Superheating

Cause

Superheating occurs when a liquid stays liquid even though it has enough heat to boil. Normally, boiling happens when water vapor bubbles can grow and burst at the surface. For that to happen, the vapor pressure inside a bubble must be higher than the surrounding air pressure. If it isn’t, the bubble shrinks and disappears.

The reason superheating can happen is surface tension. Surface tension makes a bubble act like a tiny elastic balloon, adding extra pressure inside. To make the bubble expand, the temperature must go above the normal boiling point so the vapor pressure can overcome both the air pressure and this extra surface tension force. The extra pressure from surface tension is stronger for smaller bubbles—it is inversely proportional to the bubble’s diameter.

If the biggest bubbles in a container are only a few micrometers wide, the temperature may need to be several degrees above the boiling point to start them growing. Once a bubble does start to grow, the surface tension pressure drops, and the bubble expands explosively in a feedback loop. In everyday situations, scratches on a container or tiny particles in water trap air pockets that act as starter bubbles, preventing superheating. Only a very smooth container holding purified liquid can reliably superheat.

Occurrence via microwave oven

Superheating can occur when an undisturbed container of water is heated in a microwave oven. Upon removal, the lack of nucleation sites prevents boiling, leaving the surface calm. Disturbing the water—by jostling the cup, inserting a stirring device, or adding instant coffee or sugar—can trigger violent flashing to steam, potentially spraying boiling water.

Smooth containers increase the chance of superheating, as scratches or chips house air pockets that serve as nucleation points. Repeated heating and cooling cycles of an undisturbed container, such as a forgotten coffee cup reheated without removal, release dissolved gases and make superheating more likely. Placing a spoon or stir stick (not metal) in the container beforehand or using a scratched container can prevent superheating.

Superheating in solids

Crystalline solids can be transiently heated beyond their melting point. Early theory set a stability limit at about three times the melting temperature, after which the solid becomes unstable. However, experiments with ultrafast heating have shown solids persisting above this limit.