Here's how it works:

1. Reaction: When barium chloride is added to a solution containing sulfate ions (SO₄²⁻), a white precipitate of barium sulfate (BaSO₄) is formed.

2. Observation: The formation of this white precipitate indicates the presence of sulfate ions in the solution.

3. Equation: The reaction can be represented by the following chemical equation:

BaCl₂ (aq) + SO₄²⁻ (aq) → BaSO₄ (s) + 2Cl⁻ (aq)

Important Note: This test is usually carried out in acidic conditions to prevent the interference of other anions that can also form precipitates with barium ions.