Try here...

http://en.wikipedia.org/wiki/Vapor_pressure

and here...

http://en.wikipedia.org/wiki/Triple_point

Hope this helps.

The links mjb has provided should clear up the confusion, I think.

A very short, simple answer is this: You have noticed, certainly, that wet things "dry out." (After you've washed the kitchen floor, it takes a little while to dry.) Evaporation happens at any temperature at which the compound is liquid (and sublimation happens at temperatures at which the compound is solid). Boiling is only a special case in which evaporation rate is so high that the liquid bubbles up from the rapid gas formation.

Understanding vapor pressure is a key to understanding the answer to your question, and I think the link provided by mjb is pretty good.

Molecules move continuously and exchange energy when colliding.

Even if most of them have a low speed some can reach such a speed (energy) that they can leave the liquid and go to the gaseous medium with which the fluid is in contact.

The higher the temperature the more intensive is the molecular movement and more molecules can get enough energy to leave the fluid so "evaporation" increases as temperature goes up.

This movement of molecules is called the Brownian movement.

As long as you have a fluid and a separation surface with a gas on the other side there will be vapours of the fluid in the gas.

The key to the whole thing is that it does not happen at atmospheric pressures. It will happen at a pressure below atmosphere. See the above post with a link to the explanation of the triple point of water.

Great guys thanks for the help. I think my confusion happened when I read this because of the assumption it was at atmospheric pressure. Throught my education, I've become familiar with pressure changes and the results of theose changes and can understand this concept. It was my error to assume it was at standard pressure.

I see also how part of a substances can have more energy than another and therefore can turn to a gas even at low temperatures but this wont accoutn for the enitre object right?

Thanks for clearing this up guys,

P.S. The triple point is really interesting thanks for that link!

I am afraid that it is not so.

Water(as also other liquids) has a vapour pressure at all temperature above the triple point whatever be the pressure in the system. This is an equilibrium property and not dependent on external pressure. Above the criitical point water can remain only as vapour.

Can water vapor exist below 0ºC? If so, how far below zero and how does that tie in to Frost Point (Tf)?

Thank you!

I am also interested in this topic. I was told by a manufacturer's rep that the definition of frost point is a dew point that is lower than 0ºC. The traditional definition of frost point is the temperature at which air is saturated with respect to water vapor over an ice surface.

To me these are conflicting definitions, as you can have a dew point lower than 0ºC in a process that is well above freezing. Can anyone clarify?

The dew point for the air is the temperature where the air has so much water, that it can't hold any more. This is why the second definition has the ice surface listed in it, as the ice will vaporize (sublimate is the technical name for this) to try to fill the capacity in the air.

When the temperature gets below the dew point, then the air has more water than it can actually hold. The air solves this problem by drooling all over the most water loving surface that it can find, like glass, or your lawn. If this temperature is below freezing, then the dew will form frost as soon as it comes out of the air. This means that the frost point is actually the dew point, just at lower temperatures.

I like your approach - "The air solves this problem by drooling all over the most water loving surface that it can find, like glass, or your lawn". Great mental image!

If I were to fill a vessel at room temperature with dry air, say right at 0ºC dew point, would that mean the vessel temperature would have to be lowered to 0ºC for frost to form? If so this is starting to make sense.

The next issue is whether the 'frost' point (saturation with respect to ice) or 'dew' point (saturation with respect to water) applies to the vessel at room temperature.

I fail to see how one would care about saturation with respect to ice in a process where there is no ice (e.g. the vessel temp is 25ºC). I think the distinction stems from the type of dew point sensor being used. If a mirrored surface is used that allows frost to form on its surface, then it is definitely a 'frost' point. Otherwise, it would be considered a 'dew' point.

I have a Wife that is Frigid at any Tempreture Celsious or Farenhiet. This in turn has caused a Total Vaporization ( sublimation ) of my humanity. Just a side Note: I took a viagra the other day and it got stuck in my throat, whats odd is , I had a Stiff neck for a week.

It can and does. http://hyperphysics.phy-astr.gsu.edu/HBASE/kinetic/watvap.html Here are a bunch of equations that lets one calculate the Vapor Pressure of water over liquid or solid water. http://cires.colorado.edu/~voemel/vp.html

Of course it can.

The specification for many factories' instrument air, for example, is that it shall be so dry as to have a dew point of -40degC. Above that temperature, the water is in the form of a vapour, i.e. steam, present in the air though in proportions far too low to form water droplets or ice crystals.

In another example, an aircraft flying at high altitude might be surrounded by low pressure air at similar sorts of temperatures. The vapour pressure of water present might be so low as to remain in the vapour state; alternatively it might be higher than the dewpoint and so form clouds.

Obtain a set of Steam Tables and go from there. Mayhew & Rogers is one source; Kempe's Engineers' Yearbook (any edition) is another; Perry, "The Chemical Engineer's Handbook", any edition, is another. While not exactly a cure for insomnia, any of these sources will give illuminating information about the properties of H₂O at a range of commonly-encountered temperatures and pressures.

As water vapor is cooled below the dew point, the water begins to condense out as tiny droplets. These droplet will condense at nucleation sites which are generally small dust particles of nano-micro dimensions. If those tiny paticles are not present, the vapor becomes super cooled to temperatures significantly lower than the traditional dew or frost points. Under atmospheric conditions we don't experience this supercooling because nucleation sites are normally present. All this is totally independent of any other gas involved. In 1802, Dalton showed that these condensation effects would happen the same way, even in a vacuum.

Water can also be a gas between 0 C and 100 C. Spill a little water on the floor, preferably on a non-pourous substrate. The water evaporates. The water molecules that evalorate, evaporate as a gas, regardless of how low a density. Similarly, materials such as elemental iodine sublime directly from a solid to a gas, as does dry ice.

Bob,the answer I gave you was the best I can do in a brief CR4 remark. If you are really interested in this subject, I suggest you read "Clouds in a Glass of Beer" by Craig F. Bohren. It's entertaining, educational, inexpensive and will answerer all your questions.

The process is called sublimation, which occurs with many other substances. Simply water particles escape from solidified liquid. This process is widely used to dry food by deep freezing then lowering pressure so triple point of steam chart is crossed and water then could not be in liquid form only as solid and gaseous. The reason for that is to preserve cells without breaking cell's membrane by grown crystals of water.

stan