Which one of the following metals floats in cold water?

Calcium floats in cold water because the bubbles of hydrogen gas formed during the reaction stick to the surface of the metal, providing buoyancy [1]. While potassium and sodium are physically less dense than water (0.86 g/cm³ and 0.97 g/cm³ respectively) and float due to density [1][2], they react violently with cold water, often catching fire. In contrast, magnesium does not react with cold water at all; it only reacts with hot water or steam, where it also floats due to hydrogen bubbles [1]. Calcium reacts moderately with cold water, and the resulting hydrogen gas bubbles are sufficient to make the metal float despite its density (1.55 g/cm³) being higher than that of water [1][2]. Copper is highly dense (8.96 g/cm³) and does not react with water, causing it to sink [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Activity 3.10 > p. 43
2. [2] https://www.aakash.ac.in/important-concepts/chemistry/alkali-metals