NationStates Jolt Archive

a chemistry question. can water evaporate at below 100 degrees celcius?


its not an especially stupid question - i know that to change phase the h2o molecules need enough energy to break the inter-mollecular bonds. surely below 100 degrees there just isn't enough energy.

my friend says, however, that there is enough energy at room temperature to allow some inter-mollecular bonds to be broken. so at room temperature some of the water will evaporate. he says this is why puddles and water left on worksufaces go away in a few days.

i'm not convinced. those bonds require a certain amount of energy to break and change phase. there isn't enough uniform energy in the liquid to do that, and i never heard anything about energy 'collecting' or amassing in areas allowing a few molecules to have enough energy to evaporate.

please help!

Yes of course it can. Rainwater puddles dry up in a few hours dont they?

I'm no chemistry expert but i'm pretty sure your friend is right

Yes of course it can. Rainwater puddles dry up in a few hours dont they?
i figured they just dissapate (sp?) into the ground. tarmac is pourous (sp?)

god i can't spell today :headbang:


the thing is the electrochemistry just doesn't add up as far as i can see :confused:

Yes.

1) it always evaporates, just more slowly when the temperature drops. Even ice can evaporate
2) in a lower airpressure (like high up in the mountains) water cooks under 100 C

I'm not a chemistry major but here goes. You can think of the h2o molecules as having different speeds and a temperature is the average speed. Thus some molecules will have above average speed and "break free" from the liquid form.

i figured they just dissapate (sp?) into the ground. tarmac is pourous (sp?)

god i can't spell today :headbang:


the thing is the electrochemistry just doesn't add up as far as i can see :confused:

I know very little about chemistry, but I'm pretty sure water does evaporate below 100 degrees. The water cycle would not work if it didn't. Water needs to evaporate to form clouds, etc.

yes, air pressure can affect it, also as salt can make it boil at 110

The boiling point is the temperature at which the vapor pressure of a liquid is equal to the ambient air pressure. At 1 atm, for pure water, this is 100 degrees Celsius.

This means at 100 degrees, the pressure of the water vapor escaping from the water is equal to that of the air around it. At lower temperatures, you still have evaporation but at a much lower rate.

Yes it can. 100 degrees is the point at which almost all water molecules have risen enough energy levels to break from the liquid and evaporate into the air. Puddles evaporate in sunlight, but they don't reach 100 degrees do they? And they don't get absorbed. Some will, but not many. Tarmac is surprisingly water proof, as is solid concrete, but water evaporates off that too.

It's all to do with energy levels in the substance, and the average temperature of the molecules within the sample. Can't remember exactly, I learnt it a while ago.

i figured they just dissapate (sp?) into the ground. tarmac is pourous (sp?)

Well a work surface in your kitchen isn't, but puddles disappear from there after a time.

If you want to actually see it happen with your own eyes, heat up some water to 80 or 90 degrees or so and you'll get some visible vapours, or just put your hand over it and you'll feel it.

The boiling point is the temperature at which the vapor pressure of a liquid is equal to the ambient air pressure. At 1 atm, for pure water, this is 100 degrees Celsius.

This means at 100 degrees, the pressure of the water vapor escaping from the water is equal to that of the air around it. At lower temperatures, you still have evaporation but at a much lower rate.

I was about to say something along those lines, but saw he/she had beaten me to it :D. Lucky I scrolled down the reply window slightly...

I'm no chem. major, but here's what I do know.

Water will accumulate energy if it's left outside unprotected, like in a puddle. Once that accumulation begins, molecules with the highest energy and more motion will rise to the top of the water. Because they have more motion and energy, the bonds to other water molecules are weakened. Then all ot takes is either the accumulation of enough energy through heat absorbtion, or a stiff wind across the top of the puddle, and the water molecules leave the puddle as a gas.

This is the reason people will blow on soup or coffee to cool it down, because the wind removes the hottest molecules, or the ones with the most energy, and cools the liquid.

Yes, to add to what some people have already said, the molecules in a puddle of water have energies spread across a certain range, in what I *think* is called the Boltzmann distribution (although that may very well be wrong) - imagine a curve in roughly this shape:

.
. .
. .
. .
. .
. .

with energy increasing left to right, and the number of molecules with a certain energy being the height of the curve at that point. All of the molecules to the right of a certain point of the curve will have enough energy to evaporate; as it gets hotter, the shape of the curve shifts so that a greater proportion is to the right of this point, and so a higher proportion of the water molecules have sufficient energy to evaporate.

This may not be absolutely accurate in all specifics - I haven't got time to find my kinetics notes - but the general principle is certainly correct.

Yes water can evaporate at any temperature above freezing. After freezing some molecules will still escape into the air. The process is called sublimation rather than evaporation though.

Yes, to add to what some people have already said, the molecules in a puddle of water have energies spread across a certain range, in what I *think* is called the Boltzmann distribution (although that may very well be wrong) - imagine a curve in roughly this shape:

.
. .
. .
. .
. .
. .

with energy increasing left to right, and the number of molecules with a certain energy being the height of the curve at that point. All of the molecules to the right of a certain point of the curve will have enough energy to evaporate; as it gets hotter, the shape of the curve shifts so that a greater proportion is to the right of this point, and so a higher proportion of the water molecules have sufficient energy to evaporate.

This may not be absolutely accurate in all specifics - I haven't got time to find my kinetics notes - but the general principle is certainly correct.

Well, sod that, it doesn't put my dots in the right places!! Ummm, the curve is shaped like a hill! Or perhaps a bell, if you prefer.

ok it looks like i'll have to concede.

its just that i still think the energy required to break the inter-mollecualr bonds is not enough under 100 degrees, and there's not that much energy fluctuation/difference within the liquid. oh well

http://www.straightdope.com/mailbag/mevaporation.html

*Snip*

Yeah, that's the one. Boltzmann factorand distribution.

The boiling point is the temperature at which the vapor pressure of a liquid is equal to the ambient air pressure. At 1 atm, for pure water, this is 100 degrees Celsius.

This means at 100 degrees, the pressure of the water vapor escaping from the water is equal to that of the air around it. At lower temperatures, you still have evaporation but at a much lower rate.
You are almost there! :)

Pure Metal: Your general problem is what's called a "phase equlibrium" problem where you have two phases: a liquid phase (which, for simplicity, we assume contains only water) and a vapor phase (which contains oxygen, nitrogen, and water).

Every liquid exerts a "vapor pressure" against its surroundings. Some solids (like carbon dioxide, or dry ice) will also exert a vapor pressure. This is the minimum pressure you need to exert on that substance to keep it in the liquid or solid phase. Vapor pressure increases with temperature. This is why water will boil below 100 C in the high mountains: The pressure up there is lower than at sea level, so you don't need as high a vapor pressure to boil. This is also why pressure cookers work: The pressure inside the cooker is higher than normal, so you need more vapor pressure to boil.

Now, you can have anything press in on that liquid to keep it a liquid. However, if the "anything" pressing in on the liquid isn't made from the same stuff as the liquid, itself, then molecules of the liquid are going to start diffusing slowly into the vapor. This is why a puddle will evaporate into hot, dry air, but will not evaporate into hot, moist air.

Raoults Law says that for the two phases to be in equlibrium, {the vapor pressure of the water in the liquid phase} must be equal to {the partial pressure of water in the vapor phase}. "Partial pressure" is just another expression for how much of something is in a vapor phase. Air is usually at a pressure of 1 atmosphere and consists of 20% oxygen and 80% nitrogen, so the partial pressure of oxygen is 0.2 atm and the partial pressure of nitrogen is 0.8 atm. If there is no water in the air, then {the partial pressure of water in the vapor phase} is zero. This results in liquid water evaporating even though it's not at the boiling temperature. Since the sky is so large, the water that evaporates is carried away, so {the partial pressure of water in the vapor phase} remains zero.

If it's a humid day, then {the partial pressure of water in the vapor phase} will be greater than zero, but probably less than {the vapor pressure of the water in the liquid phase}, so the puddle will still evaporate, but more slowly.

Being above the boiling point merely means that the vapor pressure is greater than the total pressure, so there's no way that any partial pressure is ever going to match it.

I could throw equations at you, but this is probably enough for you to swallow as it is.

--The Democratic States of Cogitation
"Think about it for a moment."
Founder and Delegate of The Realm of Ambrosia

ok it looks like i'll have to concede.

its just that i still think the energy required to break the inter-mollecualr bonds is not enough under 100 degrees, and there's not that much energy fluctuation/difference within the liquid. oh well
Why do you think water cools when it evaporates? The high energy particles are leaving it

You are alomst there! :)

Pure Metal: Your general problem is what's called a "phase equlibrium" problem where you have two phases: a liquid phase (which, for simplicity, we assume contains only water) and a vapor phase (which contains oxygen, nitrogen, and water).

Every liquid exerts a "vapor pressure" against its surroundings. Some solids (like carbon dioxide, or dry ice) will also exert a vapor pressure. This is the minimum pressure you need to exert on that substance to keep it in the liquid or solid phase. Vapor pressure increases with temperature. This is why water will boil below 100 C in the high mountains: The pressure up there is lower than at sea level, so you don't need as high a vapor pressure to boil. This is also why pressure cookers work: The pressure inside the cooker is higher than normal, so you need more vapor pressure to boil.

Now, you can have anything press in on that liquid to keep it a liquid. However, if the "anything" pressing in on the liquid isn't made from the same stuff as the liquid, itself, then molecules of the liquid are going to start diffusing slowly into the vapor. This is why a puddle will evaporate into hot, dry air, but will not evaporate into hot, moist air.

Raoults Law says that for the two phases to be in equlibrium, {the vapor pressure of the water in the liquid phase} must be equal to {the partial pressure of water in the vapor phase}. "Partial pressure" is just another expression for how much of something is in a vapor phase. Air is usually at a pressure of 1 atmosphere and consists of 20% oxygen and 80% nitrogen, so the partial pressure of oxygen is 0.2 atm and the partial pressure of nitrogen is 0.8 atm. If there is no water in the air, then {the partial pressure of water in the vapor phase} is zero. This results in liquid water evaporating even though it's not at the boiling temperature. Since the sky is so large, the water that evaporates is carried away, so {the partial pressure of water in the vapor phase} remains zero.

If it's a humid day, then {the partial pressure of water in the vapor phase} will be greater than zero, but probably less than {the vapor pressure of the water in the liquid phase}, so the puddle will still evaporate, but more slowly.

Being above the boiling point merely means that the vapor pressure is greater than the [i]total pressure, so there's no way that any partial pressure is ever going to match it.

I could throw equations at you, but this is probably enough for you to swallow as it is.

--The Democratic States of Cogitation
"Think about it for a moment."
Founder and Delegate of The Realm of Ambrosia
wow, excellent explaination! thanks :)
i had to read it about 4 times (haven't studied chemistry or physics in about 3 years... but it definatley makes sense. cheers


i was definatley wrong then. bah...

wow, excellent explaination! thanks :)
i had to read it about 4 times (haven't studied chemistry or physics in about 3 years... but it definatley makes sense. cheersGlad to be of service. :)

--The Democratic States of Cogitation

That was a really great summery. So why not blow your mind?

http://www.lsbu.ac.uk/water/phase.html

Water is so much more interesting that it's typically given credit for.

After checking out that cool learning, there's always a copy of Kurt Vonnegut's Cat's Cradle.

I was always under the impression that evaporation had something to do with the sun and the type of energy that it gives off.

I was always under the impression that evaporation had something to do with the sun and the type of energy that it gives off.
Sunlight will speed up evaporation by heating the liquid somewhat. A heat lamp will do the same thing. Even in a dark room evaporation goes on.

A puddle of water is made of tiny water molecules that keep boucing around. Usually they only bounce a little distance before they hit another water molecule, but near the surface a water molecule can be knocked right out of the puddle. That's evaporation. The hotter the water is, the faster the molecules move, and the more likely they are to be knocked out of the puddle. When the tempreture reaches 100 degrees, the molecules are moving fast enough to overome the attraction they have for each other and they stop sticking together and fly off and become water vapour.

The Hydrological Cycle? Evaporated water... condensing into clouds? Happening in places like the UK and Ireland... Where it rains all the time... and is never anywhere near 100 degrees 'c'.

Cambridge Major - yup, it's the Boltzmann Distribution - you beat me to that idea.

a chemistry question. can water evaporate at below 100 degrees celcius?


its not an especially stupid question - i know that to change phase the h2o molecules need enough energy to break the inter-mollecular bonds. surely below 100 degrees there just isn't enough energy.

my friend says, however, that there is enough energy at room temperature to allow some inter-mollecular bonds to be broken. so at room temperature some of the water will evaporate. he says this is why puddles and water left on worksufaces go away in a few days.

i'm not convinced. those bonds require a certain amount of energy to break and change phase. there isn't enough uniform energy in the liquid to do that, and i never heard anything about energy 'collecting' or amassing in areas allowing a few molecules to have enough energy to evaporate.

please help!

Sorry, you are wrong, your friend is right. At any temperature above 0 degrees, water can theoretically evaporate. Think of it this way: when you enter the shower, there is steam, right? That water is nowhere near 100 degrees. There you go.

(P.S. So yea, the intermolecular bonds can be broken below 100 degrees. 100 is the temperature when all bonds are broken, and water fully becomes a gas.)

One of the things I learned in Chemistry.....

.....water is the coolest stuff ever. It does so much random stuff, and breaks all the rules. Water rocks.