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
Heat *is* motion. Those little bubbles are spots where the water is so hot, and moving so fast, that it actually pushes away the water around it and becomes a little bubble of steam. The steam is water molecules zipping around bumping into the liquid water hard enough to keep it away. It’s actually lifting all of the water in the pot directly above it!
As you point out, steam is lighter than water so this bubble does start to float. As it does, it moves out of the super-hot bottom layer of water that’s right against the heating element.
As soon as the water around the bubble gets a little colder, it can’t sustain itself. The steam bumps into the cold water and loses energy. As it slows down, the water above presses in. The whole thing collapses!
Basically because the bottom of the water is the hottest, it gets to the boiling point first. Steam bubbles can only just survive there during this phase of heating. Stirring the water as it heats up will make these bubbles take longer to appear, but once they do they’ll be able to rise through to the top much faster. This is because all of the water will be heating more evenly.
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