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This is the information I found regarding orthoboric acid's solubility:

  1. Textbook (NCERT India): Sparingly soluble in water (didn't say what temperature - presumably cold) and highly soluble in hot water.

  2. Wikipedia: "Boric acid is soluble in boiling water."

My question: why is it only sparingly soluble in cold water even though it has polar $\ce{B-OH}$ bonds? Also, what happens on increasing the temperature that increases the solubility?

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    $\begingroup$ It's mostly because orthoboric acid doesn't react with water the way other acids do, while other acids dissociate into protons and conjugate bases, orthoboric acid actually absorbs the hydroxyl from the water, thus releasing protons from the water itself. I'm sure a better answer is on the way though... $\endgroup$ – Abhigyan Chattopadhyay May 3 '18 at 14:59
  • $\begingroup$ The pKa of all the hydrogens are 9.2, 12.4, and 13.3, so it doesn't dissociate in neutral water (en.wikipedia.org/wiki/Boric_acid). This also suggests the molecule is less polar, so solubility will be lower. $\endgroup$ – ericksonla May 3 '18 at 15:34
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    $\begingroup$ see section 2.1 of this: digital.library.unt.edu/ark:/67531/metadc843233/m2/1/high_res_d/… $\endgroup$ – DavePhD May 3 '18 at 18:40

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