Why does sulphuric acid react differently depending on whether it is added to water or water is added to it?
You should never do the latter...........
And I have said before here that
When an acid is added to water, the bulk of the solution, the water PLUS the aqueous acid, heats up as the acid is solvated......When water is added to acid, mixing is never instantaneous and the water droplet is solvated causing a hot spot, which might bubble and spit. With the reverse addition, acid to water, it will still get hot, but the bulk of the solution heats up, and it heats up globally not locally.
I am hesitant to relay these facts, because it might tempt the more experimentally minded to test these propositions (I know I did when I was an undergrad!, and it does spit). Anyway, whatever you do you MUST wear safety spectacles, and a laboratory coat to protect your clothing.
I have said the same thing here.