Re: the boiling point is simply the temperature such that the vapor,pressure equals total pressure, at whatever total pressure your system,is at. At normal atmospheric pressure, the vapor pressure of water is,equal to atmospheric pressure at 100C. This means instead of gas water,molecules only being able to self-diffuse through the atmosphere, they,can actually displace the atmosphere, and you get the bubbles and,roiling of boiling.

From	The Starmaker <starmaker@ix.netcom.com>
Newsgroups	sci.physics
Subject	Re: the boiling point is simply the temperature such that the vapor,pressure equals total pressure, at whatever total pressure your system,is at. At normal atmospheric pressure, the vapor pressure of water is,equal to atmospheric pressure at 100C. This means instead of gas water,molecules only being able to self-diffuse through the atmosphere, they,can actually displace the atmosphere, and you get the bubbles and,roiling of boiling.
Date	2017-07-20 20:35 -0700
Organization	Aioe.org NNTP Server
Message-ID	<5971766C.5930@ix.netcom.com> (permalink)
References	<okrk68$1m9h$1@gioia.aioe.org>

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What I don't unnderstand is why do you pretend to do the explaining? Why the cut-and-paste?

https://www.quora.com/Why-does-steam-come-when-you-boil-water#!n=18

(see Shams Shimu, Still learning..
Answered Feb 3, 2016)


https://www.reddit.com/r/askscience/comments/2ng06u/why_does_my_hot_drink_have_steam_rising_from_it/





Serg io wrote:
> 
> Glad you asked Claudius, many are so confused like you are on this subject.
> 
> Even though 100Â°C is the actual temperature needed for water to boil,
> the nature of matter is such that even under that temperature, some
> molecules have enough kinetic energy to convert to gaseous form.
> Temperature is merely the average kinetic energy of a system, so some
> molecules are above that temperature and some are below at all times.
> Under 100Â°C, say at 80Â°C, some molecules would still have enough kinetic
> energy to convert to gaseous form.
> 
> This is why water still evaporates over time, albeit very very slowly,
> from say, a puddle, on a cooler day. Increasing temperature just
> increases the rate of a favorable reaction that is naturally
> occurring/provides the conditions for it to spontaneously occur.
> Reactions favor the direction that increases the number of micro states
> available to the system (think in terms of wanting personal space - in
> general, gaseous form is more "comfortable" for a compound vs. being
> smooshed as a liquid or even more smooshed as a solid), some reactions
> just don't happen spontaneously without certain conditions or a
> catalyst, i.e. a specific temperature.
> 
> The "steam" you see is a mist* of the gaseous particles being condensed
> into tiny liquid particles suspended in the air, which is cooler than
> the system the gas just left.
> 
> For any substance, the energies of the molecules are spread out in a
> bell curve. So while the mean energy (what we call temperature) is 0Â°C
> or less, a proportion of molecules have energies that would correspond
> to 10Â°C or even 100Â°C. This is given by the formula:
> 
> fraction of molecules with energy greater than Îµ = e-Îµ/kT where k is a
> constant, the Boltzmann constant, and T is the current temperature in
> Kelvin (Celsius + 273). e is 2.7182818.....
> 
> Since it's exponential, the proportion drops of rapidly though. The
> energy at a given temperature is 1.5kT, so try putting e-1.5*373/273
> into your calculator. It comes out around 12.8%, so that many molecules
> behave like they have a temperature of 100Â°C.
> 
> PS: for physicists: I understand that 1.5k*373 is not the same as the
> energy needed to sublimate. This is just to get across the idea that a
> significant proportion of molecules have much higher energy than the mean.
> 
> All solids and liquids have something called a "vapor pressure", which
> is essentially the average amount of gas molecules existing near the
> surface of the solid or liquid. If this vapor pressure of that gas at
> the puddle surface is higher than the number of molecules (of that gas)
> existing in the ambient, it will diffuse into the ambient, and there is
> a net transfer from the puddle to the atmosphere, hence the puddle shrinks.
> 
> This is why puddles evaporate quicker on drier days, and slower on humid
> days. And if the temperature drops (vapor pressure depends on
> temperature) and ambient humidity rises, this can go the other
> direction, and you get dew.

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