An example of an alkaline gas is

1. Analyze the question

The question asks us to identify which of the given gases is "alkaline." In chemistry, an alkaline substance is one that acts as a base. Specifically, for a gas to be alkaline, it must be able to react with water or acids to produce hydroxide ions ($OH^-$) or accept protons ($H^+$).

2. Explain the concept

• Alkaline/Basic Gases: These are gases that dissolve in water to form an alkaline solution (a base). The most common example is Ammonia ($NH_3$). When $NH_3$ dissolves in water, it reacts to form ammonium hydroxide ($NH_4OH$), which dissociates to release $OH^-$ ions: $$NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$$
• Acidic Gases: Gases like $HCl$ (hydrogen chloride) and $NO_2$ (nitrogen dioxide) are acidic. $HCl$ dissolves in water to form hydrochloric acid, and $NO_2$ dissolves to form a mixture of nitrous and nitric acids.
• Neutral Gases: $N_2$ (nitrogen) is generally inert and does not exhibit acidic or basic properties under standard conditions.

3. Show the solution

• Option A ($HCl$): This is a strong acid gas. It turns blue litmus paper red.
• Option B ($N_2$): This is a neutral, diatomic gas that makes up most of our atmosphere. It is neither acidic nor alkaline.
• Option C ($NH_3$): Ammonia has a lone pair of electrons on the nitrogen atom, which allows it to act as a Lewis base (proton acceptor). It turns red litmus paper blue, confirming its alkaline nature.
• Option D ($NO_2$): This is an acidic oxide of nitrogen.

4. Identify the answer

Based on the chemical properties, Ammonia ($NH_3$) is the only gas listed that exhibits alkaline properties.

5. Tip

Exam Strategy: Remember the "Litmus Test" rule for gases! If you are asked about the nature of a gas, think about how it affects litmus paper. Alkaline gases (like $NH_3$) turn red litmus paper blue, while acidic gases (like $HCl$ or $SO_2$) turn blue litmus paper red.