Question

what happens when we add HCL to BaCl2+Na2SO4=BaSO4+Nacl

Answer 1

During the reaction of BaCl₂ with Na₂SO₄, HCl is added to the reaction mixture.

As during the reaction, barium salt that is BaCl₂ can react with water and can form Ba(OH)₂ , and that can remove some of Ba²⁺ ions before it can precipitated with SO₄²⁻ . HCl is added to prevent this , that is it removes Ba(OH)₂ and changes it back to BaCl₂.