A question about the special case of calcium hydroxide

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Discussion Overview

The discussion revolves around the classification of calcium hydroxide as a strong base despite its limited solubility in water. Participants explore the definitions of solubility and dissociation, and how these concepts relate to the strength of a base. The conversation also touches on the net ionic equation for acid-base neutralization involving calcium hydroxide and a strong acid.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how a substance that is only slightly soluble can be considered a strong base, suggesting that calcium hydroxide should be classified as a weak base due to its limited dissociation into ions.
  • Another participant clarifies that solubility and dissociation are distinct concepts, indicating that the classification of a base as strong or weak depends on the dissociation constant K_B.
  • A follow-up question is raised regarding a web page's claim that not all hydroxides are strong bases due to solubility, prompting a discussion about the ambiguity in defining base strength.
  • Some participants note that there are multiple ways to assess the strength of a base, including K_B and the maximum pH achievable, which may lead to varying interpretations of base strength.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between solubility and base strength, with some emphasizing dissociation constants while others reference solubility. The discussion remains unresolved regarding the implications of these concepts on the classification of calcium hydroxide.

Contextual Notes

Participants highlight that the definitions of solubility and dissociation may depend on specific contexts, and there is no consensus on how these definitions impact the classification of bases.

Dong Aleta
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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!
 
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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.
 
DrDu said:
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!
 
DrDu said:
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!
 
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.
 
DrDu said:
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!
 

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