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My teacher told me if it reacts with an acid and a base, then the compound is amphoteric.

However, I am not satisfied. Is there any other way of telling? For example we are give a compound say BeO, how can we say if it's amphoteric or not?

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    $\begingroup$ Examine the structure? Do you mean, experimentally? $\endgroup$
    – Buck Thorn
    Jun 16, 2019 at 19:40
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    $\begingroup$ socratic.org/questions/… $$$$ If the substance can act like both an acid and a base, i.e., both give and accept protons, then it's amphoteric. $\endgroup$ Jun 16, 2019 at 19:41
  • $\begingroup$ Sorry to tell you but your teacher's opinion is not right, just because the "react" is not enough. You may say that if an oxide reacts with an acid and forms a salt or if an oxide reacts with a base forms a salt then you can say that it is an amphoteric oxide. Take an example of aluminum oxide, when dissolved in HCl it will form aluminium chloride, but we can also dissolve it in NaOH, where it forms sodium aluminate. $\endgroup$
    – AChem
    Jun 16, 2019 at 22:03

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Two rules that partially solve the problem:

1) Predominantly ionic oxides, such as those of magnesium and heavier alkaline earth metals, will be only basic in water, even if only weakly so due to limited solubility. You need significant covalent bonding of the metal to oxygen to get the metal to take on additional hydroxide ions in strong bases.

2) Notice I say "metal" above. Nonmetals generally form oxides that are only acidic ($\ce{CO2}, \ce{SO2}$) or inert ($\ce{CO}, \ce{NO}$). You need a metal for the oxide to react with water or acid as a base would.

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Sorry to tell you but your teacher's opinion is not right, just because the word "react" is not enough. Usually you label oxides as "amphoteric". You may say that if an oxide reacts with an acid and forms a salt or if an oxide reacts with a base forms a salt then you can say that it is an amphoteric oxide. Take the example of aluminum oxide, when dissolved in HCl it will form aluminium chloride, but we can also dissolve it in NaOH, where it forms sodium aluminate.

However, we can broaden this definition further (good for advanced classes). One formal definition is "A chemical species that behaves both as an acid and as a base is called amphoteric. This property depends upon the medium in which the species is investigated: H2SO4 is an acid when studied in water, but becomes amphoteric in superacids."

See: https://goldbook.iupac.org/html/A/A00306.html

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