When small, equal-sized pieces of lithium, sodium, and potassium are added separately to troughs of water, sodium and potassium melt into shiny spheres that dart across the surface, whereas lithium reacts steadily without melting.
Which statement correctly explains why sodium and potassium melt during this reaction while lithium remains a solid?
The reaction of lithium with water is endothermic, which cools the metal, whereas the reactions of sodium and potassium are exothermic, heating the metals above their melting points.
Lithium has a higher melting point due to stronger metallic bonding, and its reaction with water is slower, releasing heat at a lower rate so the metal does not reach its melting point of 181 ∘C181\ ^\circ\text{C}181 ∘C.
Lithium reacts with water to form insoluble lithium oxide (Li2O\text{Li}_2\text{O}Li2O) on its surface, which acts as a thermal insulator and prevents the metal core from absorbing the heat of reaction.
Lithium is more dense than water and sinks, allowing the surrounding water to rapidly absorb the heat of reaction, whereas sodium and potassium float.
