[urtula]

Dear all,

I have always known that when making a dilution from something very reactive or dangerous you add the substance you want to dilute to the water and not the other way around.
So eg: HCl 99%, you want to dilute it, then you add the HCl to the water.
Now I know that it can react strongly, exothermic reaction or splashes.

But how do you explain it?
I always just see it as: when pouring the HCl in the water you pour slowly, so the HCl gets diluted pretty fast and thus less possibilities on an accident happening.
The other way around: the water is slowly poured on strong HCl causing less of a diluting effect and thus more possibilities on an accident.

But how do you explain this in a more scientific way?

[Arkcon]

It is all about the exothermic reaction, and the high heat capacity of water, as it dilutes a reagent.  We keep large amounts of water around without incident, and likewise, dilute solutions of acids and other reagents are safe left around.  On the other hand, concentrated acids, whether 99% pure, or 90% pure or 88.75% pure we treat with care, against the possibility of a spill or splash.  So you understand, why we dilute into a large volume of water.

[urtula]

It is all about the exothermic reaction, and the high heat capacity of water, as it dilutes a reagent.  We keep large amounts of water around without incident, and likewise, dilute solutions of acids and other reagents are safe left around.  On the other hand, concentrated acids, whether 99% pure, or 90% pure or 88.75% pure we treat with care, against the possibility of a spill or splash.  So you understand, why we dilute into a large volume of water.

Eum, I am not sure I understand you.

I understand that concentrated solutions are dangerous and you dilute them when working with them.
But thats not really my question or problem.


But its not really an explanation to my question?
Or is the eplanation simply that water has a high heat capacity and thus you should dilute in water (and not the other way around: dilute by adding the water because the HCl (eg) has not a high heat capacity thus causing more gas or splashes to come free while pouring the water in the HCl ?)

[Arkcon]

The real problem is sulfuric acid, I've seen on PBS shows that pouring water onto concentrated sulfuric acid results in the water skittering about on the surface, splashing away from its own steam.  Try to see if YouTube has a video like that for you to view.  It may even show adding acid to the water, and the hot acid pours down the water, diluting as it goes.  Try to compare the two scenarios with the description I tried to make above -- what sort of dilution is happening in each case.

[Babcock_Hall]

The density of concentrated HCl* is greater than the density of water.  If you add it to the water, as it sinks, it starts to mix in.  If you added water to the acid, the mixing might not be as easy.
EDT
*By concentrated HCl I meant commercial hydrochloric acid, which is roughly 12 M.

[mjc123]

First, to be pedantic, pure HCl is a gas. Concentrated hydrochloric acid is ca. 37% HCl.

You should be careful with concentrated acids in general, but as Arkcon says, sulfuric acid is particularly bad. There are 3 main reasons for this:
1. The mixing of water and sulfuric acid is highly exothermic, but this is not symmetrical - sulfuric acid "likes" water more than water "likes" sulfuric acid. Adding a small amount of water to conc H₂SO₄ generates about twice as much heat as adding the same amount of H₂SO₄ to water.
2. The heat capacity of H₂SO₄ is about a third that of water. So with twice the heat, you get 6 times the temperature rise. (I tried this once, adding 5 mL of one component to 50 mL of the other, with good stirring. The water temperature rose by ca. 10°, the acid by ca. 60°!)
3. Conc H₂SO₄ is very viscous, so without good stirring you don't get good mixing, or dispersal of the heat - you get a hot spot where the water is added, that can heat it above its boiling point, causing the effects described by Arkcon.

[urtula]

First, to be pedantic, pure HCl is a gas. Concentrated hydrochloric acid is ca. 37% HCl.

You should be careful with concentrated acids in general, but as Arkcon says, sulfuric acid is particularly bad. There are 3 main reasons for this:
1. The mixing of water and sulfuric acid is highly exothermic, but this is not symmetrical - sulfuric acid "likes" water more than water "likes" sulfuric acid. Adding a small amount of water to conc H₂SO₄ generates about twice as much heat as adding the same amount of H₂SO₄ to water.
2. The heat capacity of H₂SO₄ is about a third that of water. So with twice the heat, you get 6 times the temperature rise. (I tried this once, adding 5 mL of one component to 50 mL of the other, with good stirring. The water temperature rose by ca. 10°, the acid by ca. 60°!)
3. Conc H₂SO₄ is very viscous, so without good stirring you don't get good mixing, or dispersal of the heat - you get a hot spot where the water is added, that can heat it above its boiling point, causing the effects described by Arkcon.

Yes, I gave a bad example with the HCl, but the idea is the same.

Thanks for the explanation, however what do you mean with "likes", the reaction is not symmetrical. Any explanation for this? Or is this too difficult to explain in an easy way?
I supose it has to do something with a certain reaction between the elements?

[mjc123]

The simple explanation (I guess it's probably more complicated than that in reality) is that mixing is exothermic because water and sulfuric acid form strong hydrogen-bonding interactions between them. However, in order to do so, the existing hydrogen bond network in the pure compounds must be disrupted to some extent. This hydrogen bonding is stronger in water than in sulphuric acid, hence the addition of H₂SO₄ to water is less exothermic than the addition of water to H₂SO₄. Thus the graph of enthalpy versus composition is unsymmetrical - see attached plot.