Redox and reactivity in group 2 metal-water reactions

Steps:

• Group 2 metals (Ca, Sr) react with water: 2M + 2H₂O → 2MOH + H₂, where M is oxidized (loses electrons to M²⁺) and water reduced (gains electrons to H₂ and OH⁻).
• Oxidation half: M → M²⁺ + 2e⁻; reduction half: 2H₂O + 2e⁻ → H₂ + 2OH⁻.
• Reactivity increases down group 2: Sr more reactive than Ca, so Sr + water more vigorous (faster H₂ evolution).
• Match to options: Substance reduced is water, oxidized is Ca or Sr; vigorous reaction is Sr + water.

Why D is correct:

• D accurately identifies water reduced (to H₂) and Sr oxidized (to Sr²⁺), with Sr's higher reactivity causing vigorous reaction per group 2 trend.

Why the others are wrong:

• A: Reverses redox (metals oxidized, not reduced) and picks less vigorous Ca.
• B: Reverses redox (water oxidized, not reduced) despite correct vigorous Sr.
• C: Correct redox but wrongly picks less vigorous Ca over Sr.

Final answer: D