Identifying X as Sr via solubility trend and nitrate decomposition

Steps:

• Group 2 elements in Period 3 (Mg) or 5 (Sr); hydroxide solubility increases down the group.
• Mg(OH)₂ is less soluble than X(OH)₂, so X = Sr (Period 5).
• Group 2 nitrates thermally decompose: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
• Match choices: X = Sr, equation must produce oxide (XO), not metal or nitrite.

Why B is correct:

• Balanced equation 2Sr(NO₃)₂ → 2SrO + 4NO₂ + O₂ follows standard decomposition for heavier Group 2 nitrates.

Why the others are wrong:

• A: X = Mg (wrong, contradicts solubility); products form nitrite (XNO₃), not oxide.
• C: X = Sr (correct) but products include metal (X), not oxide.
• D: X = Sr (correct) but equation unbalanced (missing coefficient or stoichiometry mismatch).

Final answer: B