# 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?

$\ce{Zn + H2SO4 ->[H2O] ZnSO4 + H2 ^}$