chem project, acetone/anything evaporating

[brianmay27]

ok, so this i cant seem to get a clean answer.

 

So why is it that something like water having a boiling temp of 100 still evaporates at room temp?

 

i came up with two answers and want to see if they are right

1) the pressure of the water particles is a bit more then that of 1 atm and thus slowly break free,

or

2) the substance bonds with the air and slowly evaporates that way.

 

is one of thous correct?

[Darkblade48]

It has to do with equilibrium, actually.

 

Water in a closed environment (say a container), as you know, will also evaporate to some extent, but then the air becomes saturated with water moisture. The amount of water that condenses from the air to water is equal to the amount of water that evaporates from the water to the air. As a result, no more evaporation occurs.

 

Conversely, in an open environment, the amount of water in the air is not in equilibrium (i.e. the rate at which water goes into the air is faster than the rate at which water from the air condenses back into your beaker), so the water eventually evaporates.

 

Of course, if you had an air tight room, and had enough water, you could eventually saturate the air in the room so that it would be at equilibrium...

 

Hope that helps.

[brianmay27]

well, kinda

 

i am more saying why does water evaporate even if it does not boil.

 

so like you need to add J. to water to get it to boil before it starts steaming, so why is it that the water will evaporate when it is not hot enough to steam.

 

are you saying that it tries to defuse so everything in the area (lets say it is in a jar) is the same quantity?

[Melvin]

Some percentage of water molecules have enough energy to evaporate, even below the boiling point. This percentage increases as the temperature increases.

 

Try this:

 

Wet a cotton ball with water. Rub some water on the palm of your hand. Wave you hand around in the air. Your hand should feel cooler as the water evaporates off your skin.

 

Also, try it again with rubbing alcohol (isopropanol) on one side of your hand and water on the other. The alcohol side should feel cooler faster because alcohol is more volatile (evaporates easier).

 

Melvin

[Externet]

well, kinda

 

i am more saying why does water evaporate even if it does not boil.

 

so like you need to add J. to water to get it to boil before it starts steaming, so why is it that the water will evaporate when it is not hot enough to steam.

 

Well, the misconcept is the water does NOT evaporate at 100 Celsius.

The water evaporates at ANY temperature.

 

What happens at 100 Celsius is NOT evaporation, is change of state to STEAM, which is NOT vapor.

 

The misuse of the word vapor -even in text books- under and over 100 Celsius produces the misconception.

 

If vapor meant being over 100 Celsius, clouds would have to be hot, and they are not. They are vapor, not steam.

 

Miguel

[DrDNA]

The answer is vapor pressure.

Wiki does a pretty good job of describing it.

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

 

But I think this is an even better description:

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html

[John Cuthber]

What happens at 100C is that the vapour pressure (and that is vapour in spite of what miguel says) reaches 1 atmosphere.

There seem to be 2 conflicting ideas about the distinction between water vapour and steam. As far as science is concerned, the clouds are an aerosol of liquid water in air. The stuff you can see is liquid.

Water vapour is not visible- nor is the vapour from acetone or most other things).

The problem arises because we have all seen the steam comming out of a boiling kettle and we know that steam drives the pistons in steam engines. Unfortunately, these aren't the same thing.