When you’re boiling a pot of water, right before the water starts to boil if you watch carefully at the bottom of the pot there will be tiny bubbles that form and disappear. Why do they just disappear instead of floating up to the top once they’re already formed??
In: Physics
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Small bubbles of water boiling – but during this time not all water is 100° C so not all water is at boiling temperature. The little bubbles of steam travel up and the water above it at a bit lower temperature cools the steam down back to water, so they don’t escape from the top.
Interestingly, if you use cold water and watch as it slowly heats up, you will see bubbles form that DO make it to the top.
Air dissolves more easily in cold water. As it heats up, the air has to come out. These bubbles will often sit on the bottom of the pot until they’re big enough and float to the top. Typically they are all gone by the time steam actually starts to develop (the disappearing bubbles).
A follow-up question on the same lines
Why then do the bubbles appear to form at specific single points at the bottom of the pan? It’s like a steady stream of bubbles from that location.
Based on the answers I went through this thread bubbles form because the vessel is heated and so adjacent water turns to vapor, but then why only specific locations?
I’m a materials engineer so here goes.
Water boiling is a phase change reaction. In any phase change there has to be nucleation of the new phase in the current phase.
Nucleation basically means small (sometimes spherical) bubbles of the second phase trapped in the first.
Since all reactions are governed by kinetics and thermodynamics both, it is possible that a reaction may be thermodynamically favorable but not kinetically.
This means, right before the water boils, it has enough energy to turn into vapor, but it does not have enough to escape the liquid
Also, there is a critical nucleation size, beyond which the second phase is stable in the first one. This is due to the different internal and surface energies of the 2 phases.
Simply, when 2 phases are in contact, they have some surface energy. Both phases also have some internal energy.
Both these energies are negative (so higher means more negative)
If the surface energy is higher than the internal energy, the nucleus will be stable and grow. OTOH, if the surface energy is lower, the nucleus will collapse.
Again, kinetics says the reaction should probably happen, since we have enough energy to grow a nucleus. But until thermodynamics comes in and says “you have crossed the critical nucleus size, you may grow further,” the nucleus will always collapse on itself
To further expand, there’s different ‘regimes’ of boiling. As the bottom surface is heated above the boiling point, we don’t actually see any bubbles until the surface is a few degrees past boiling. The larger the temperature difference, the more bubbles we see form and eventually make it to the top. This is what we refer to as nucleate boiling.
If the temperature of the surface gets too high, a stable vapor film actually forms over the surface and traps the bubbles.
This is referred to as film boiling.
[Boiling curve](https://www.engineersedge.com/heat_transfer/water_boiling_graph_curve_13825.htm)
Finally something I can answer lol.
So the heating element (or fire if you use a gas range) is obviously very hot. Much hotter than the 100 degrees C water needs to start evaporating. As you heat up the pan, that heat from the stove goes to the pan and subsequently, starts heating the water. The water at the bottom of the pan will heat up much faster than the water at the top of the pan (since it takes time for the heat to go up the water).
As the bottom of the pan gets to 100C and water starts boiling, that bubble (steam, aka water vapour and not air (though there is some small amount of it)) will start rising but immediately get in contact with more water that is not at 100C yet (further from the bottom of the pan), and ‘cool’ down and that water vapour will go back to liquid and the bubble disappear. As you keep heating it for longer, you’ll notice the bubbles get higher and higher before they disappear up until they can reach the very top and start bubbling and you’ll say it’s boiling.
You’ll also see these bubbles on the walls of the pan since the heat from the bottom of the pan can conduct through the walls of the pan and heat from the sides, and those will also be much hotter than the water itself.
Fun fact, if you want your water to boil faster, you can agitate the water as it heats up (mix with spoon or something) and it will help the water at the bottom to mix with the water at the top and transfer that heat faster.
EDIT: The laSt part about cooking was not 100% corrEct aNd therefore, I removeD it. As poiNted oUt by others, cooking has other processes coming in play such as maillarD reactions and hEat destroying pathogenic germS.
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