A colourless gas with characteristics odour which dissolves in water to form an alkaline solution is

Step 1: Analyze the question We need a gas that is:

1. Colourless
2. Has a characteristic (pungent) odour
3. Dissolves in water to form an alkaline solution

Step 2: Concept Gases that dissolve in water to form alkaline solutions are basic gases. Most common gases are acidic (non-metal oxides, hydrogen halides). The main basic gas you need to know at this level is ammonia (NH₃).

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

The OH⁻ ions produced make the solution alkaline. It turns red litmus paper blue.

Step 3: Check each option

• A. Ammonia: colourless, pungent smell, forms an alkaline solution (ammonium hydroxide). ✔
• B. Carbon(IV) oxide: odourless, forms weakly acidic carbonic acid. ✘
• C. Hydrogen chloride: colourless and pungent, but forms hydrochloric acid (acidic). ✘
• D. Sulphur(IV) oxide: colourless and pungent (choking), but forms sulphurous acid (acidic). ✘
• E. Sulphur(VI) oxide: forms sulphuric acid (strongly acidic). ✘

Step 4: Answer The correct option is A. ammonia.

Tip: Ammonia is the only common alkaline gas. If a question says "gas turns moist red litmus blue" or "forms an alkaline solution", think ammonia immediately. All the others (CO₂, HCl, SO₂, SO₃, NO₂) are acidic.