Answer is: BaCO₃ (barium carbonate).
barium carbonate is white powder with low solubility in water, but good solubility in acids (with the exception of sulfuric acid), because it forms solunle salts and carbonic acid (decomposes on carbon(IV) oxide and water).
For example if we add hydrochloric acid in barium carbonate:
BaCO₃(s) + 2HCl(aq) → BaCl₂(aq) + H₂CO₃(aq).