hi, I was looking at a video of water explosions or vapor explosions. And came to my mind is there any measurements made on how long it takes for example a molecule of water to change to a molecule of vapor after it has reached its vaporized temperature point? I assume is instantaneous but has to be some time in the micro seconds I assume. any one knows?.

 

Are you aware of the 'Kinetic Theory' and its implications?

What do you see as the difference between a molecule of water and a molecule of vapour?

 

I'm no physicist, but I know that proteins fold in about 100ms, when folding@home first started the initial amount of time they could simulate per day was something like a nanosecond, I think I remember reading that they'd broken the millisecond barrier at some point.

In any case, I would guess somewhere between 1Ms and 100ns. I'd make a guess it depends on how far they can move apart in a given time given their surrounding atmosphere and how much energy is going into the water to keep them moving.

 

You might want to ponder studiot's post a bit longer...

 

While the unit molecule of water is the same in the condensed phases and in its gas phase, the condensed phases are characterized by hydrogen bonding.

Going from a hydrogen bonded phase to a gas would require breaking those bonds, which probably occurs at the rate of a molecular vibration (e.g., 10E-16 seconds).

According to Wikipedia:

The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at the timescales faster[emphasis added] than 200 femtoseconds.[6]

Thus, while that does not answer the question of how long it takes a single molecule to convert from a condensed phase to the gaseous phase, we can assume it is pretty fast, and is certainly not milliseconds. I would not be surprised if vaporization were even faster than that limit and approached that of simple proton transfer reactions.

John

 

Are you aware of the 'Kinetic Theory' and its implications?

What do you see as the difference between a molecule of water and a molecule of vapour?

Just the mini basics.
No difference in structure but different in state.

 

While the unit molecule of water is the same in the condensed phases and in its gas phase, the condensed phases are characterized by hydrogen bonding.

Going from a hydrogen bonded phase to a gas would require breaking those bonds, which probably occurs at the rate of a molecular vibration (e.g., 10E-16 seconds).

According to Wikipedia:


Thus, while that does not answer the question of how long it takes a single molecule to convert from a condensed phase to the gaseous phase, we can assume it is pretty fast, and is certainly not milliseconds. I would not be surprised if vaporization were even faster than that limit and approached that of simple proton transfer reactions.

John

Thanks, so can we say the change is instantaneous? At what speed can something be said to occurred instantaneous?.
Proton transfer reactions occur in a much smaller space.

 

No difference in structure but different in state.

Which means what exactly?

What state is for instance a single isolated molecule in? Solid? Liquid? Vapour?

Just the mini basics.

I think you already know the answer and so I'm trying to lead you to answer your own question.

 

Which means what exactly?

What state is for instance a single isolated molecule in? Solid? Liquid? Vapour?



I think you already know the answer and so I'm trying to lead you to answer your own question.

You mean vaporization time has to do with the potential energy stored in the molecules and is release after the vaporized temperature is reached?
I have no idea what you mean with your questions.
My question is simple and was what is the time it takes for a molecule of water to change from liquid to gas after it has reached its vaporized point 212F. or 100C.I assumed is in a few milliseconds because it must leave the surface of the water to became vapor plus the time it takes to change to vapor. Also I asked what could it be considered instantaneous measured in time.

 

I assumed is in a few milliseconds because it must leave the surface of the water to became vapor plus the time it takes to change to vapor. Also I asked what could it be considered instantaneous measured in time.

That's funny, I always assumed that the liquid to vapor transition could happen anywhere within the liquid mass, hense all the bubbling when boiling. Are not those raising bubbles small areas of vaporised water in gas phase?

I don't think the transistion from liquid to vapor state is limited to the surface layer of water. So does your transition time include the time to raise to the surface? Could take many millisec. Or is the transition time the time to convert from liquid to gas bubble within the liquid? Thats probably very fast.

Lefty


Lefty

 

That's funny, I always assumed that the liquid to vapor transition could happen anywhere within the liquid mass, hense all the bubbling when boiling. Are not those raising bubbles small areas of vaporised water in gas phase?

I don't think the transistion from liquid to vapor state is limited to the surface layer of water. So does your transition time include the time to raise to the surface? Could take many millisec. Or is the transition time the time to convert from liquid to gas bubble within the liquid? Thats probably very fast.

Lefty


Lefty

A good observation, the molecules that vaporize inside the liquid as soon as they vaporize since they are lighter than water they still has to raise up and separate somehow from the surrounding water as soon as they became vapor so this minuscule distance must take additional time too. What is still water wants to stay uniform hence its has potential energy that is transferred into kinetic energy and into vapor by the raise in temperature. At the surface of water or inside they raise up as soon as they became vapor.
You think it could be said that it happens instantaneous?.

 

lighter than water

Do you really think a molecule of water in the gaseous state is lighter than a molecule of water in the liquid state?

I have no idea what you mean with your questions.

You keep talking about a single molecule. So I asked if a single isolated molecule can be considered as a solid, liquid or gas.

You said you have basic knowledge of the kinetic theory. Study of this theory would go a long way to answering you question.

One of the key concepts of the Kinetic Theory is that we can't consider individual molecules in aggregates of matter. We have to consider averages.
It is the average properties of an aggregate that decidses whether the body of water should be considered solid, liquid or gas. The term is meaningless for an individual molecule.

A second key concept of the Kinetic Theory is that of dynamic equilibrium.

Once you have a good hold of these you can proceed to the third concept which answers your specific question. This proposes that energy is exchanged between molecules by collisions. When two liquid mlecules collide and one gains enough energy to be considered a gas it is then by definition a gas. The change can be regarded as instantaneous at the moment of collision. Within any body of liquid there are always some molecules inthis situation and most of the change to gas occurs within the liquid. However most of these molecules then loose energy in further collisions and never make it out into the atmousphere.

If we now return to the concept of averages, we can calculate the average % of molecules at any one time that have enough energy to be considered as a gas. We can also calcualte the average time for a molecule to gain enough energy and the average number of collisions to do this.
This figure might be closer to what you are seeking.

 

If we now return to the concept of averages, we can calculate the average % of molecules at any one time that have enough energy to be considered as a gas. We can also calcualte the average time for a molecule to gain enough energy and the average number of collisions to do this.
This figure might be closer to what you are seeking.

Can you throw some numbers into your calculations to provide a ballpark estimate of just the rate of conversion of liquid water to water(g) at STP? Obviously, the net conversion is zero at equilibrium, so you can add other reasonable assumptions. For simplicity, assume there is negligible hydrogen bonding in the gas phase.

The initial question, however, asked for the time it took for vaporization in a non-equilibrium situation in which energy was being added.

And came to my mind is there any measurements made on how long it takes for example a molecule of water to change to a molecule of vapor after it has reached its vaporized temperature point?

I suggested in that situation that the time from when a molecule attains sufficient energy to be a gas, say from absorption of a photon of infrared light, to vaporization is not instantaneous, but rather is limited by the time it takes to break the hydrogen bonding. A fairly large range for that time was suggested earlier based on statements in Wikipedia and from what is known of proton transfer reactions from other sources. Nevertheless, that period is neither instantaneous nor is it several milliseconds.

John

 

Do you really think a molecule of water in the gaseous state is lighter than a molecule of water in the liquid state?

No I did not said that, I said MoleculeS as a group.
Yes your are right my first post was stated wrong. A single molecule is the exact same thing no matter in what state is on.

 

Can you throw some numbers into your calculations to provide a ballpark estimate of just the rate of conversion of liquid water to water(g) at STP? Obviously, the net conversion is zero at equilibrium, so you can add other reasonable assumptions. For simplicity, assume there is negligible hydrogen bonding in the gas phase.

The initial question, however, asked for the time it took for vaporization in a non-equilibrium situation in which energy was being added.

I suggested in that situation that the time from when a molecule attains sufficient energy to be a gas, say from absorption of a photon of infrared light, to vaporization is not instantaneous, but rather is limited by the time it takes to break the hydrogen bonding. A fairly large range for that time was suggested earlier based on statements in Wikipedia and from what is known of proton transfer reactions from other sources. Nevertheless, that period is neither instantaneous nor is it several milliseconds.

John

Thanks a lot so you think is not instantaneous now, ok when can something be considered to occur instantaneously? Something in the nanoseconds o picosecond, femtosecods?
I have the answer and I think we can say it happens instantaneous because it happens at the moment of change I mean there is nothing in between when the molecules reach certain temperature the next is just to become gas.

 

So do you want to discuss the Kinetic Theory, in relation to your question?

 

So do you want to discuss the Kinetic Theory, in relation to your question?

If you are refering to me yes I do I find it interesting.
You think we can say it happens instantaneously?
I think so.

 

I am trying to get you to think about what you mean by your question.

Is the question how long does it take to boil a kettle dry? i.e. are you talking about the bulk or aggregate of water? The answer obviously depends upon your stove and kettle.

In terms of the Kinetic theory, this states the following.

All molecules are in a state of motion.

These molecules exhibit a range of velocities and therefore energies at all temperatures.
This range obviously has an average with some molecules above the average and some below.

At any one time some of these molecules will possess enough velocity to be considered gaseous.

As we heat the mass of water (increase the temperature) the proportion of such molecules will increase.

At some point there will be more molecules with such energy than without and we say the water boils. At boilong point the average energu corresponds to this.

Even when the average energy is above boiling point some molecules will be below the average sufficiently to be considered liquid.



So there is no clear cut transition between the two states for an aggregate of water.
There is a dynamic equilibrium with exchange going on all the time, the balance depending upon energy input.

 

At some point there will be more(emphasis added) molecules with such energy than without and we say the water boils. At boilong point the average energu corresponds to this.

I followed your argument up to that point, I think. Can you provide a citation for those two sentences? In particular, if the average kinetic energy equals the minimum for the gaseous state, then how can more than half have greater than that amount of energy?

What causes that skewing. Intuitively, which I agree is dangerous to introduce here, it would seem that because the upper temperature/energy is not bounded or is at least quite high, but the lower clearly is bounded (i.e., 0°K), that any skewing would be in the opposite direction. That is, a few very hot molecules would offset many more colder molecules in calculation of the average.

Why does water not boil dry, unless you continually add heat to it? (I am asking how you explain the heat of vaporization with your model.)

John

 

Studiot what you said is as a general idea but what I meant was in a given time when an avarage of molecules have gain enough energy to vaporize that is a temperature of 100C. At that instant is not liquid anymore it becames gas. What is the transition time?
I already have the anwswer it happens in an instant because does not occurr anything in between that change after 100C is reached for that particularr avarage of molecules are in the gaseous form.
Maybe I don't understand enough about this but it makes sense to me.
Jphalt Is going to a tangent I think, but nevertheless is interesting.