Why should all the acid be used up when making zinc sulphate crystals?
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.