Magnesium phosphide fits hydrolysis to slightly soluble hydroxide and combustion to +5 oxide

Steps:

• Q hydrolyzes with water to a slightly soluble hydroxide, identifying Y as Mg (Mg(OH)₂ sparingly soluble) over Na (NaOH highly soluble).
• Q contains Period 3 elements Y (metal) and Z (non-metal), forming a binary compound like a phosphide or sulfide.
• Q burns in moist air to an oxide with oxidation state +5, matching phosphorus in P₄O₁₀ (P = +5), not sulfur (S = +4 in SO₂ or +6 in SO₃).
• Thus, Q is Mg₃P₂, hydrolyzing to Mg(OH)₂ + PH₃, and PH₃ combusts to P₄O₁₀ + H₂O.

Why A is correct:

• Mg₃P₂ + 6H₂O → 3Mg(OH)₂ (slightly soluble) + 2PH₃; 4PH₃ + 8O₂ → P₄O₁₀ (+5 for P) + 6H₂O.

Why the others are wrong:

• B: Na₂S hydrolysis yields soluble NaOH, not slightly soluble hydroxide.
• C: Na₃P hydrolysis yields soluble NaOH, not slightly soluble hydroxide.
• D: MgS combustion yields SO₂ (S = +4) or SO₃ (S = +6), not +5 oxide.

Final answer: A