When a pot of water is waiting to heat up on the stove, small bubbles are the first sign that it is ready to boil. As the water heats up, the bubbles get bigger and boiling begins when the water reaches 212 degrees Fahrenheit (100 degrees Celsius).
Or is it? Anyone who has boiled water in a microwave will notice that there are no bubbles. So why do bubbles form when water is boiled in a way other than a microwave?
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“The boiling point means that above that temperature the molecules are happier as a vapor than as a liquid,” said Jonathan Boreyko, a fluid mechanics expert at Virginia Tech. Above 212 °F, the specific energy of water molecules, known as the chemical potential, is lower in gases than in liquids, making vapor the most stable form.
“But to actually perform boiling, you need to generate bubbles, and that has an energy cost,” Boreyko told Live Science. “So just because you’re happier with steam doesn’t mean it boils well.”
Therefore, the temperature at which water actually boils is a trade-off between the chemical potential energy saved by becoming a gas and the energy expended in forming bubbles.
Importantly, a bubble is not just a mass of gas, but also an interface between the gas and liquid phases. And like all liquid interfaces, this surface is affected by surface tension.
Surface tension is a force that constantly tends to compress the gas-liquid boundary to the smallest possible area. For foam, this means that it completely collapses back into a homogeneous liquid. Therefore, a stable bubble must contain enough gas such that the savings in chemical potential energy is greater than the interfacial surface tension, making large bubbles more stable.
“Surface tension is basically the energy cost per area,” Boleyko says. “Really small bubbles have a very large surface area to volume ratio, while large bubbles have a small area relative to their volume. The larger they get, the more volume dominates and outweighs the cost of surface tension.”
As a result, water often doesn’t boil until just above 212 F. This is a phenomenon known as overheating. The boiling point indicates the temperature at which the gas becomes more stable than the liquid, and the extra temperature corresponds to the activation energy required to produce sufficiently large bubbles.
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However, Mirko Gallo, a fluid mechanics researcher at Sapienza University of Rome, told Live Science that various factors influence how easily these bubbles form.
“Dissolved gases, impurities in the water, and the surface of the container can all lower the energy barrier for bubble formation,” Gallo explained. These irregularities within the bulk liquid provide distinct nucleation points around which bubbles form, reducing the surface tension penalty in forming perfectly spherical bubbles.
“When you form a bubble at the edge, it’s only half a sphere, so it has less surface area and requires less energy,” he added. Therefore, the first bubbles always begin to appear from the border of the pot.
boil water in microwave
Conversely, a microwave oven can superheat water up to 36 F (20 C) as the unusual heating conditions effectively suppress the formation of bubbles.
“The electromagnetic waves penetrate the entire volume and excite the water molecules, so they heat the water very quickly and evenly. On the stovetop, on the other hand, the bottom wall of the pot gets the hottest,” Boreyko explained. “You have that tendency too. [heat up things in a microwave] Because it is in a very smooth container, for example glass, there are no local hot spots that can help overcome the energy barrier and form the initial interface. ”
This huge chemical potential energy stored in the superheated liquid is spontaneously released in the form of huge explosive bubbles as soon as the container is disturbed, making microwaved water amazingly dangerous.
But overheating is not limited to water. Gallo says any liquid can be used.
“Water has a very high surface tension compared to most liquids, but basically the higher the surface tension, the more dramatic the effect,” Boreyko added.
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