When you add a sodium sulphate (Na₂SO₄) solution to a barium chloride (BaCl₂) solution, you observe the formation of a white precipitate. This precipitate is barium sulphate (BaSO₄).

Here's why:

* Chemical Reaction: The reaction is a double displacement reaction:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

* Solubility: Barium sulphate (BaSO₄) is insoluble in water, meaning it doesn't dissolve readily. This is why it forms a solid precipitate.

* Observation: The white precipitate will cloud the solution, and over time, it may settle to the bottom of the container.

Important Note: This reaction is often used as a demonstration of precipitation reactions in chemistry. It's also relevant in analytical chemistry for detecting the presence of barium ions.