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(Please excuse the wrong usage of words, I'm not a trained physicist. Please point out any misuse.)

So far, my understanding of water evaporating is the following:

  • Water always evaporates, whatever the temperature is.
  • The higher the temperature, the higher the vapor pressure, therefore the faster water vaporizes.
  • At sea level, water boils at 100°c. Boiling temperature decreases as athmospheric pressure decreases.
  • It takes a fixed amount of heat / energy to evaporate water (once it reaches boiling point?)

Consider the following scenario:

  • I'm at sea level
  • I have water boiling (hence at 100°c)

What happens if, instantly (< 0.01s), I give my boiling water enough heat to evaporate it all? Suppose that nothing prevents me from actually transferring the heat into the water. No pressure issue, no leidenfrost effect etc.

  1. Will the water instantly vaporize?
  2. Or will it breach the 100°c "barrier" and vaporize later?

Also, side question: if water can evaporate at 20°c, how much energy does it take at that temperature? Is it more/less than at 100°c?

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    $\begingroup$ Related to this $\endgroup$ – lucas Jun 20 '16 at 17:45
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    $\begingroup$ Define "instantly" $\endgroup$ – OrangeDog Jun 20 '16 at 17:56
  • $\begingroup$ Defined. That's the best I can do... Feel free to suggest definitions. $\endgroup$ – aspyct Jun 20 '16 at 17:57
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    $\begingroup$ I feel the answer is "yes" by definition of "enough" $\endgroup$ – OrangeDog Jun 20 '16 at 17:59
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    $\begingroup$ At some values of "instantly" we get nuclear fusion. $\endgroup$ – Aron Jun 21 '16 at 4:20
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From the original assumptions:

So far, my understanding of water evaporating is the following:

The higher the temperature, the higher the vapor pressure, therefore the faster water vaporizes.

The rate of water boiling, assuming a constant boiling temperature, is dependent on the rate of heat transfer to the water, not the vapor pressure.

At sea level, water boils at 100°c. Boiling temperature decreases as athmospheric pressure decreases.

It is somewhat more correct to say that the boiling temperature decreases as ambient pressure decreases, no matter what the source of the ambient pressure.

It takes a fixed amount of heat / energy to evaporate water (once it reaches boiling point?)

The heat of vaporization of water decreases as the boiling temperature (and boiling pressure) increases, up to the critical temperature (and critical pressure) of water. This means that if pressure changes substantially during the boiling process, the heat of vaporization of the water will also change substantially during the boiling process.

To answer the question directly, there are a few industrial situations where water is given enough heat to instantly evaporate if. For the case of hot oil systems in industry, a furnace heats up the oil to 450 deg F (maybe hotter), and sends the oil to process equipment for the purpose of boiling the process in a heat exchanger. Very infrequently (approximately 5 year cycles), the hot oil system is shut down for maintenance, and water is used to hydroblast equipment in order to clean it. This means that water collects in low points in the associated piping. After maintenance is complete, the proper start-up procedure calls for slowly heating the oil, checking all low points in the piping for water, and draining all water before the hot oil gets above the boiling point of water. On very rare occasions, there have been cases where a small slug of water was trapped in piping during startup without anyone knowing it. This slug of water was isolated from the hot oil by closed valves, so its temperature was too low to cause boiling. Unfortunately, in the event of a process operator opening those valves (for whatever reason), the trapped slug of water immediately contacts 450 deg F hot oil, causing an extremely rapid evaporation rate, and resulting in an explosion. And yes, this has indeed happened. So the answer is: if the water has access to enough heat, at a high enough temperature, it will indeed instantly vaporize.

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  • $\begingroup$ Great! I was hoping someone would review my assumptions! That's a terrific answer, and it calls for a follower question regarding the vapour pressure then. Thank you, that was awesome :) $\endgroup$ – aspyct Jun 20 '16 at 20:21
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The answers boils down to yes, the larger the rate of of heat (assuming you can transfer it at any rate you want), the larger the vaporization rate. The rate of change in internal energy at constant volume is $U=U_0+\dot Q\Delta t$, where $\Delta U$ is the total internal energy necessary to change the phase, and $\dot Q$ is the rate of heat transfer. Thus $\Delta t=\frac{\Delta U}{\dot Q}$, and you can make the evaporation time as short as you like.

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  • $\begingroup$ "boils down to": I see what you did there! Great answer though, thanks. $\endgroup$ – aspyct Jun 20 '16 at 19:42
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My answer is based on our everyday experience.

The simple answer is no.

Suppose you have a hot pan, now put a drop of water over it and you will see that it will sustain for a while. This is because the steam makes a insulating layer between the pan and water drop. This phenomena is always present which (may not be the only reason) is one of the reasons that water is not vaporized instantly.

EDIT:

After some discussion I change my answer to maybe.

Use of high power microwaves can heat the water to sperheated states which upon liberating (by small disturbance) can generate an avalanche of the evaporation and water may evaporate instantly.

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  • $\begingroup$ Good point. Suppose there was no insulation and that all the heat was effectively transferred to the water, what would happen then? $\endgroup$ – aspyct Jun 20 '16 at 17:45
  • $\begingroup$ I can not say. May be you can use microwaves of very high power and the water may instantly vaporize. Now the question is how fast is instantly. 1 second, 1 mili second ? My guess is within few seconds it can be vaporized. In this case it will act as steam bomb blast. $\endgroup$ – hsinghal Jun 20 '16 at 17:49
  • $\begingroup$ But then the point being, the water will never exceed its boiling point? $\endgroup$ – aspyct Jun 20 '16 at 17:51
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    $\begingroup$ Microwaves can heat water to super heated state, which result in sudden and large evaporation of the water. Never put a glass of water in microwave oven which do not have rotating bottom plate for a long time. It is more likely that water become super heated and upon touching an avalanche of evaporation occur and a large amount of it become steam which may burn the hand or even blast the glass. $\endgroup$ – hsinghal Jun 20 '16 at 17:57
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    $\begingroup$ Thermal inkjet printheads use the superheat principle to create vapor explosions in small volumes of ink; the resulting vapor bubble's expansion serves as the "piston" which ejects ink through a nearby nozzle. By heating the ink at a rate of about 300C per microsecond, it is possible to boost the temperature of the ink (which is mostly water) to between 260C and 280C before boiling begins. the stored superheat enthalpy furnishes the heat of vaporization necessary to fuel the resulting explosion. $\endgroup$ – niels nielsen Nov 11 '17 at 6:22
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If water in some volume $V$ becomes gas (the attraction between water molecules is switched off instantly), then the pressure of this gas will be enormous. This is how explosive works.

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  • $\begingroup$ Let's blow sh*t up! More seriously, what if the water was free to move? Like out of gravity, somewhere in outer space? If the water was in a sphere, and the heat started spreading from the center, do you think the outer layer of water could become a strong enough container to make an explosion out of this? $\endgroup$ – aspyct Jun 20 '16 at 19:45
  • $\begingroup$ Yes, the spherical volume will fly apart with a velocity close to a corresponding shock-wave velocity or somewhat faster. $\endgroup$ – Vladimir Kalitvianski Jun 20 '16 at 20:11
  • $\begingroup$ Along that line of thought check out en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion $\endgroup$ – whatsisname Jun 20 '16 at 23:20

protected by Qmechanic Jun 21 '16 at 3:51

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