I've been taught a LONG time ago to AAA "Always Add Acid". I've recently come across what feels like an exception to that rule when making piranha solution (you add the H2O2 to the acid). I'm not a chemist so this surprised me. I'm wondering, in general, when should I not add acid to solution and instead add solution to acid?

Of course the safest thing is to always read SOPs when doing something I've never done, but I'd like to understand the exceptions so I can sanity check things too.

  • $\begingroup$ This rule of thumb is usually irrelevant, unless you're diluting highly concentrated acids. It shouldn't be repeated in such vague form, but I guess it's catchy. $\endgroup$
    – Mithoron
    Commented Oct 16, 2023 at 20:56
  • 1
    $\begingroup$ AAA alludes to concentrated (98%) sulfuric acid. Other mineral acids already contain significant amounts of water. You would also normally add them to water, but with H2SO4, it's not just dangerous, but guaranteed to go wrong, badly, every time. Other mineral acids, it's not even really dangerous. $\endgroup$
    – Karl
    Commented Oct 16, 2023 at 21:01
  • 1
    $\begingroup$ The said rule does not apply when the reasons behind it fail to apply, and understanding those reasons is a mandatory prerequisite to actually doing such things in practice. $\endgroup$ Commented Oct 16, 2023 at 21:39
  • $\begingroup$ Most mineral acid solutions are denser than water. Adding tge acid -> it tends to sink through, which improves mixing. $\endgroup$ Commented Oct 16, 2023 at 23:19

1 Answer 1


For the major case for application of the "Always Add Acid" rule, the reasons are obvious. Adding water to concentrated sulfuric acid usually leads to local overheating of water. That causes local violent boiling and splashes of concentrated acid. That is avoided by adding the acid to water as there is a lot of heat capacity to avoid boiling.

From personal experience, it is good to do it in two steps, if applicable, as it is less challenging, considering cooling and manipulation with hot acid solutions.

  • Pouring the rest of the older diluted acid to water and cooling it down.
  • Pouring the concentrated acid to this new mixture.

When concentrated sulfuric acid and $\text{30+ %}$ hydrogen peroxide are to be mixed, it is choosing the smaller evil of pouring the hydrogen peroxide to the acid. This still can cause eventual smaller splashes of the acid or surface hydrogen peroxide decomposition. But it avoids local overheating and dehydrating of hydrogen peroxide solution. That could destabilize it and making it violently decompose in the whole volume, splashing most of the liquid.

Note that sulfuric acid has kinetically stabilization effect due acidic environment and at the same time destabilization effect by heating up and bonding with water. That latter increases the thermodynamic activity of the peroxide.

Preparation and manipulation of this piranha solution is always dangerous and should be reserved to experienced chemists, considering all needed precautions. It is not called so without a reason.


Not the answer you're looking for? Browse other questions tagged or ask your own question.