For dilution of cone. H2SO4 , the cone, acid is always added to water and water is never added to the cone, acid even though heat is evolved in both cases.
Reason: If water is added to cone. H2SO4 the heat produced is sufficient to spontaneously vaporise a part of the few drops of water added. This is because the amount of water is very small and bioling point of water is much lower than cone. H2SO4 , which is in bulk. Due to this sudden vaporisation of water cone, acid tend to spurt out and cause serious injuries.
On the other hand, if cone. H2SO4 is added to water, the heat produced in this case can only raise the temperature of water slightly because water is an bulk. Thus, in this case spurting of the cone, acid is avoided.
