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I saw as an example that water is amphoteric (to be more precise amphiprotic), and I came up with the following questions:

Is there any substance that acts as a weak acid and a strong bases at the same time, or vice versa?

If so, or if not so, please give some explanations on the possible reasoning using their structures.

Also I have another question which is quite related but too short to be posted as an independent question, so I will ask here. The necessary condition for a substance to be amphoteric, as I heard, is that for it should have a hydrogen atom in it, and also the molecule should either have a lone pair of electron or be negatively charged. I understand the conditions about hydrogen atom and an electron pair, but why can the negatively charged property possibly be a necessary condition for a molecule to be amphoteric?

Thanks in advance.

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    $\begingroup$ OH- in NaOH is a very weak acid and O^2- extremely strong base, but NaOH melted with Na would produce Na2O and H2. $\endgroup$ – Poutnik Feb 11 at 7:51
  • $\begingroup$ @Poutnik So is there any systematic rule for amphoteric substances or do properties differ for every substance? $\endgroup$ – curious Feb 11 at 7:53
  • $\begingroup$ Aside of amphoteric properties they share, their properties differ, but that is obvious. There must be 2 conjugate acid/base pairs that share 1 member. $\endgroup$ – Poutnik Feb 11 at 7:54
  • $\begingroup$ @Poutnik Sorry I was being ambiguous, I was talking specifically about amphoteric properties $\endgroup$ – curious Feb 11 at 7:55
  • $\begingroup$ Ask yourself, what do acidities of 2 different hydrogens of the same molecule/ion structure have in common ? Nothing in general. They can reside on the same atom as one extreme, or can be separated by many different atoms as the other extreme. $\endgroup$ – Poutnik Feb 11 at 8:18

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