Barium oxide reacts with ammonium sulfate to form a white precipitate

Steps:

• Reactants are solid BaO and aqueous (NH4)2SO4.
• Reaction: BaO(s) + (NH4)2SO4(aq) → BaSO4(s) + 2NH3(g) + H2O(l).
• BaSO4 is insoluble, forming a white precipitate in the test tube.
• Warming drives the reaction forward, producing the observable precipitate.

Why B is correct:

• Barium sulfate (BaSO4) is a white, insoluble solid per solubility rules, precipitating immediately upon reaction.

Why the others are wrong:

• A: Ammonia gas evolves and turns damp red litmus blue, but "initially" implies before warming, when no gas forms yet.
• C: No nitrogen oxides form; brown gas (NO2) requires nitrates and strong oxidation, absent here.

Final answer: B