Concentrated sulfuric acid (oil of vitriol, H₂SO₄): the strong acid at about 96–98 % by weight, density about 1.84 g/cm³. A powerful dehydrating agent and strong acid used in pigment making, metal pickling, batteries and chemical reactions that need a non-volatile acid. Extremely corrosive and gives off great heat with water: always add acid to water, never water to acid.