What’s the difference between water boiling and it evaporating normally? Don’t both end up the same, ie. water turning into gas form?
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Water vapor wants to expand – all gasses do that – and the pressure it exerts to do that is called the “vapor pressure.”
Liquids *also* have a vapor pressure, called the “saturation vapor pressure” that depends on the temperature of the liquid. More temperature means that water vapor will “leak out” of the surface of the liquid at a higher temperature.
At room temperature this saturation vapor pressure is low, only about 1/50th of atmospheric temperature. So water vapor leaks out slowly. And if you have humid air, the pressure of water vapor going back into the liquid can balance it out.
If water is hot enough its saturation vapor pressure reaches the total absolute pressure. Newly formed water vapor is capable of pushing other things out of the way. So it pushes water away from the bottom of a kettle and makes steam bubbles, or a jet of steam can push through air and blow a whistle. Those things don’t happen during low-temperature evaporation.
If the ambient pressure is lower then you need less saturation vapor pressure and the boiling temperature is also lower.
There are several other interesting effects, like what happens when a container contains *only* water vapor and water with very little air or other gases. If you do that the water will quickly boil-evaporate at any temperature between about 0 and 370 and very quickly spread that heat to the rest of the container by condensing on cool spots. This effect is used to cool electronics.
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