Magnesium reacts with steam to form magnesium oxide (MgO)

Steps:

• Identify the reaction: Mg + H₂O → MgO + H₂, so compound J is MgO.
• Calculate relative molecular mass (Mr): Mg (24.3) + O (16.0) = 40.3.
• Recall physical state: MgO is a white solid at room temperature.
• Determine chemical nature: MgO is a basic oxide, reacting with acids.
• Check solubility: MgO is insoluble in water, forming a slightly soluble hydroxide slowly.

Why D is correct:

• MgO has low solubility in water (solubility product indicates poor dissolution), unlike highly soluble compounds like NaCl.

Why the others are wrong:

• A: Mr matches exactly (24.3 + 16.0 = 40.3).
• B: MgO appears as a white powder or solid.
• C: As a Group 2 oxide, MgO neutralizes acids, confirming basicity.

Final answer: D