Superheating is a fascinating occurrence where a liquid is warmed beyond its boiling point, yet it doesn't boil. This precarious or metastable state can cause the liquid to begin boiling at any point, stimulated by internal or external influences.
One can achieve superheating by warming a uniform substance in a pristine container that lacks any nucleation sites. It's crucial not to agitate the liquid during this process.
When we say "water boils," it means that the water vapor bubbles are growing unrestricted, eventually bursting at the surface. For a vapor bubble to enlarge, the temperature must be sufficiently high so that the vapor pressure surpasses the surrounding pressure, which is primarily atmospheric. If the temperature falls below this threshold, a water vapor bubble will shrink and disappear.
However, superheating contradicts this basic principle. There are instances where a liquid doesn't boil, even though its vapor pressure is greater than the surrounding pressure. This is due to an additional force called surface tension, which hinders the growth of bubbles.
Surface tension makes the bubble behave like an under-inflated rubber balloon that still has elasticity. The pressure inside the bubble increases slightly due to the "skin" trying to contract. To make the bubble expand, the temperature must be slightly higher than the boiling point in order to generate enough vapor pressure to counter both the surface tension and the surrounding pressure.
What makes superheating potentially explosive is the fact that inflating a larger bubble is easier than a small one, similar to blowing up a balloon where the hardest part is getting started. Interestingly, the additional pressure caused by surface tension is inversely proportional to the bubble's diameter.
This implies that if the largest bubbles in a container only have a diameter of a few micrometers, overcoming the surface tension might require the temperature to exceed the boiling point by several degrees Celsius.
Once a bubble starts to grow, the pressure caused by the surface tension decreases, leading to an explosive expansion. Most containers are imperfect and contain scratches that trap air pockets, which serve as starting bubbles. However, a container with only microscopic bubbles can experience dramatic superheating.
Superheating can happen when a still container of water is microwaved. Upon removal, the absence of nucleation sites prevents boiling, leaving the surface calm. But, once the water is disturbed, it can flash into steam violently, potentially spraying boiling water out of the container.
Triggering the boiling can occur by shaking the cup, introducing a stirring device, or adding substances like instant coffee or sugar. The likelihood of superheating is higher with smooth containers as scratches or chips can harbor small air pockets, which act as nucleation points.
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