Confusion about boiling point and vapor pressure [duplicate]

It is said that a liquid boils when its vapor pressure equals to the external pressure of its environment (e.g. atmospheric pressure). That’s because the bubbles of water vapor that form will be crushed if the vapor pressure is below the atmospheric pressure. However, it’s the fluid’s pressure that actually does the crushing, isn’t it? And the fluid pressure varies with depth, with only the fluid at the top of the liquid being equal to atmospheric pressure. Shouldn’t we say that the liquid boils when its vapor pressure is equal to the greatest fluid pressure in the liquid?

• Let's put it this way: true, water at precisely $100^\circ\rm C$ will boil at the top and not deep down below, but as it boils, the upper layers vanish and new layers become free from the liquid pressure and also start to boil. So it goes. Nov 9, 2016 at 20:16
• @IvanNeretin So at precisely the boiling point, there won't be any bubbles, because vaporization only occurs at the surface of the liquid? Nov 10, 2016 at 6:00
• Strictly speaking, yes. To have bubbles in the entire volume of your kettle, all the way to the bottom, you might need something like $100.05^\circ\rm C$. Nov 10, 2016 at 6:42
• @IvanNeretin Did you use C-Clap Eq? For 1 m deep, I got $102.7 ^\circ \, \mathrm{C}$ Supposing constant entalphy...; I don't know what happens if you have $1\,m$ of water at $T_B$, will it boil because of vessel irregularities? Otherwise, I guess it won't...the system will be hold at 100...
– user43021
May 8, 2018 at 20:48
• @santimirandarp I didn't use a thing, other than my common sense. Wait, did you just measure the temperature inside a living geyser? May 8, 2018 at 20:52

1 Answer

Shouldn’t we say that the liquid boils when its vapor pressure is equal to the greatest fluid pressure in the liquid?

No. If you heat up water at the ocean's surface to 100 °C, it will boil, even though the pressure at the bottom of the ocean is far higher.

As you note, pressure is not guaranteed to be homogeneous in space (i.e. the same everywhere), especially for deep bodies of water. But neither is nearly any other fluid property. Temperature can vary with position in poorly-mixed waters. So can whether the water is boiling or not.

A given small-ish region of water with nearly constant temperature and pressure will boil if and only if the (P, T) condition of the water is at or above the water's vapor pressure at the same temperature.