The solution, which is formed by dissolving the gaseous oxide in water, stains the litmus red. What kind of gas could it be?

Litmus is colored red by solutions that have an acidic medium, for example, acid solutions. Acids, in turn, can be formed by the interaction of acidic oxide with water.

Thus, this description is satisfied:

carbon monoxide (IV) (carbon dioxide): CO2 + H2O = H2CO3
sulfur (IV) oxide (sulfur dioxide): SO2 + H2O = H2SO3
nitric oxide (IV) (brown gas): 2NO2 + H2O = HNO3 + HNO2

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