[gavindor]

If CaO is placed in water then  we get Ca^2+(aq) and 2OH-(aq)

If we evaporate the water off, do we get  CaOH(s)  or CaO(s)?  I'm guessing CaOH(s)

So i'm wondering if it's possible to get CaO from CaOH(s)?

Thanks

[Aldebaran]

You could look up decomposition of calcium hydroxide on Wikipedia

[gavindor]

Thanks.. I see. You'd get Ca(OH)2(s) if boiling  Ca(OH)2(aq).   And yeah wikipedia mentions that given Ca(OH)2(s) you can get CaO, which I understand is thermal decomposition and happens at 512C.

So would you say it doesn't dissolve in water, (since there's a reaction)  ?

[Hunter2]

No. CaO dissolves in water together with an reaction. CaO is not existing in water, it is Ca(OH)2.

[gavindor]

No. CaO dissolves in water together with an reaction. CaO is not existing in water, it is Ca(OH)2.

ok thanks, so when you say something dissolves, do you mean, there might or might not be a reaction, but when there is it's reasonably mild?

e.g. suppose potassium or lithium is added to water, would you say it dissolves? or would you say no because it gives an explosive reaction.. not mild enough to call dissolving?

Or would you say that potassium or lithium does dissolve in water?

[Borek]

I would say it is a gray area. To some extent every dissolution is kind of a chemical reaction - ions get hydrated, organic compounds often create hydrogen bonds with water and so on. Despite of what we teach in basic chemistry there is no precise line between what is purely a physical change and what is a chemical reaction, there is a continuity of changes (easy to realize when you think in terms of reaction heat - some processes/reactions are clearly exothermic, some are clearly endothermic, for some the effect is close to zero - but it doesn't mean nothing happened, could be there are two effects canceling out).

In most cases when we talk about dissolution we mean "solid disappears into the solution and there are no obvious signs of chemical reaction occurring". Yes, it is rather handwavy.

[gavindor]

...
...
In most cases when we talk about dissolution we mean "solid disappears into the solution and there are no obvious signs of chemical reaction occurring". Yes, it is rather handwavy.

I think some people would say, NaCl breaking into its ions, isn't forming a new substance.. and even though it is technically a chemical reaction.. 'cos even dissolving can be endothermic or exothermic.. it still gets classed as a physical change(at least in basic chemistry which makes the distinction between chemical and physical change)..

But I think there is a chemical reality that an ionic compound breaking into its ions, or covalent compound breaking into ions(as some polar covalent might),  or a covalent compound breaking into molecules,  And not forming something else. And what it breaks into , forming a solution and that involves also getting solvated. That's dissolving..

CaO  I think reacts to form a suspension of CaOH_2 (and I suppose partly a solution of Ca^2+ + 2OH-)  but because it forms something else(The O^2- of it reacting with H2O to form 2OH-), many would say it doesn't dissolve. Plus with the suspension part, the word dissolve wouldn't be used, from what I understand, and the suspension is the bulk of it.  And the solution would be Ca^2+ + 2OH- so one would say CaOH_2(s) dissolves.  But not CaO.

Na2O  forms a solution of 2Na+ + 2OH-   But since the Na2O has become a solution, not of sodium oxide, but a solution of sodium hydroxide,  Na2O reacted to form something else, and the something else dissolved. So the Na2O didn't dissolve.

When it comes to HCl, it's an unusual one 'cos HCl reacts  but people still use the word dissolves.  I think maybe that is because people often write   HCl  --->  H+(aq) + Cl-(aq)      Even though the H+(aq) is really H3O+(aq). But since they write HCl solution like that, as if it didn't react(into something else), they speak of it as if it didn't react into something else, "solution of hydrogen chloride" and just say it dissolved ..    Perhaps if they were to not write H+(aq) and if they were to call it solution of hydronium chloride, then they'd not say it dissolved. So HCl is often modelled as if it dissolves eg like NaCl. (unlike Na2O where people say it reacts rather than it dissolves 'cos  with Na2O, we know O^2-(aq) doesn't exist, but also nobody writes O^2-(aq) and claims it's a shorthand for OH- , nobody writes O^2-(aq) at all. In contrast, with HCl,  H+(aq) doesn't exist but people write it as if it does, or as a shorthand for H3O+(aq) .  So they model HCl as if it were a salt, but that custum/convention hasn't been used with Na2O and so the language of "dissolving"/"solubility", gets applied to HCl but not to Na2O  ).

Do you think that understanding of the term "dissolving" is right.. And alternative to the handwavy "solid disappears into the solution and there are no obvious signs of chemical reaction occurring"?

I spoke to two chemists that took the view that NaCl dissolves(we'd all agree on that, but I include it to make a point),   Na2O doesn't dissolve 'cos it reacts to form another compound, and that compound it reacts into dissolves.  Na2O dissolving would mean getting Na+ + O^2- which we don't get.   HCl they thought   maybe an inconsistency, 'cos it does react to form something else,  H3O+ is formed in a reaction. BUT, I think that can be answered in that it is often modelled as if it dissolves  in the manner that NaCl does, of just dissociating/splitting up. It's modelled as if there's no reaction forming anything different, so, as if H+(aq) exists.   HCl --> H+(aq) + Cl-(aq) 

And NH3 dissolves because it breaks up into molecules, and while there is a reaction NH3+H2O-->NH4+ + OH-,  the equilibrium is to the left, there isn't much NH4+ + OH-.. There's enough for a pH test to pick up the high proportion of OH- relative to H+ but the vast bulk of what is there is NH3.  So, solution of NH3.. and the reaction to NH4+ + OH-, can in some sense  be considered insignificant 'cos there's so little NH4+ + OH-.

So I think dissolving is where the substance breaks into either ions, or molecules, and forms a solution of that substance. Not reacting to form something else. (with slight inconsistency with HCl but only because it often gets modelled as if it doesn't react to form something else).  What do you think of that as a definition/explanation of the meaning of the term "dissolving"?  (as opposed to  "solid disappears into the solution and there are no obvious signs of chemical reaction occurring")?

Thanks

[Borek]

No matter how precisely you will try to define, you will found counterexamples. Best approach is to use more or less lousy definition that covers most of the cases, and to remember things get complicated when you dig deeper.

Otherwise you will just waste time on nitpicking. As I often repeat: reality is (in most cases) continuous and laughs at our attempts to classify it in binary terms.

[gavindor]

No matter how precisely you will try to define, you will found counterexamples...

I'd be very interested in seeing some counter examples.. if you happen to know of any..

Thanks

[Babcock_Hall]

@OP,

You mentioned potassium in one comment.  If you mean potassium metal, that is something very different versus CaO, in that some redox chemistry happens.

[gavindor]

@OP,

You mentioned potassium in one comment.  If you mean potassium metal, that is something very different versus CaO, in that some redox chemistry happens.

yes  I was using that as an example of something that one would say doesn't dissolve, as it reacts to form something else, and is always written as a reaction..

There is a redox reaction there, but I wasn't commenting on whether or not there was a redox reaction there. But indeed a dissociation/dissolving wouldn't be redox either.  Na2O+H2O  isn't spoken of as dissolving 'cos there's a reaction there converting it to something else..  It   happens to not be a redox.  People speak of the solubility of NaOH not of Na2O. And the solution is of NaOH.  The criteria i'm using to see if something is said to dissolve or not, is   It doesn't react to form something else,  Or, it does but it's unusual in that it tends to get written as if it doesn't. Or, it does but there's such a strong reverse reaction that it's almost all as is. And it forms a solution.