Question

why are aqueous solutions of sodium sulphate and Barium Chloride used ? why not in solid form?

Answer 1

a chemical reaction between any elements/ compounds occurs when an exchange of ions between the reactants takes place.
elements/compounds undergo inonization only in presence of H2O (moisture/water)
hence aqueous solution of the respective reactants are used.

(note. Also the amount of water in the aqueous solution plays an imp. role as one can dilute or concentrate the solution as required by adding required amount of water to it)