Barium's vigorous water reaction vs. barium oxide's basicity

Steps:

• Barium metal reacts with water to produce hydrogen gas rapidly, observed as effervescence.
• Barium oxide reacts with water to form barium hydroxide, a strong base with pH around 13.
• Match choices to properties: effervescence describes barium's reaction; 13 indicates barium oxide's solution pH.
• Eliminate mismatches: A and C pair effervescence with 8 (neutral pH, incorrect for oxide); B pairs surface formation (oxide layer, not key property) with 13.

Why D is correct:

• D pairs barium's rapid hydrogen effervescence with water (2Ba + 2H₂O → 2Ba(OH)₂ + H₂) and barium oxide's high pH 13 from BaO + H₂O → Ba(OH)₂.

Why the others are wrong:

• A: "Few gas bubbles" understates barium's vigorous reaction; 8 is neutral pH, not basic for barium oxide.
• B: "Form on the metal surface" describes passivation, not barium's key reactivity; 13 mismatched to barium.
• C: "Rapid effervescence" incorrect for barium oxide (no gas produced); 8 wrong for barium.

Final answer: D