H2SO4 is not hot at room temperature.It is the reaaction of water that causes a highly exotheric reactioni.e The hydration reaction of sulfuric acid is highly exothermic. If water is added to the concentrated sulfuric acid, it can react, boil and spit dangerously. One should always add the acid to the water rather than the water to the acid. The necessity for this safety precaution is due to the relative densities of these two liquids. Water is less dense than sulfuric acid, meaning water will tend to float on top of this acid. The reaction is best thought of as forming hydronium ions, by
- H2SO4 + H2O → H3O+ + HSO4−,
- HSO4− + H2O → H3O+ + SO42−.