CuSO4(aq) + H2(g)}$$
$$\ce{H2SO4(aq) + CuO(s) -&gt; CuSO4(aq) + H2O(l)}$$
$$\ce{2H2SO4(aq) + Cu2O(s) -&gt; 2CuSO4(aq) + H2O(l) +H2(g)}$$