Explain why a solution of salt in water is considered a mixture and not a compound.

A solution of salt in water considered as a mixture because when salt is dissolved in water it does not undergo any chemical change but only dissociates into its ions.

NaCl + $H_2$ O $\to$ N$a^{+}$ + C$l^{-}$ + $H^{+}$ + O$H^{-}$