Understanding Metals and Their Reaction with Cold Water

Metals react with water in different ways depending on their reactivity. Some metals react vigorously with cold water, some react only with hot water or steam, and others do not react with water at all.

When a metal reacts with water, it typically forms a metal hydroxide or metal oxide and hydrogen gas:

• Metal + Cold Water $\rightarrow$ Metal Hydroxide + Hydrogen gas
• Metal + Hot Water $\rightarrow$ Metal Hydroxide + Hydrogen gas
• Metal + Steam $\rightarrow$ Metal Oxide + Hydrogen gas

The hydrogen gas produced during the reaction can stick to the surface of the metal. If enough hydrogen bubbles are formed and attach to the metal, they can decrease the overall density of the metal-bubble combination, potentially causing the metal to float on the water surface.

Analyzing the Options for Floating in Cold Water

Let's examine how each of the given metals reacts with cold water and whether they float:

• Magnesium (Mg): Magnesium reacts very slowly with cold water. It reacts more readily with hot water or steam. While some hydrogen bubbles are produced in cold water, they are usually not enough to make it float significantly or consistently.
• Calcium (Ca): Calcium reacts with cold water to form calcium hydroxide and hydrogen gas. The reaction is less vigorous than that of alkali metals like sodium or potassium, but the hydrogen bubbles produced are enough to stick to the calcium metal, causing it to float on the water surface.
• Potassium (K): Potassium is a very reactive alkali metal. It reacts violently and exothermically with cold water, producing potassium hydroxide and hydrogen gas. The reaction is so vigorous that the hydrogen gas produced often catches fire. Due to the rapid production of gas and its low density, potassium also floats and moves rapidly on the water surface. However, the question asks which *one* metal floats. While Potassium floats, Calcium also floats due to attached hydrogen bubbles and is a common example of a less reactive metal (compared to Potassium) that exhibits this behavior.
• Copper (Cu): Copper is a relatively unreactive metal. It does not react with cold water, hot water, or steam. Copper is also much denser than water and will sink immediately without any reaction or gas production to make it float.

Comparing Reactivity and Floating Behavior

Let's summarize the behavior of these metals with cold water:

Both Calcium and Potassium float in cold water. However, Calcium is often highlighted as a metal that floats specifically due to the hydrogen bubbles sticking to its surface, while Potassium's floating is part of a much more violent and rapid reaction. In the context of typical questions, Calcium's behavior of floating with attached bubbles is a distinct characteristic.

Conclusion

Based on the reactivity with cold water, Calcium reacts and produces hydrogen gas bubbles that stick to its surface, causing it to float. While Potassium also floats, Calcium is a standard example of a metal that floats in cold water because of the attached hydrogen gas.

Revision Table: Metals and Water Reaction Summary

Additional Information: Factors Affecting Floating of Metals

Several factors determine if a metal will float or sink in water:

• Density: If the density of the metal is less than the density of water (approx. $1 \text{ g/cm}^3$), it will float. Lithium ($0.534 \text{ g/cm}^3$), Sodium ($0.968 \text{ g/cm}^3$), and Potassium ($0.856 \text{ g/cm}^3$) are alkali metals with densities less than water, and they float.
• Reactivity with Water: Metals that react with water produce hydrogen gas. These gas bubbles can attach to the metal's surface.
• Adhesion of Hydrogen Bubbles: If the hydrogen bubbles produced during the reaction stick effectively to the metal surface, they can increase the buoyancy, potentially making the metal float even if its density is greater than water (like Calcium, density $1.55 \text{ g/cm}^3$). The attached bubbles reduce the effective density of the metal-gas combination.
• Surface Tension: Surface tension can play a role for very small pieces of metal, but for larger pieces reacting with water, the reaction and gas production are the primary factors for floating, especially if the metal's density is greater than water.

In the given options, Calcium's behavior of floating in cold water is mainly attributed to the hydrogen gas bubbles that are produced and adhere to its surface during the reaction.