IGCSE Chemistry Practice Exam 2025 - Free IGCSE Chemistry Practice Questions and Study Guide

The sequence that accurately describes the test for sulfate ions begins with the addition of dilute hydrochloric acid (HCl) followed by barium chloride (BaCl₂). When BaCl₂ is introduced to a solution containing sulfate ions, a white precipitate of barium sulfate (BaSO₄) forms. This reaction is based on the principle that barium ions react with sulfate ions to produce an insoluble salt, which is indicated by the formation of a white solid.

The initial addition of dilute HCl is crucial because it removes any carbonate ions that might be present in the solution, which could otherwise lead to the formation of a white precipitate due to the reaction with barium, misleading the test for sulfate ions. Thus, ensuring that the solution is free from carbonate impurities is essential for the accuracy of this qualitative test.

Having set the stage for the test in this manner allows for a clear identification of sulfate ions based solely on the formation of the white precipitate of barium sulfate, confirming the presence of sulfate ions in the solution.