The reaction between barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄) is called a precipitation reaction because it results in the formation of a solid precipitate.

Here's why:

1. Double Displacement: When barium chloride and sodium sulfate react, they undergo a double displacement reaction, meaning the positive and negative ions switch partners.

2. Formation of Barium Sulfate: This exchange leads to the formation of barium sulfate (BaSO₄) and sodium chloride (NaCl).

3. Insolubility of Barium Sulfate: Barium sulfate is insoluble in water, meaning it doesn't dissolve to a significant extent. Therefore, it forms a solid that settles at the bottom of the reaction mixture, known as a precipitate.

4. Visible Evidence: The formation of a white, cloudy precipitate (barium sulfate) is a clear visual indication that the reaction has occurred.

In summary: The reaction between barium chloride and sodium sulfate is a precipitation reaction because it results in the formation of an insoluble solid (barium sulfate) that precipitates out of the solution.