[lip]

Hi all,

I want to understand boiling of liquid in molecular level. As per my understanding when a liquid in a closed system reach  the equilibrium, rate of vaporization of liquid equals to rate of condensation. So  the energy supply to the system after this point is utilized to increase the temperature of the liquid phase and establish a new equilibrium with a higher vapor pressure. This process will continue till  critical temperature where there is no difference between two phases.

What will happen to equilibrium when another type of gas introduced in to gas  phase(increase Pressure by p). how it effect the vaporization   ? how this alter the boiling temperature?


Thank for your time on this

[lip]

Hi
I think about this more and come up with a following concept.

I think about two cases
Open system  under external pressure P
When the energy is supplied  to system, some of this energy is utilized to vaporize the molecules trough the phase boundary. but if external pressure P is greater than temperature T,   liquid is unable to form bubbles and evaporate more liquid phase molecules. so excess energy is utilized to increase the kinetic energy (temperature ). With the increase of T equilibrium vapor pressure will increase and come to a point where it equal to external pressure. At this point liquid is able to form bubbles and utilized entire energy to evaporate liquid without increasing the temperature. there is no liquid phase after this point due to several factors. As the system is open this will not reach the equilibrium water pressure outside. so  all the liquid will evaporated.

Is this correct way of thinking
   

[Borek]

As per my understanding when a liquid in a closed system reach  the equilibrium, rate of vaporization of liquid equals to rate of condensation.

OK, that would mean saturated vapor.

So  the energy supply to the system after this point is utilized to increase the temperature of the liquid phase and establish a new equilibrium with a higher vapor pressure.

OK

This process will continue till  critical temperature where there is no difference between two phases.

No boiling on the way up?

What will happen to equilibrium when another type of gas introduced in to gas  phase

Think if the fact that water boils at different temperatures at different heights above sea surface is not a hint.

[Borek]

With the increase of T equilibrium vapor pressure will increase and come to a point where it equal to external pressure. At this point liquid is able to form bubbles and utilized entire energy to evaporate liquid without increasing the temperature. there is no liquid phase after this point due to several factors.

Before liquid phase disappears you will have a period when the temperature of the system doesn't change and amount of liquid diminishes. Please remember under constant pressure boiling temperature is constant.

[lip]

Thanks Borek,

I think my real question is why liquid boils at boiling point under atmospheric pressure even though the system is an open one.

Now I understand  bit more . One point so bother me is why liquid need to achieve temperature where  it saturated vapor pressure equal to  external pressure to boils even the system is open one and  what so special about boiling point.

so now I think this way. when a liquid is heated, some of the energy provide is utilized to vaporize liquid. but as there is an external pressure P, liquid is unable to form bubbles so most of energy is utilized to increase the kinetic energy. Process will go on till saturated vapor pressure reach atmospheric   pressure.

After this point liquid is able to form bubbles within the fluid that now have enough internal pressure to expand against the external pressure. so entire energy spend on vaporization  instead of increasing the T(Entropy driven ?). SO no Temperature increase at this point until all the liquid vaporized. but if somehow we are able to build water pressure in atmosphere to saturated water pressure. then increasing Temperature would resume.    right?

[Borek]

but if somehow we are able to build water pressure in atmosphere to saturated water pressure. then increasing Temperature would resume.

Up to this moment you were right, but you lost me with this statement.

[lip]

Hi borek,

What I meant was that if we somehow able to build up the liquid vapor pressure (by closing the system )which is equal to saturated vapor pressure of the liquid, then again system is in equilibrium with its vapor phase and no net  vaporization occurs. So any addition of  energy would be utilized to increase the (Kinetic energy)temperature and to establish a new equilibrium right?

Example :We have a closed system with liquid water and system in equilibrium at 100 C. so water in this system would not boil under these condition and any additional energy would be used to increase the T . right?   

[Borek]

Example :We have a closed system with liquid water and system in equilibrium at 100 C. so water in this system would not boil under these condition and any additional energy would be used to increase the T . right?

If you heat the system T goes up, no doubt about it. Whether the liquid will not boil - I am not sure. I see two possible scenarios - in one, heating increases the pressure in the gas phase fast enough to not allow boiling. In the other, pressure doesn't rise fast enough, and added heat boils the liquid, till the pressure is high enough to stop boiling. Net effect is in the end the same, we just get there by slightly different path. To find out which answer is the correct one you would need to do some simulations using exact values of specific heats and the boiling pressure vs temperature curve.

Could be the answer is the same for all liquids, I just don't know.