[AhBeng]

A variety of sources, including Wikipedia and several chemistry websites, stated (without explanation) that the pH of both anhydrous and hydrated calcium chloride, are basic/alkaline.

Based on the polarizing power of Mg2+ ion, we expect a solution of MgCl2 to be slightly acidic. Based on similar charge density of the Ca2+ ion with Na+ ion, we expect a solution of CaCl2 to be neutral (like NaCl).

So why is CaCl2(s) and CaCl2(aq) basic/alkaline?

Thanks in advance for any replies!

[BluRay]

A variety of sources, including Wikipedia and several chemistry websites, stated (without explanation) that the pH of both anhydrous and hydrated calcium chloride, are basic/alkaline.

Based on the polarizing power of Mg²⁺ ion, we expect a solution of MgCl₂ to be slightly acidic. Based on similar charge density of the Ca²⁺ ion with Na⁺ ion, we expect a solution of CaCl₂ to be neutral (like NaCl).

So why is CaCl₂(s) and CaCl₂(aq) basic/alkaline?

Because it's not...
Ca²⁺ + 2H₂O <--> Ca(OH)₂ + 2H⁺

[MrTeo]

Ca²⁺ + 2H₂O <--> Ca(OH)₂ + 2H⁺

Er... what do you think of this?

Cl^-+H₂O=HCl+OH^-

I've never seen something like this... probably beacuse both HCl and Ca(OH)₂ are strong acids/bases so no hydrolysis occurs 

Anyway I don't think that CaCl₂ is alkaline too (or if it is there's a reason unknown to me)...

[renge ishyo]

This reminds me of that topic where I got into trouble suggesting to a beginning chem student that NaCl was slightly basic:

http://www.chemicalforums.com/index.php?topic=35427.msg135549#msg135549

The best answer is that CaCl₂ is neither basic nor acidic (it's a salt).

If you were pressed to explain why the CaCl₂ solution would be "slightly basic" then you can "rationalize it" I suppose by the fact that for every one calcium ion that is released you release two chloride ions. These chloride ions are VERY weakly basic and so they can liberate OH^- ions to some extent. The problem there as before is that Cl^- is simply very weak. So much so that the acidic byproduct of the reaction of Ca²⁺ with water actually competes with it. In fact, you can argue as Borek did before that the final result should be the other way around. So it is best to classify CaCl₂ as neither acidic or basic if you possibly can.

In reality, I think the basic pH of 8-9 that can be observed when CaCl₂ is added to solution probably arises in a complex way. For example, after dissolving calcium chloride in water it can form calcium hydroxide as shown by blue ray above. Calcium hydroxide can further react with CO₂ in the air to form calcium carbonate which in water will dissolve to some extent to form bicarbonate ion (HCO₃^-). The presence of this ion would also contribute to a slightly raised pH. The extent of all of the above depends on the concentrations of the species involved as well. For instance, the wiki also lists that CaCl₂ can be present at a slightly acidic pH (see the wiki for the hydrated compound which can be between 6.5-8) depending on the circumstances. Saying CaCl₂ is neither acidic or basic is still the best answer.

[BluRay]

Ca²⁺ + 2H₂O <--> Ca(OH)₂ + 2H⁺

Er... what do you think of this?

Cl^-+H₂O=HCl+OH^-

No.
Cl^- does not hydrolize (in significant amount), Ca²⁺ does (even if very little).
Remember that Ca(OH)₂ solubility is small:
Ca(OH)₂  Ca²⁺ + 2OH^-;   Ksp ~ 10^-5

However the following equilibrium is more clarifying:
Ca²⁺ + H₂O  (CaOH)⁺ + H⁺;   Ka ~ 10^-12.7
(from: G.Charlot - "Qualitative Chemical Analysis")

With the previous reaction only, an 1M solution of CaCl₂ would have a pH = 6.4 (if I made the computation correctly ). So it's just slightly acidic, but it is.

[MrTeo]

However the following equilibrium is more clarifying:
Ca²⁺ + H₂O  (CaOH)⁺ + H⁺;   Ka ~ 10^-12.7
(from: G.Charlot - "Qualitative Chemical Analysis")

With the previous reaction only, an 1M solution of CaCl₂ would have a pH = 6.4 (if I made the computation correctly ). So it's just slightly acidic, but it is.

Now that's more like it, even if I think that Mr.Charlot should write those H⁺ as H₃O⁺ 
Anyway I simply wanted to state that an equation like that was wrong (you can't image Ca²⁺ hydrolysing that way, moreover taking 2OH^- from water... just as you can't think Cl^- taking H⁺ (or you can hardly imagine it  ))

[Borek]

Just below 7 would be my bet without checking.

Could be whoever states that it is slightly alkalic refers to CaCl₂ industrially available in bulk quantities - it is produced by heating CaO/H₂O/NH₄Cl mixture in Solvay process, so it can contain some tiny amounts of Ca(OH)₂. But that's my guess only.

[BluRay]

Just below 7 would be my bet without checking.

Could be whoever states that it is slightly alkalic refers to CaCl₂ industrially available in bulk quantities - it is produced by heating CaO/H₂O/NH₄Cl mixture in Solvay process, so it can contain some tiny amounts of Ca(OH)₂. But that's my guess only.

Actually, I think I've understood what the OP and renge ishyo refers to: wikipedia says the the *pKa* of calcium chloride is ~ 8-9, not that the solution has such values of pH!!!!!!

http://en.wikipedia.org/wiki/Calcium_chloride

For the OP: pKa = 9 means Ka = acidity constant = 10^-9.

For comparison, ammonium ion NH₄⁺ has a Ka = 5.8*10^-10 and so would be less acid than Ca²⁺.

[renge ishyo]

You are correct Blueray! Yeah, i just glanced at the numbers and didn't even notice the pKa next to it! That would be where the confusion comes from. Yeah, that's not equal to the pH ranges for the salt.

[AhBeng]

Thanks a lot, BluRay and everyone else who replied!

Amazing how misreading / misinterpreting a little word can lead to sooooo much frustration. Heh.