 In the double displacement reaction of a metal sulfite with an acid, we'll start out with white sodium sulfite and add an aqueous solution of hydrochloric acid. The sulfite and the hydrochloric acid react to form sulfurous acid and sodium chloride. The sulfurous acid is not stable and decomposes to form water and sulfur dioxide. The sulfur dioxide escapes from the solution. We can test for the formation of sulfur dioxide by bringing in a moist piece of blue litmus paper. The sulfur dioxide dissolves in the water to form sulfurous acid which turns the blue litmus paper red.