Unraveling the Science Behind Bubbling Boiling Water and Its Absence in Microwaves
Have you ever gazed at a pot of water as it starts to boil on the stove? The initial hint of boiling is the appearance of tiny bubbles. As the water's heat intensifies, these bubbles grow larger until a full rolling boil indicates the water has reached 212 degrees Fahrenheit (or 100 degrees Celsius). But have you noticed that when you heat water in a microwave, there are no bubbles? What's the science behind this?
Why Do Bubbles Appear in Boiling Water?
When water is heated over a heat source like a stove, small bubbles, at a nanoscale, begin to form and collapse. However, it's interesting to note that visible bubbles usually start forming at temperatures significantly higher than the boiling point of water.
Jonathan Boreyko, a fluid dynamist, explains, "The boiling point is the temperature at which water molecules prefer being in a vapor state rather than a liquid one." When the temperature goes beyond 212 F, the inherent energy of the water molecules, also known as the chemical potential, is lower for the gas than the liquid, making vapor the most stable form.
However, to actually initiate boiling, a bubble needs to be created, which requires energy. Therefore, the boiling point is a balance between the energy saved by turning into a gas and the energy expended to form a bubble.
The Role of Surface Tension
It's crucial to understand that a bubble isn't just a volume of gas but also serves as a boundary between gas and liquid phases. Like all liquid interfaces, this surface is subject to surface tension. Surface tension is a force that constantly tries to reduce the gas-liquid boundary to the smallest possible area. In the case of a bubble, this would mean collapsing entirely back into a uniform liquid.
Boreyko further explains, "Surface tension is essentially an energy cost per area. Smaller bubbles have a very large surface-area-to-volume ratio, whereas a bigger bubble has a smaller area relative to its volume. The volume dominates the bigger you get, which outcompetes the surface tension cost."
The Phenomenon of Superheating
As a result, water often doesn't boil until it's a bit hotter than 212 F, a phenomenon known as superheating. The boiling point marks the temperature at which the gas becomes more stable than the liquid, and the extra temperature corresponds to the activation energy required to form a sufficiently large bubble.
However, numerous factors affect how easily these bubbles can form. "Dissolved gases, impurities in the water, the surface of the container can all lower the energy barrier for the formation of the bubble," Mirko Gallo, another fluid dynamist, explained. These irregularities within the liquid provide a distinct nucleation point around which bubbles can form, reducing the surface tension penalty of forming a completely spherical bubble.
Why No Bubbles In Microwaved Water?
On the contrary, in a microwave, the unique heating conditions suppress bubble formation so effectively that it's possible to superheat the water by up to 36 F (20 C).
"The microwave's electromagnetic waves penetrate and excite the water molecules throughout the entire volume, heating the water quickly and uniformly, unlike a stovetop, where it's the bottom wall of the pot that gets hottest," Boreyko explained.
Moreover, microwaves typically heat things in smooth containers, like glass, which lack localized hotspots that help get over the energy barrier to create the first bubble interface.
This vast store of chemical potential energy in the superheated liquid is suddenly released in the form of a large, explosive bubble as soon as the container is disturbed, making water heated in the microwave unexpectedly dangerous.
Superheating: Not Just for Water
However, it's important to note that superheating isn't exclusive to water; it can occur with any liquid. "Water has a very high surface tension compared to most liquids, but essentially, the higher the surface tension, the more dramatic the effect," Boreyko concluded.