How is barium sulfate formed in a precipitation reaction?

Barium sulfate (${\text{BaSO}}_4$) is formed through a metathesis reaction, a type of precipitation reaction where compounds exchange their anion and cation partners.

In this process, a solution containing barium ions (${\text{Ba}}^{2+}$) is mixed with a solution containing sulfate ions (${\text{SO}}_4^{2-}$). Because barium sulfate is virtually insoluble in water, the ions combine to form a solid precipitate that separates from the solution.12

A common laboratory method to demonstrate this is by adding a solution of barium nitrate, ${\text{Ba(NO}}_3{)}_2$, to a solution of sodium sulfate, ${\text{Na}}_2{\text{SO}}_4$. This reaction is highly significant in chemistry, serving as both a qualitative and quantitative test to detect the presence or measure the concentration of sulfate or barium ions in a sample.1