Question

Water is considered an amphoteric substance. Which of the following other substances can also be considered amphoteric?
(A) ${H_2}S{O_4}$
(B) $HSO_4^ - $
(C)$SO_4^{2 - }$
(D) $CO_3^{2 - }$

Answer 1

Hint: The term amphoteric means the given substance can react with both acid and as a base. That means it can act as a proton donor as well as a proton acceptor. We need to see which species can donate protons as well as accept protons.

Complete answer: We know that a substance which acts as a proton donor and proton acceptor is called an amphoteric substance. It also acts as an acid and a base both. The most common amphoteric substance in the world is water. A hydrogen atom has an electron and a proton. It is made by the removal of an electron from it. So acid is called a proton donor and base as a proton acceptor. In the case of water it auto ionises to and it reacts with another to form hydroxide ion and hydronium ion while auto ionising. So water itself acts as a proton donor and proton acceptor. So it is an amphoteric substance.
Now coming to the options we have ${H_2}S{O_4}$ , as it is a strong acid the equilibrium favours the complete ionisation of sulphuric acid and $HSO_4^ - $ ions cant gain ${H^ + }$ and reform into ${H_2}S{O_4}$. So it is not an amphoteric substance.
Now we have bisulphate ion which acts as a base and an acid both. The reaction is given as follows
$HSO_4^ - + {H_3}{O^ + } \to {H_2}S{O_4} + {H_2}O$
Here bisulphate ion accepts a proton from hydronium ion.
 $HSO_4^ - + OH \to {H_2}O + SO_4^{2 - }$
Here bisulphate ion donates a proton to hydroxide ion. So it acts both as an acid and base.
So $HSO_4^ - $ is an amphoteric substance.

Hence the correct answer is option (B).

Note: Amphoteric behaviour depends on the oxidation state in case of oxides. Also, there is a category of amphoteric species such as amphiprotic species. These are the molecules which can either donate or accept proton examples that include amino acids and proteins.