Alkali metal properties identify potassium

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 the highly exothermic reaction producing sufficient heat.
• The reaction with water is vigorous, characteristic of Group 1 metals.

Why D is correct:

• Potassium (K) reacts with water via 2K + 2H₂O → 2KOH + H₂, releasing hydrogen gas for floating and intense heat that melts its low-melting-point solid (63°C).

Why the others are wrong:

• A. Calcium is harder (cannot be cut easily) and reacts moderately without melting.
• B. Copper does not react with water and is too hard to cut with a knife.
• C. Magnesium reacts slowly with water, sinks due to density >1 g/cm³, and does not melt.

Final answer: D