Answer :
Balanced chemical reaction : It is defined as the number of atoms of individual elements present on reactant side must be equal to the product side.
Double-displacement reaction : It is defined as the reaction in which the cation of two reactants molecule exchange their places to give two different products.
(a) The balanced chemical reaction will be:
[tex]2HBr(aq)+NiS(s)\rightarrow NiBr_2(aq)+H_2S(g)[/tex]
(b) The balanced chemical reaction will be:
[tex]NH_4I(aq)+NaOH(aq)\rightarrow NaI(aq)+NH_3(g)+H_2O(l)[/tex]
(c) The balanced chemical reaction will be:
[tex]2HBr(aq)+Na_2S(aq)\rightarrow 2NaBr(aq)+H_2S(g)[/tex]
(d) The balanced chemical reaction will be:
[tex]2HClO_4(aq)+Li_2CO_3(aq)\rightarrow 2LiClO_4(aq)+CO_2(g)+H_2O(l)[/tex]
