how to know a compound is amphoteric?

Question

How do I know a compound is amphoteric? I know the definition Is it either lose or gains an H+, but how do I identify it in for example the following compounds?

CH3NH2

HSO4-

CN-

H3O+

Answer 1

I think most of this comes from experience, but you basically need to know something about Lewis structure, valence, octets, etc. Let's look at the examples given:

CH₃NH₂:

it can gain H and become CH₃NH₃⁺ but is not likely to lose H and become CH₃NH so it would not be amphoteric

HSO₄^-:

It can gain H and become H₂SO₄ (sulfuric acid) and it can also lose H and become SO₄²- (sulfate anion) so it is amphoteric.

CN^-:

It can gain H to become HCN (hydrogen cyanide, prussic acid), but it clearly can't lose H. It is not amphoteric.

H₃O⁺:

It cannot gain H but it can lose H to become H₂O. It is not amphoteric. H₂O is amphoteric however as it can both gain H and become H₃O⁺ or it can lose H to become OH^-.