What is indicated by the formation of a white precipitate when adding barium chloride to a solution?

The formation of a white precipitate upon adding barium chloride to a solution indicates the presence of sulfate ions. Barium chloride reacts with sulfate ions to form barium sulfate (BaSO4), which is insoluble in water and appears as a white precipitate. This reaction can be summarized by the following equation:

[ \text{Ba}^{2+} (aq) + \text{SO}_4^{2-} (aq) \rightarrow \text{BaSO}_4 (s) ]

Here, the barium ions from barium chloride combine with sulfate ions to create solid barium sulfate.

In contrast, the other ions mentioned in the choices do not lead to the formation of a white precipitate with barium chloride. Chloride ions react with barium to form soluble barium chloride, nitrate ions do not typically precipitate with barium, and hydroxide ions can form a different barium compound that is also soluble. Therefore, the specific formation of a white precipitate provides strong evidence of the presence of sulfate ions in the solution.