Which of the following gases has a characteristic pungent smell, turns red litmus paper blue and forms dense white fumes with hydrogen chloride gas?

Analyze the question: We need a gas with three properties:

1. A characteristic pungent smell
2. Turns red litmus paper blue (so it is basic/alkaline)
3. Forms dense white fumes with hydrogen chloride gas

Explain the concept: Most common gases are neutral or acidic in water. Only a few are basic. The best-known basic gas is ammonia (NH₃). It dissolves in water to form ammonium hydroxide, which releases OH⁻ ions:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

These hydroxide ions make the solution alkaline, so red litmus turns blue.

Check each option:

• A. N₂ is odourless, colourless and neutral. It does not affect litmus, so it is ruled out.
• B. N₂O (laughing gas) has a faint sweet smell and is neutral. It is ruled out.
• C. Cl₂ is pungent, but it is acidic and bleaches litmus (turns it colourless) rather than turning red litmus blue. It does not form white fumes with HCl. It is ruled out.
• D. NH₃ is pungent, basic (turns red litmus blue), and reacts with HCl as follows:

NH₃(g) + HCl(g) → NH₄Cl(s)

Ammonium chloride is a dense white solid smoke, which is the white fumes seen. This is the standard laboratory test for ammonia.

Answer: The gas is ammonia, option D.

Exam tip: Remember "Ammonia = the only common basic gas." If a question mentions red litmus turning blue or white fumes with HCl (a glass rod dipped in conc. HCl held near the gas), the answer is almost always NH₃.