# How can we know that which salt is soluble or which is not ?

1. Jul 23, 2011

### sankalpmittal

There is one topic in my chemistry book which is driving me nuts .

There is given a solubility chart showing that all Nitrates of metals are soluble without exception .

All chlorides are soluble except AgCl and PbCl2

I am wondering how do we know which is soluble or which is not (or checking a precipitate ) ?
By its Bond energy ?

Please kindly illustrate it comprehensively .

( I'm 14 years , 10th class . So I may not know to that extent as you people. )

2. Jul 23, 2011

### SpectraCat

For the most part, these solubility rules are known experimentally from trial and error. I am not aware of any simple rules to predict solubilities of salts from first principles. When I teach this to my students, I simply have them memorize the same empirical rules that you are encountering now.

The most useful ones are:

1) All salts involving sodium (Na+), potassium (K+), ammonium (NH3+), nitrate (NO3-) or acetate (C2H3O2-) are always soluble.

2) Almost all salts involving chloride (Cl-) , bromide (Br-), and iodide (I-) are soluble .. notable exceptions being salts of silver (Ag+) or lead (II) (Pb2+).

3) Almost all sulfates (SO42-) are soluble, with the notable exceptions being barium (Ba2+), strontium (Sr2+) and calcium (Ca2+). Lead (II) sulfate is only slightly soluble.

4) Almost all hydroxide salts are *insoluble*. Hydroxides of calcium, strontium and barium are slightly soluble.

5) All carbonate (CO32-) and phosphate (PO43-)salts are insoluble.

The way you use a table like this is work down from the top. So, if you have to deal with something like sodium phosphate (Na3PO4), you know it is soluble, since you apply rule 1 before rule 5.

Last edited: Jul 23, 2011
3. Jul 24, 2011

### sankalpmittal

I know you are pretty correct , Spectra cat .
When I searched it on google the same problem which I was countering , One guy said that it has to do with the capacity of ionic equilibrium .

Now I am far more confused .

4. Jul 24, 2011

### SpectraCat

What are you confused about? I might be able to help you understand, but I need to know more details about what is confusing you.

5. Jul 24, 2011

### sankalpmittal

Is it the weight of Silver in AgCl because of which Agcl forms precipitate and dont dissolve or is it its bond type or pattern . ?

Can you please explain it ?

6. Jul 24, 2011

### Mike H

If you are familiar with electronegativity, you can - very approximately - rationalize why some salts are insoluble or sparingly soluble. I am doubtful if you can explain away all exceptions using this method off the top of my head, though.

Silver chloride, if one looks at the difference in electronegativities, is more covalent in nature than something like sodium chloride but more polar than something like methane.

7. Jul 25, 2011

### sankalpmittal

AgCl <-----------------------> Ag+ + Cl-

Now Ag is heavy , right . So why don't it settle down ?
How can AgCl be covalent ? It is purely ionic or electrovalent . Nothing is been shared .
Please illustrate it in more depth .

Does it has some kind of theory illustrated in Solubility equilibrium or something ?
http://web.fccj.org/~ethall/2046/ch19/solubility.htm [Broken] ?

Last edited by a moderator: May 5, 2017
8. Jul 25, 2011

### SpectraCat

Purely ionic bonds are hard to find. Almost all bonds have some covalent character. The amount of covalent character can be estimated by comparing the Pauling electronegativities of the bonded atoms, as explained by Mike H already. The more similar the electronegativities, the more covalent character the bond will have. The simple rules you learned about electron "transfer" or "sharing" are simply guidelines that help you understand the limiting cases. For many molecules, the actual bonding situation is quite close to a given pure electrostatic or pure covalent cases, but sometime it is not. The classic example is hydrogen fluoride (HF) ... would you say that is covalent or electrostatically bonded?

Your ideas about silver being "heavy" are not proceeding along the right track. For example, gold chloride is soluble, and gold is in the same group but is much heavier than silver.

Solubility equilibria exist for all salts, whether or not we describe them as "soluble". In real terms, what determines whether or not a salt is soluble is whether or not the Gibbs free energy of the system (solute and solvent) is lowered by having the solid salt dissociate into the solvent. Remember, for a process to be spontaneous at a given temperature, the Gibbs free energy change: $\Delta G=\Delta H - T\Delta S$ must be less than zero. In all cases, the entropy of the system is raised ($\Delta S > 0$) when the salt dissolves, but sometimes this is counterbalanced by a large enthalpy penalty ($\Delta H > 0$). Basically, the salt crystal has a large lattice stabilization associated with the organization of so many charged particles into a very favorable structure, so sometimes (particularly when you have multiply-charged ions) it takes more free energy to break the lattice apart than is regained from the positive entropy change associated with the dissociated state at the appropriate temperature, and so the dissolution process is not spontaneous.

Last edited by a moderator: May 5, 2017
9. Jul 25, 2011

### sankalpmittal

I am too young for all this . However you do mean that we cannot even say for sure which type of bond a particular compound possess ? You mean that there is no actual proof of question I have put up ? You mean that these are just observations or experimental proofs , right because the human knowledge is still limited .

Last edited: Jul 25, 2011
10. Jul 25, 2011

### Mike H

As SpectraCat noted, nearly all bonds have covalent nature. It's not a matter of ionic vs covalent - the question will frequently be, "how much ionic character and how much covalent character to this particular chemical bond?" That's why there is, for example, the notion of a "polar covalent" bond to indicate that it is not purely a covalent bond.

11. Jul 29, 2011

### sankalpmittal

Ok , How can AgCl possess the characteristics of covalent bond ?

:)

12. Jul 29, 2011

### Mike H

I think it would be very helpful to read the following webpages by Phil Grandinetti (Professor of Chemistry at Ohio State University) to get a short introduction to electronegativity and its use in understanding chemical bonding.

http://www.grandinetti.org/Teaching/Chem121/Lectures/Electronegativity

http://www.grandinetti.org/Teaching/Chem121/Lectures/BondPolarity

If you still have questions afterwards, feel free to ask.

13. Jul 30, 2011

### sankalpmittal

The site tells that ionic or electrovalent bond does not necessarily depend on ionization potential or electron affinity but on the difference of charge in two systems given by (delta)+ and (delta)- , electric dipole movement for particular leading to attraction .

So let me draw out the conclusion .
AgCl<--->Ag+ + Cl-

Ag is very low in activity series :
K
Na
Ba
Ca
Mg
Al
Zn
Fe
Pb
H
Cu
Hg
Ag
Au
Pt

So there isn't much attractive force developed and its compounds are more covalent in character and so it settles in aqueous solution

Right ?

:)

14. Jul 31, 2011

### Mike H

That's correct - silver chloride is more covalent in character and it is less preferable for it to give up an electron when in solution than something like sodium or potassium.

Of course, this sort of very crude analysis works well for silver chloride, not all exceptions in the solubility rules. If you want a more general understanding, you will need to progress in your studies and begin to understand the issues of solubility equilibria and the thermodynamics thereof, as was briefly mentioned above. In the meantime, learning descriptive/qualitative chemistry is surprisingly helpful in many cases.

15. Aug 1, 2011

### sankalpmittal

Haha !!:rofl:

If I started studying solubility equilibria now , then it may disturb my class 10th performance as it is the case of board . ( Boards are the exams in India held at national level in class 10th and class 12th . They are the exams on 12 subjects in class 10th : History, geography , Evs , bla bla . Boards give question only from text books, from any council prescribed text books and they want ie the council , the bookish answers ! )
That is why it may spoil my class performance . Solubility equilibiria comes in class 12th in India.

16. Aug 1, 2011

### Mike H

There are other concepts which can be helpful in understanding solution chemistry - one notable example is the idea of hard and soft acids & bases - but it is usually something that is introduced when studying organic/inorganic chemistry at a university level (in my experience, at least). And quite frankly, even that is fraught with its own problems and exceptions.

I realize it can be frustrating that the reasoning isn't more transparent and straightforward. Things do start to make more sense after you spend more time thinking about them, I promise. :)

17. Aug 2, 2011

### sankalpmittal

:)

Last edited by a moderator: Apr 26, 2017
18. Aug 2, 2011

### Saitama

I think the boards for class 10th are removed, so i think you will have a lot of time for other studies too.

(BTW, i just passed out 10th)

You are going to come across them in class 11th.

19. Aug 3, 2011

### sankalpmittal

Are you in CBSE board ? I think that in CBSE , they have made boards optional in class 10th .

But I am in ICSE board . The boards are compulsory there .

Anyways , which one is better ?

20. Aug 3, 2011

### Saitama

Yes, i am in CBSE board, i thought that you are also in CBSE board.

Can't say, i have never gone through the ICSE syllabus.
May be mishrashubham can help you, i don't know whether he is from ICSE or CBSE but you can still ask him.
Or try contacting other Indian members here on the board.
I am not sure but i think PiBond is also Indian.