Boiling water is the same thing. The water molecules are
trapped in the pot and kept in place by their own fence—the
pressure of the air in the atmosphere pushing down on them.
Add energy to the pot in the form of heat, and water
molecules begin to start leaping off the surface of the water.
This is called evaporation. Eventually the pressure produced
by the water molecules trying to escape becomes equal to or
greater than the pressure of the atmosphere pushing down
on it. The fence breaks, the floodgates open, and water
molecules rapidly jump from a liquid state to gas, bubbling
up violently. This conversion of liquid water to water vapor
(steam) is what you see when you look at a pot of boiling
water; with pure water at sea level, this occurs at 212°F
(100°C).
Here’s a quick rundown of what happens when you bring
a pot of water to a boil:
- Quivering: At between 130° and 170°F, tiny bubbles of
water vapor begin forming at nucleation sites (more on
those later) along the bottom and sides of the pot. They
won’t be large enough to actually jump and rise to the
surface of the water, but their formation will cause the top
surface to quiver a bit.