What happens when moist blue litmus paper is introduced in the jar of sulphur dioxide?

Sulphur becomes sulphur dioxide when sulphur is burned in a blue flame of oxygen gas. Sulphur dioxide reacts to the addition of water, becoming acidic, that is an acid solution. It is then correctly names sulphuric acid. It will change the blue litmus paper to red, but if the litmus paper is red to begin with, the colour does not change.

When combined with oxides other colours occur. This is why experiments with sulphur are popular with science teachers because the result is easily visible, prompts questions from the class and the various results are easily recorded.