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There is a rule stating that we shall add a strong acid to water, and not the other way because of safety; if we would add water to the acid, the reaction could be dangerous (boil).

What about adding strong bases and water? Which is the safer order to mix them?

I would like to know the safer addition orders, and the justification, for water+acid or water+base(?)/base+water(?). Thank you!

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    $\begingroup$ Same guidelines. Another big issue is that if something splashes, it'll be whatever you're adding into, so much safer to add to water. $\endgroup$ – Zhe Apr 26 '17 at 18:47
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The reason for adding strong acids or bases to water rather than the other way around is that the dissolution/reaction of these compounds with water tends to be very exothermic and can result in splattering or even boiling of the strong acid or base, particularly if water is added to the concentrated acid or base.

On the other hand, if you slowly add the acid or base to water, you will never have a concentrated acid or base present to splatter as it will be rapidly diluted by the much larger amount of water. Even if you accidentally add the acid or base too quickly, at least the splattering will primarily be of water or dilute acid/base rather than splattering concentrated strong acid or base. It is still important to do the addition slowly so that you only slowly increase the acidity/basicity of the solution. This way, the rate of reaction (and thus also the heat evolution) will happen at a safe level and greatly reduce the risk of splattering.

As a side note, this concept does not only apply to the addition of strong acids and bases to water. While mixing any chemical reagents (unless otherwise specified of course), you should add the more reactive or concentrated solution to the less reactive or more dilute solution, for the same reasons as described for the addition of strong acids and bases to water.

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  • $\begingroup$ I was thinking about that... and I predict that there are some exceptions... so you should rewrite your last paragraph, to advert that there are exceptions! For instance, image chain reactions between a common and a "more dangerous" material... $\endgroup$ – sissi_luaty May 2 '17 at 0:07
  • $\begingroup$ @sissi_luaty , I'm sorry but I don't understand what you are asking of me. Could you give an example? Should I take the last paragraph out so that the answer sticks to your question of just acids and bases. I'm glad to help, I just need some clarification. $\endgroup$ – airhuff May 2 '17 at 0:12
  • $\begingroup$ The fact is that the words "in general" can be tricky in this context... it's not clear from your text if the last rule is for all the cases or for most of the cases. - after pondering about that I suppose that there are some exeptions... $\endgroup$ – sissi_luaty May 2 '17 at 4:13
  • $\begingroup$ Do not remove the extra part, but please clarify that (because one exception could mean a bang!) with a minor edit... thank you! $\endgroup$ – sissi_luaty May 2 '17 at 4:16
  • $\begingroup$ Yea, I've gotten into the habit of (over) using terms like "in general" because there are so few absolutes in chemistry! I can definitely see how that backfires here and makes people wonder what the exceptions are. But I honestly can't imagine a case in which you would do things the other way around, that is adding a less concentrated reagent to a more concentrated one. That's the kind of thing that if there is an exception out there, it would need to be made perfectly clear on the bottle, in training, the SOP, etc. I think the thing for me to do here is word it in more definitive terms. $\endgroup$ – airhuff May 2 '17 at 4:28

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