An experiment showing burning of sulfur in oxygen. A flow-chamber joined to gas washing-bottle (filled with solution of methyl orange) is being used. The product is sulfur dioxide (SO2) with some traces of sulfur trioxide (SO3). The 'smoke' that exits the gas-washing bottle is, in fact, a sulfuric acid fog generated in a reaction represented by:
SO3(g) + H2O(l) = H2SO4(aq).
The oxides are acidic, as can be witnessed by the colour change of the indicator.