Identity a metal based on its properties A metal X can be cut with a knife. When placed in water, metal X floats and melts. What is X? The options are calcium, copper, magnesium and potassium.


I think it can't be potassium because it will react vigorously with water, So I believe it's calcium. But the only problem is that calcium is denser than water so it will sink.
Copper is much denser than water and it does not react with it. Magnesium is also denser than water and reacts only very slowly. Calcium will also sink in water, although it will react with it producing hydrogen, so small pieces of the element may float on the surface. However, it would definitely not melt. The correct answer is potassium - it is a very soft metal, less dense than water and reacts with it producing enough heat to melt it (potassium melts at only $\pu{63.5^\circ C}$).