Acidic Gas Containing Hydrogen

Steps:

• pH 4 indicates an acidic aqueous solution, produced by X dissolving in water.
• Acidic gases that dissolve to form H+ ions typically contain hydrogen atoms, like HCl or HF.
• Hydrogen halides are common examples of room-temperature gases yielding acidic solutions.
• Thus, X must contain hydrogen atoms to directly contribute H+.

Why C is correct:

• Hydrogen-containing gases (e.g., HCl) ionize in water as HX → H+ + X-, per Arrhenius acid definition, producing low pH.

Why the others are wrong:

• A: Acidic solutions contain ions (H+ and anions), so they conduct electricity.
• B: Acidic gases involve non-metals (e.g., halogens), not metals.
• D: Room-temperature gases like HCl are covalently bonded, not ionic.

Final answer: C