What are the white fumes produced when ammonium hydroxide reacts with sulphuric acid ?

Answer 1

Since the reaction produces water + (a solution of) ammonium sulfate, I would assume that the fumes are from the ammonium sulfate (since water 'fumes' are invisible). However, it's possible that the heat produced from this exothermic reaction would vaporize some of the water forming steam (invisible( which cools & condenses to water vapor (the white "fumes".)

Answer 2

2NH₄OH + H₂SO₄ ==> (NH₄)₂SO₄ + 2H₂O

Hydrolysis of the NH4+ ion proceeds as follows:

NH₄⁺ + H₂O <==> NH₃ + H₃O⁺

So, you see ammonia (NH3) fumes