Sulfer is a non-metal elements on burning in the presence at oxygen, it is forms SO2 and SO3 non-metal oxides.
S8(g) + O2(g) \(\overset{Burn}{\longrightarrow}\) SO2 + SO3
On dissolving these non-metal oxide in water, they are formed corresponding acids.
SO2(g) + H2O(l) → \(\underset{\text{Sulfurous acid}}{H_2SO_3(aq)}\)
SO3(g) + H2O(l) → \(\underset{\text{Sulfuric acid}}{H_2SO_4(aq)}\)
Hence, non-metal oxides in water acidic in nature.