How can you distinguish copper sulfate and copper carbonate?

• A. Acidify with hydrochloric acid, then add barium chloride; white precipitate forms with sulfate but not with carbonate.
• B. Add sodium hydroxide; both form blue precipitates.
• C. Add silver nitrate; both give precipitates.
• D. Heat with acid; carbonate decomposes.

Answer: (A)

The test hinges on how sulfate and carbonate ions behave in solution. Sulfate ions react with barium ions to form barium sulfate, which is an insoluble white precipitate, so a solution containing sulfate will show a solid when BaCl2 is added. Carbonate ions, when you first acidify, are converted to carbon dioxide and water, so after acidification there are no carbonate ions left to form a sulfate precipitate with barium.

So, acidify the sample with hydrochloric acid, then add barium chloride. If you see a white precipitate, the unknown was a copper salt containing sulfate (copper sulfate). If no precipitate forms, the unknown was copper carbonate, because the acid removed the carbonate as CO2 and no BaSO4 can form. This approach cleanly separates the two by exploiting the distinct reactions of sulfate versus carbonate under acidic conditions.