A question about the special case of calcium hydroxide

[Dong Aleta]

Hi!

I am having trouble understanding how a substance only slightly soluble in water is considered as a strong base. Isn't the definition of a base a substance that will increase the amount of OH^- in a solution? In that case, shouldn't calcium hydroxide be considered a weak base because of its limited ability to dissociate into Ca⁺ and OH^- ions? Can someone explain this please?

I am seeking to understand this because I don't know how to express the net ionic equation for the acid-base neutralization involving Ca(OH)₂ and a strong acid.

Will it be:

Ca(OH)₂ (aq) + 2H⁺ (aq) -> Ca²⁺ (aq) + H₂O (l)

or just simply:

OH^- (aq) + H⁺ -> H₂O (l)

Thanks so much!

 

[DrDu]

Solubility and dissociation are two different things. One is described by solubility constant, the other one by dissociation constant. Whether a base is a strong base, depends solely on the dissociation constant K_B.

 

[Dong Aleta]

Solubility and dissociation are two different things. One is described by solubility constant, the other one by dissociation constant. Whether a base is a strong base, depends solely on the dissociation constant K_B.

Ah, of course! I get it now. Thanks a lot for pointing that out. I really appreciate the help!

 

[Dong Aleta]

Solubility and dissociation are two different things. One is described by solubility constant, the other one by dissociation constant. Whether a base is a strong base, depends solely on the dissociation constant K_B.

Sorry, I'd like to add a follow-up question. I just came across this web page that said "Not all hydroxides are strong bases since not all hydroxides are highly soluble" suggesting that solubility has something to do with the basicity of the substance. Is the web page incorrect with its statement?

Here is a link to that web page: http://ch302.cm.utexas.edu/chemEQ/ab-theory/selector.php?name=strong-bases#q1b

Thanks so much!

 

[DrDu]

Well, there are simply several way to judge the strength of a base. Either via the K_B or via the maximal pH you can reach. Hence talking about the strength of a base in plain language, this may sometimes be ambiguous.

 

[Dong Aleta]

Well, there are simply several way to judge the strength of a base. Either via the K_B or via the maximal pH you can reach. Hence talking about the strength of a base in plain language, this may sometimes be ambiguous.

I see! Thank you again for the response! Much appreciated!