Correct Option
The correct option is Calcium
Explanation
The reaction of metals with water depends on their position in the reactivity series. When reactive metals come into contact with water, they displace hydrogen, forming a metal hydroxide and releasing hydrogen gas. The physical behavior of the metal during this reaction-such as floating or sinking-is determined by the intensity of the reaction and the interaction of the evolved gas with the metal surface.
Detailed Analysis
- Calcium ( Calcium reacts with cold water. The reaction is less violent than that of sodium or potassium, and the heat evolved is insufficient for the hydrogen to catch fire. Instead, bubbles of hydrogen gas adhere to the surface of the metal. These bubbles provide buoyancy, causing the calcium to float on the water's surface.
- Magnesium: Magnesium does not react with cold water but reacts with hot water to form magnesium hydroxide and hydrogen. While it also floats due to hydrogen bubbles sticking to its surface, this occurs only under specific conditions (hot water), whereas calcium exhibits this behavior in standard cold water conditions.
- Zinc: Zinc is less reactive and does not react with cold or hot water. It reacts only with steam to form zinc oxide and hydrogen gas.
- Copper: Copper is a noble metal relative to hydrogen in the reactivity series and does not react with water (cold, hot, or steam) at all.
Key Takeaway: Calcium is the metal that reacts with cold water and floats because the evolved hydrogen gas bubbles stick to its surface.