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A question about the special case of calcium hydroxide

  1. Jul 20, 2015 #1

    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!
    Last edited: Jul 20, 2015
  2. jcsd
  3. Jul 20, 2015 #2


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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.
  4. Jul 20, 2015 #3
    Ah, of course! I get it now. Thanks a lot for pointing that out. I really appreciate the help!
  5. Jul 20, 2015 #4
    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!
  6. Jul 20, 2015 #5


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    Science Advisor

    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.
  7. Jul 20, 2015 #6
    I see! Thank you again for the response! Much appreciated!
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