Eli5: when the water is in a state of rolling boil, why do the bubbles appear to ascend from particular pinholes in the bottom.

Question

These said pinholes change from each boiling session.

In: 139

Answer 1

That’s not steam, it’s air!

Water has a lot of dissolved air in it. As the pot approaches boiling temperature, there’s a significant temperature difference between the water in the middle of the pot and water in a tiny crack. The water in the crack begins to turn into a steam bubble, but as it expands the top pokes out of the crack and reaches cold circulating water.

That makes the bubble collapse (which makes the popping sounds at the start of boiling). But there was a tiny bit of dissolved air that ended up in the bubble, and that air stays as an air bubble. The process repeats until the air bubble gets so big it detaches and floats up.

Eventually there’s enough heat added that you get genuine boiling, which is a roiling boil.

If you turn off the heat and let the water cool and then restart the process, you won’t see any of the little bubbles because you’ve de-aired the water.