Substance J is barium metal, identified by gas evolution and precipitate formation

Steps:

• Substance J + water produces gas (H2) and increases pH, indicating a reactive metal forming soluble hydroxide.
• Resulting solution contains Ba(OH)2, which is basic and soluble.
• Adding H2SO4 forms white BaSO4 precipitate, confirming Ba²⁺ ions.
• Only barium metal matches all reactions: Ba + 2H₂O → Ba(OH)₂ + H₂.

Why A is correct:

• Barium metal reacts with water to evolve H₂ gas and form soluble Ba(OH)₂ (pH rises); Ba(OH)₂ + H₂SO₄ → BaSO₄ (insoluble white precipitate, per solubility rules).

Why the others are wrong:

• B: Barium oxide + H₂O → Ba(OH)₂ (no gas evolved).
• C: Magnesium oxide + H₂O → Mg(OH)₂ (sparingly soluble, no gas); MgSO₄ is soluble (no precipitate).
• D: Identical to C (magnesium oxide).

Final answer: A