A, B and C are three elements which undergo chemical changes as shown below:
\[
  {\text{(i) }}{{\text{A}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2 B }} \to {\text{ }}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2 A}} \\
  {\text{(ii) 3CS}}{{\text{O}}_4}{\text{ + 2 B }} \to {\text{ }}{{\text{B}}_2}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_3}{\text{ + 3 C}} \\
  {\text{(iii) 3 CO + 2 A }} \to {\text{ }}{{\text{A}}_2}{{\text{O}}_3}{\text{ + 3 C }}
\]
Name the most reactive and the least reactive element.

Hint:A more reactive element can displace the less reactive element from its salt solution. A less reactive element cannot displace the more reactive element from its salt solution.

Complete answer:
In the reaction \[{\text{(i) }}{{\text{A}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2 B }} \to {\text{ }}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2 A}}\] the element B replaces the element A from its salt solution. Hence, the element B is more reactive than the element A.
In the reaction \[{\text{(ii) 3CS}}{{\text{O}}_4}{\text{ + 2 B }} \to {\text{ }}{{\text{B}}_2}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_3}{\text{ + 3 C}}\] the element B replaces the element C from its salt solution. Hence, the element B is more reactive than the element C.
Thus, among the elements A, B and C, the most reactive element is the element B.
In the reaction \[{\text{(iii) 3 CO + 2 A }} \to {\text{ }}{{\text{A}}_2}{{\text{O}}_3}{\text{ + 3 C}}\] the element A replaces the element C from its salt solution. Hence, the element A is more reactive than the element C.
Hence, the order of reactivity of elements is \[{\text{B > A > C}}\]

Thus, among the elements A, B and C, the least reactive element is the element C and most reactive element is B.

Note:
The reactions in this question are single displacement reactions. In these reactions, one element displaces another element from its salt solution. Similar type of reaction is double displacement reactions in which both cations and anions are exchanged between two reactants to form different products. Write the general reaction for the double displacement reaction as follows:
\[{\text{AB + CD}} \to {\text{AC + BD}}\]