Question

While mixing strong sulphuric acid and water, why is the acid always poured slowly into water instead of water into the acid?

Answer 1

When concentrated sulphuric acid is mixed with water, it evolves a considerable amount of heat. This heat is liberated due to the hydration of hydrogen ions derived from the acid.

H⁺ + H₂O ⟶ H₃O⁺ + Heat. 

Due to the production of heat, the water may be vaporised into steam and spill out corrosive drops of acid. To avoid this, the acid is always slowly added to water.