Why should all the acid be used up when making zinc sulphate crystals? So to make zinc sulphate crystals you add granulated zinc to heated sulphuric acid. However, you have to make sure all the acid is used up and you know so when the zinc stops reacting and producing bubbles "hydrogen". You then filter out the solution and set it on a water bath to crystallise it. The question is : if all the acid is not used up in the reaction, what will happen when I start to evaporate the solution of zinc sulphate?
I don't think you have really explained the experiment correctly. I'd guess that you're using a diluted solution of sulfuric acid.

$\ce{Zn + H2SO4 -&gt;[H2O] ZnSO4 + H2 ^}$

It would be difficult to have exactly the right amounts of zinc and sulfuric acid, so one or the other will be in excess. 


If zinc is in excess the solution can be filter to easily remove the
excess Zn.
If Sulfuric acid is in excess then when all the water is gone you get zinc
sulfate which is wet with sulfuric acid. The sulfuric acid won't evaporate 
on a water bath like water.