Which of the following metals reacts vigorously with cold water to produce hydrogen gas and a metal hydroxide?
Correct: C
Highly reactive metals like Potassium (K) and Sodium (Na) react vigorously and exothermically with cold water, producing hydrogen gas and their respective hydroxides. The reaction for sodium is: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Heat.
Magnesium reacts slowly with cold water but faster with hot water/steam. Iron reacts only with steam. Copper does not react with water.