[rn5a]

I have read that concentrated acids should always be diluted by adding acid to water & not by adding water to acid directly. Why so?

Also are concentrated acids heavier than water?

Thanks,

Ron

[nj_bartel]

The protonation of water by concentrated strong acid is an exothermic reaction.  It can potentially release enough heat to cause the water/acid to boil violently.

Generally, but no hard fast rule that says they have to be.

[rn5a]

Thanks mate for your response. Do you mean to say that adding concentrated acid to water will release lesser heat energy as compared to the heat energy released when water is added to a concentrated acid?

Also I am just 12 years old. I don't understand the terms "protonation" & "exothermic". I would be obliged if you could please explain me in a simpler language.

Thanks,

Ron

[Borek]

Amount of energy is the same, however, when you add acid to water evolving heat is distributed (dispersed) much faster, thus the temperature is not going that high up locally.

Protonation - its how the acids behave. They give away a proton (which is nothing else but nucleus of the hydrogen atom). Strong acids force other molecules to accept that proton, that process is called protonation.

Exothermic means that reaction produces heat (so the mixture gets hot) as opposed to endothermic - when the reaction consumes heat (and the mixture becomes cold).

[rn5a]

Do you mean to say that when you add water to acid, evolving heat is distributed much slower (as compared to adding acid to water) which may cause the mixture (of acid & water) to boil violently?

What if we add water very slowly to the concentrated acid to dilute the acid? Isn't that advisable as well?

Thanks,

Ron

[nj_bartel]

Water distributes heat within itself very well very quickly.  When you drop the water in the acid, the localized heating of the water can cause it to boil almost immediately.  The rapid formation of the steam (which occupies a much larger amount of space than the liquid water) causes it to squirt acid with it.

Slowly adding water to a concentrated acid is actually the worst thing you can do, as you don't adequately dilute in time to beat the localized heating.

[Jorriss]

You have super good grammar and writing for being 12. Kudos.

[nj_bartel]

Concur!

[Caeldom]

Likewise, the internet could do with more twelve year olds like you Ron.

Two handy ways you can remember it too:
1. Alphabetical order - A to W, or
2. "First the water, then the acid, otherwise it won't be placid!" - Not sure who came up with that one

Just adding on to that, exactly why is the protonation of water by concentrated strong acid an exothermic reaction?