When sulfur powder burns, it reacts with oxygen in the air to form sulfur dioxide gas. This gas is soluble in water, and when it dissolves, it forms sulfurous acid. Sulfur dioxide gas is a pungent, irritating gas that has a choking odor. It is harmful to breathe, and it can cause respiratory problems, such as bronchitis, asthma, and pneumonia.

The chemical equation for the reaction between sulfur powder and oxygen is:

S + O2 → SO2

The sulfurous acid that is formed when sulfur dioxide gas dissolves in water is a weak acid. This means that it only partially dissociates in water, and it does not release many hydrogen ions (H+). The chemical equation for the dissociation of sulfurous acid is:

H2SO3 → H+ + HSO3-

The hydrogen ions that are released when sulfurous acid dissociates are what make the product formed on burning sulfur powder acidic.