Alkali metal reactivity and physical properties

Steps:

• Element X is soft enough to cut with a knife, indicating low hardness typical of alkali metals.
• X floats on water, showing low density less than 1 g/cm³.
• X melts during the reaction, due to intense exothermic heat from water reactivity.
• These traits match potassium's vigorous reaction: 2K + 2H₂O → 2KOH + H₂, producing hydrogen gas and sufficient heat to melt the metal.

Why D is correct:

• Potassium (density 0.86 g/cm³) is soft, floats, and its reaction with water (2K + 2H₂O → 2KOH + H₂) generates enough heat (ΔH exothermic) to raise its low melting point (63°C) above ambient temperature.

Why the others are wrong:

• A. Calcium: Too dense (1.55 g/cm³) to float and harder, not cut by knife.
• B. Copper: Inert to water, no reaction or melting.
• C. Magnesium: Reacts slowly with water, sinks (density 1.74 g/cm³), and doesn't melt.

Final answer: D