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??
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.