The reaction of water (H2OO) and sulfuric acid (H2OSO4O) is very exothermic and gives out heat. If water is added to the concentrated sulfuric acid, it can boil and spit dangerously. One should always add the acid to the water rather than the water to the acid.
Acids reacts with water
Some of the learners also asked: Why do we need to add water slowly to the container having concentrated acids?