Answer:
A reaction is said to occur if there is a formation of an insoluble solid or a precipitate(s) or a liquid (l) or a gas(g).
If both the reactants and products are in aqueous state, No reaction takes place.
[tex]$2 \mathrm{NaCl}(a q)+M g B r_{2}(a q)>M g C l_{2}(a q)+2 \mathrm{NaBr}(a q)(\mathrm{NO} \text { REACTION })$[/tex]
All chlorides and Bromides are soluble except that of Ag, Hg and Pb.
Hence, No reaction takes place since all the reactants and products are in aqueous states.
[tex]${KOH}(a q)+{NaCl}(a q)>K C l(a q)+{NaOH}(a q)(\mathrm{NO} \text { REACTION })$[/tex]
Salts of Group IA are soluble. Hence No reaction takes place
[tex]$M g S(a q)+2 {NaOH}(a q)>M g(O H)_{2}(s)+N a_{2} S(a q)$[/tex]
(REACTION TAKES PLACE)
All hydroxides are insoluble except that of Group IA, ammonium ion and Group IIA down from Calcium.
Hence Reaction takes place with the formation of [tex]Mg(OH)_2[/tex] precipitate