AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(C) Hydrogen
When calcium reacts with cold water, hydrogen gas is evolved. The bubbles of H₂ stick to the metal's surface, making calcium float.
$$\text{Ca(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}$$
Source: Chapter 3, Section 3.2.2
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The key fact is that all metals reacting with water produce hydrogen gas, not oxygen or CO₂. The floating is caused by H₂ bubbles clinging to the metal surface — a detail specifically mentioned for calcium in the textbook. Examiners may test whether students confuse the gas evolved (H₂) with the product in solution (Ca(OH)₂).