Barium Sulphate Formula and Properties

The formula BaSO₄ is derived by combining the barium cation and the sulphate anion:

• The Barium ion is an alkaline earth metal in Group 2, so it has a valency of +2 (Ba²⁺).
• The Sulphate ion is a polyatomic anion with a charge of -2 (SO₄²⁻). Because the +2 and -2 charges perfectly balance each other out, they combine in a 1:1 ratio to form the neutral compound BaSO₄.

Barium Sulphate is notoriously insoluble in water. Because of this, it is used as the standard laboratory test to detect the presence of sulphate ions. If you mix aqueous Barium Chloride (BaCl₂) with aqueous Sodium Sulphate (Na₂SO₄), a double displacement reaction occurs: BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq) The BaSO₄ instantly drops out of the clear solution as a thick, milky white precipitate.

Even though Barium is highly toxic to humans, Barium Sulphate is perfectly safe to swallow. Why? Because it is completely insoluble, meaning it does not dissolve into the bloodstream. In medicine, a patient swallows a thick liquid called a 'Barium Meal' before taking an X-ray. The heavy Barium Sulphate coats the stomach and intestines, blocking the X-rays, which creates a highly detailed, bright white image of the digestive tract on the X-ray film.