[poobear]

Hi!

I have always been teached that you should put acid in water, and never water in acid. However I have never been given an answer other than it can boil up if you add water in acid.

When googling I find pages like http://antoine.frostburg.edu/chem/senese/101/safety/faq/always-add-acid.shtml, also saying that it can boil up.

But could anyone please explain this reason? For me, whether I add a little acid to water or a little water to acid, the little is always the limiting factor. So if you just put the same volume of little acid or water it should not make much difference.

Example: 10 HCl molecules added to 100 H2O molecules -> 10 of the H2O molecules of the solvent will react with the 10 HCl molecules. 10 H2O molecules added to 100 HCl molecules -> 10 of the HCL molecules of the solvent will react with the 10 H2O molecules.

Thanks

[nj_bartel]

When you add an acid to a base, you form a weaker acid and a weaker base, releasing energy.

Example:  Sulfuric acid + water -> Hydronium ion + sulfate ion

Sulfuric acid = stronger acid
water = stronger base
hydronium ion = weaker acid
sulfate ion = weaker base

High energy reactants to the left, lower energy products to the right

The released energy is heat

When you add the right amount of water to an acid, you can heat the acid-water interface, releasing enough heat around that boundary to boil the water, which rapidly expands into a gas, making the acid shoot out from the container due to the volume increase

[poobear]

When you add the right amount of water to an acid, you can heat the acid-water interface, releasing enough heat around that boundary to boil the water, which rapidly expands into a gas, making the acid shoot out from the container due to the volume increase

Thanks for the reply!
But why does not the water at the interface boil up when I add acid to water?

[Borek]

Acid and heat get "diluted" quickly, so you don't get local high temperatures. This process is much slower when water is added to acid, which means local pockets with temperatures high enough to start water boiling.

[poobear]

This process is much slower when water is added to acid

Thanks for the additional reply! Do you (or anyone else) happen to know why one of the reactions is much slower? That would make me sort of understand the whole picture.

[Borek]

It is all in kinetics of mixing, water has much lower viscosity, so acid gets diluted fast, while water dropped into acid stays in its "pocket" for much longer. Plus acid is usually much denser, so it tends to go down, while water stays on the surface.

[poobear]

Ah, thanks!