Many acids generate a great deal of heat when added to water; in fact, some generate enough heat to actually boil the water. Therefore, adding a drop or two of water to concentrated acid generates enough heat to spatter the water along with some of the acid. That may result in burns to the face, hands, and eyes. Adding acid to water means it is the weakest at the beginning and that much heat is generated but it is slower. Therefore, you are going from weakest to strongest if adding acid to water but going strongest to weakest if adding water to acid.
