Which of the following gases is alkaline?

Step 1: Analyze the question We need the gas that is alkaline, meaning it dissolves in water to form a basic solution (one that turns red litmus blue).

Step 2: Concept

• Most non-metal oxides are acidic. They dissolve in water to form acids.
• Very few common gases are basic. The main one is ammonia, NH₃.

Step 3: Check each option

• A. NO₂ (nitrogen dioxide): It is acidic. It dissolves in water to form nitric acid and nitrous acid: 2NO₂ + H₂O → HNO₃ + HNO₂
• B. CO₂ (carbon dioxide): It is weakly acidic. It forms carbonic acid: CO₂ + H₂O → H₂CO₃
• C. NH₃ (ammonia): It is alkaline. It dissolves in water and produces hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ The OH⁻ ions make the solution basic, and moist red litmus paper turns blue.
• D. N₂O (dinitrogen monoxide, "laughing gas"): It is a neutral oxide. It does not react with water to form an acid or a base.

Step 4: Answer Only NH₃ forms an alkaline solution, so the correct option is C.

Exam tip: NH₃ is the only common basic gas, so remember "Ammonia = the alkaline gas." You can also test for it with moist red litmus paper turning blue, and it has a pungent smell. Most other gases that come up in exams, such as CO₂, SO₂, NO₂ and HCl, are acidic.