IGCSE Science Paper 6 Practice Test 2026 - Complete Study Resource

When sulfate ions are present, adding a source of barium ions causes them to combine and form barium sulfate. This compound is almost insoluble in water, so it appears as a white solid that settles out. Because BaSO4 has a very low solubility, it forms specifically in the sulfate test, making it the characteristic precipitate. Other salts would require different anions (sulfite or carbonate or a soluble salt like BaCl2 would not precipitate under these conditions), so the white precipitate you observe is barium sulfate.