barium chloride + sodium sulfate → sodium chloride + barium sulfate.
BaCl2(aq) + Na2SO4(aq) → 2NaCl(aq) + BaSO4(s).
The hydrochloric acid is added first to remove any carbonate ions that might be present - they would also produce a white precipitate, giving a false positive result.
Barium nitrate solution can be used instead of barium chloride solution. However, nitric acid is added first to acidify the test solution. Sulfuric acid cannot be used because it contains sulfate ions - these would interfere with the second part of the test.
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