The physics behind the Dew Point phase shift

[karen_lorr]

Many websites gives the results of Dew Point but not the reason.

What is going on at the dew point? Not the result (at a certain VSP vapour becomes liquid if it can find nuclei to coat) but the reason for the result.

I have been reading up on this – just out of curiosity – and would like someone to tell me if I’m on the right lines or way off.

As far I understand it when Water vapour molecule slow down (due to a heat loss) they can no longer be a gas simply as they are moving too slowly to maintain their distance apart. When they move closer they become a liquid.

Is that right?

Or can anyone give a simple to understand reason behind phase shift.

Thank you

K

 

[russ_watters]

That's relates, but I like to look at it a different way: The air's moisture holding capacity varies with temperature. Dew point is the lowest temperature a parcel of air can be and still hold on to the moisture it has.

 

[karen_lorr]

I know that at a certain saturation vapour pressure the gas will shift to liquid.

What I'm trying to work out is why?

There must be something going on with the molecules - I think it may be down to their speed (maybe they slow down and at a certain speed they can't rebound so much - but that doesn't sound right :)
But also why (with no nuclei) will the H2O stay as gas all the way down to around -37 or less

I know the results but what I'm trying to get an insight into is what is really going on

The "air" doesn't really have a moisture holding capacity as the H2O gas can exist in a vacuum (apart from itself)

 

[256bits]

Perhaps if you look at it from a liquid point of view.
Well, what does the vapour pressure of a liquid depend upon. Surely temperature of the liquid has some say in the matter.
And if you have heard of the chemical forces that hold the molecules of a liquid together, such as hydrogen bonds and the like, you realize that as the molecules of the liquid jostle about, some may obtain enough kinetic energy to overcome these bonds, and escape the liquid surface and become vapour. Increase the liquid temperature, and a greater percentage of molecules would have enough kinetic energy to escape.
If a quantity of the liquid is sealed in container, the vapour pressure above the liquid will represent the percentage of molecules that have enough energy to escape at a certain temperature.

Why does the vapour pressure only reach a certain value at a particular temperature. Surely if the vapour is saturated, one can promote more evaporation by just increasing the volume above the liquid.
Good point. But that reduces the number of collisions of the molecules in the vapour state with the liquid surface. Any vapour molecule of any kinetic energy can become part of the liquid. a molecule of high kinetic energy can barge right in, and one of low kinetic energy can be attracted by the other liquid molecules.

Karen lorr, can you connect the dots now? on saturation pressure and dew point?

 

[pmacias]

udos to you for taking the initiative to research and understand the physics behind the dew point phase shift. You are indeed on the right track with your understanding of the process.

To put it simply, the dew point is the temperature at which the air is saturated with water vapor. When the temperature of the air drops to the dew point, the water vapor molecules in the air start to slow down and lose energy. This causes them to come closer together and condense into liquid droplets, forming dew or fog.

The reason for this phase shift is due to the properties of water molecules. Water is a polar molecule, meaning it has a slight positive charge at one end and a slight negative charge at the other end. This makes water molecules attract each other and form weak bonds called hydrogen bonds. When water molecules in the air slow down and come closer together, these hydrogen bonds become stronger and cause the molecules to stick together, forming liquid droplets.

Furthermore, the temperature at which the phase shift occurs is dependent on the amount of water vapor present in the air. The more water vapor there is, the higher the dew point temperature will be. This is why the dew point is often used as an indicator of humidity.

I hope this explanation helps you understand the physics behind the dew point phase shift. Keep up the curiosity and enthusiasm for science!