Discussion

A flask contains water and bromphenol blue indicator. A sample of yellow powdered sulfur in a deflagrating spoon is ignited until it burns spontaneously in air with a light blue flame. The burning sulfur is then inserted into the flask. The sulfur reacts with oxygen in the flask to form sulfur dioxide gas, which then dissolves in the water, forming sulfurous acid. The solution turns to a yellow color, indicating the acid formation. As more gas dissolves in solution, the solution becomes more acidic.

S(s) + O₂(g) → SO₂(g) (exothermic)

SO₂(g) + H₂O(l) → H₂SO₃(aq)

Narration

Water and bromphenol blue indicator are stirred in a flask. Sulfur, an impurity of coal, is burned in a deflagrating spoon. When the spoon is inserted into the flask, sulfur dioxide is formed in the synthesis reaction. The gas dissolves, forming aqueous sulfurous acid. The formation of the acid is indicated by the color change of the solution.