When does instantaneous boiling occur in a superheated liquid?

Instantaneous boiling in a superheated liquid occurs when the temperature of the liquid exceeds its boiling point, entering a state beyond the superheat limit. At this point, the liquid is capable of undergoing rapid phase transition from liquid to vapor.

In a superheated state, the liquid molecules have absorbed energy beyond what is necessary to maintain a liquid state at a particular pressure, and thus are in a metastable condition. If a disturbance or nucleation site is introduced—such as an impurity, bubble, or sudden drop in pressure—this can trigger rapid vaporization, resulting in the phenomenon of instantaneous boiling.

It's essential to understand that the boiling point is defined under specific conditions of pressure; raising the temperature significantly above this point while maintaining the same pressure allows the liquid to exist in this unstable, superheated state. Therefore, reaching or exceeding the superheat limit is crucial for the rapid transition to occur.