How do I identify an amphoteric compound based on the acid form?

Just look at it and try adding a proton or taking away a proton. If you generate a species that has a reasonable charge or is neutral, then it probably is a reasonable amphoteric compound.

In general, the conjugate base of a polyprotic acid is a good choice.

#"HCO"_3^(-)# is amphoteric because it can react as either an acid or a base.

This shows it acting as a Bronsted base, since it accepts a proton to become its conjugate acid, carbonic acid, which decomposes into carbon dioxide gas and water liquid:

#"HCO"_3^(-)(aq) + "H"^(+)(aq) rightleftharpoons "H"_2"CO"_3(aq) rightleftharpoons "CO"_2(g) + "H"_2"O"(l)#

This shows it acting as a Bronsted acid, since it donates a proton to become its conjugate base, carbonate:

#"HCO"_3^(-)(aq) rightleftharpoons "H"^(+)(aq) + "CO"_3^(2-)(aq)#

Some other examples are:

• #"H"_2"PO"_4^(-)# (dihydrogen phosphate)
• #"HSO"_4^(-)# (hydrogen sulfate)
• #"H"_2"C"_6"H"_5"O"_7^(-)# (a form such as monosodium citrate can generate this species in water)

Can you write reactions for these, in a similar manner to the above?