Are boiling and evaporation the same?
If it takes 212 degrees to change liquid water into a vapor, how is it that a mud puddle can evaporate even when the temperatures outside are well below 100 degrees?
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Used knowledge is even more valuable.
Replies
Boiling occurs throughout the volume of the liquid, evaporation occurs only at the surface.
Boiling can only occur at a temp above the boiling point of that liquid. Evaporation can occur at any temp.
Molecules exert a certain amount of attraction or molecular force between them and their neighboring molecules. The amount of force dictates what state the substance will achieve at a given temperature.
A little force, then the molecules move freely, the substance may be a gas.
Medium force? The molecules stay close to one another, but still can slide around, it might be a lquid.
Strong force? Molecules are held tight aginst one another, it may be a solid.
When a liquid is at room temp, the body of liquid as a whole may appear to be just sitting there. However, on a molecular level, molecules are in motion, bumping against each other. If you were to look at the molecules that make up the surface layer of the liquid, every once in a while one of those moving molecules will get enough energy to ovecome the molecular force holding it to its neighboring molecules. When it overcomes that force, it breaks away into free air. That's evaporation.
Cool the liquid below room temp, and the energy level in the liquid decreases, fewer molecules gain enough energy to escape the bonds of their neighbors, there is less evaporation.
Raise the liquid above room temp, or in your case, a puddle in the sun, and the energy level at the surface is raised, more molecules are able to escape, and the rate of evaporation increases.
Raise the temperature of the liquid to the boiling point of that liquid? Now you have a case where the AVERAGE motion of the particles is fast enough to overcome the forces holding them close together. Since the liquid is heated evenly throughout, molecules or particles of the liquid are escaping not just from the microscopic surface level of the liquid, but from within and throughout the volume of liquid itself.
The mixture "boils."
It's easier for evaporation to occur, as molecules of the liquid that make up the surface layer have fewer molecules surrounding them, and thus a lesser force is being exerted on them.
Molecules of liquid within the volume of the liquid are surrounded by other molecules, each exerting a holding force on it, thus the retaining force can be higher.
Edited 5/12/2007 10:31 pm ET by Mongo
Not as consise as piffen's explanation, but from an old chemistry major,.... bravo!
Dave
More a matter of physics than chemistry isn't it?
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Biology is mostly chemistry, chemistry is mostly physics, physics is mostly math and math is just something that we made up.
"Biology is mostly chemistry, chemistry is mostly physics, physics is mostly math and math is just something that we made up."
If that paradigm is correct, then life as we know it is "logically" just a figment of our imagination. New knowledge is priceless.
Used knowledge is even more valuable.
So why did the math teacher always want to see my work?LOL
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"No, no, you're not thinking; you're just being logical." - Niels Bohr;-)
They both get mushed together at the atomic level. Surfaces tensions, intermolecular attraction/repulsion forces, specific heat, and whole bunch of ther physical stuff can be figured out with quantum physics, and so can reaction chemistry. If you are smart enough, that is.
I'm not. That is why I am a carpenter/electrician :)
Dave
> That is why I am a carpenter/electrician :)That's a scary combination. Are you a "pretty good carpenter for an electrician" or a "pretty good electrician for a carpenter"? ;)
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
The state of KY says I am a Master Electrician, but I was a carpenter first, and will always think of myself that way. Working for a utility company and becoming an electrician is kind of a hand in glove thing.
Still think I am a better carpenter than electrician, but that may not be saying much compared to some who hang out here, in either trade.
Dave
Air can hold a certain amount of moisture at any temperature. When the amount of moisture in the air is less than the air can absorb, evaporation occurs. The temperature at which the air is completely saturated is called the "dew point." If the air temperature falls below the dew point, you get fog or precipitation.
Relative humidity is a direct measurement of how saturated the air is, and it takes these temperatures (dew point and air temperature) into account. If the relative humidity is high, that puddle will stay a puddle for a long time. If it's low, that puddle will evaporate rapidly.
George Patterson
Boiling is the action when the entire volumn of liquid gets to that temperature and energy level.
Evaporation hppens one molecule at a time.
one moecule of water gets up enough energy and takes a run at the surface and breaks free, looking back over his shoulder at Nurse Cratched with a gleedful laugh as he soars into the air - "I'm FREE!"
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Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
You missed your calling! You should be teaching physics. No kidding--that's an explanation that would stick with even the most bored student.
I've been told many a time I would be a good teacher.Maybe when I retire....Same skills apply to salesmanship
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Picture Robin Williams explaining it that way! a little facial expression....verbal emphsis...
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LOL
Evaporation is when water molecules bacome part of the air. Boiling is when water turns to steam.
So they're 2 completely different things.
No, they're very much the same thing, only at different rates.http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Here's an interesting thing to note about the boiling issue. Water, like any fluid, has a unique vapor pressure curve, exponential in nature, and at any temperature it exerts a certain pressure. Boiling occurs when the partial pressure at the temperature equals the total pressure above the water, so that the air above it is displaced totally by the steam formed. Now, take a closed, insulated vessel partly filled with water, having a thermometer that reads the water temperature and pressure gauge that shows absolute pressure, connect a good vacuum pump to the top, and turn it on. Depending on the starting water temperature, the water starts to boil when the pressure gets down to what the vapor pressure curve indicates for that temperature. Since turning water to steam takes roughly 1000 BTU/lb, and the vessel is insulated, the boiling process results in the temperature of the water becoming lower, and the pump must draw the pressure down even more to maintain boiling. This lowering of the temperature and pressure will continue until the temperatue is 32 F and the pressure is 0.08854 psi (absolute, or about 14.6 psi of vacuum below standard atmospheric pressure at seas level). At that point, boiling will continue at constant temperature, and the heat removal will result in turning some of the remaining water to ice. Eventually, only ice will remain, and further pumping will result in sublimation of the ice, drying it up some and making it even colder. Now that's something every Fine Home Builder needs to know!
It all comes down to energy. The colder it is the less energy, ergo the slower the evaporation rate. Those little water molecules need to get energy from somewhere to be able to make the jump into vapor state.
Once you hit the boiling point most all the water molecules can jump at once.
I heard or read once that Stanley Livingstone, in searching for the source of the Nile, knew that water flowed downhill (duh). Apparently he would measure the temperature at which water boiled to determine his altitude. The higher up you are the lower the atmospheric pressure, ergo the water will boil at a lower temperature.
Let's not confuse the issue with facts!
If you put your mud puddle into an airtight box, the air would reach 100% relative humidity and no more water would evaporate.
But since the sun is shining on your puddle (warming it), and air at LESS than 100% humidity relative to the temperature of the mud puddle is blowing by, the mud puddle evaporates.
If the pressure at the surface of a liquid is equal to its vapour pressure, it boils. The normal boiling point is the temperature at which the vapour pressure equals atmospheric pressure. Reduce the pressure in the container enough and you can boil water at room temperature.
Boiling does not take place "throughout the liquid", since the pressure in the liquid increases with the depth of the liquid. If the liquid is heated uniformly to the point at which it just boils, the liquid at the surface would boil while the liquid a foot below would not. It's by providing liquid head in this way that we can pump boiling liquids from stills etc. without tearing up the pumps.
You can get explosive boiling by heating the bottom of a container of liquid hotter than the top, or by having two different liquids, with the one in the bottom of the pot having a lower boiling point. The combo heats quietly until the first little bubble forms, then explosively boils.(This is how one form of Turkish coffee is made, in fact, using a stratified mixture of coffee grounds, water, and sugar.)Explosive boiling is of course a hazard often warned against when heating water in a microwave. A less familiar but much more hazardous situation can occur in stored liquified natural gas, as the more volatile components tend to turn into denser liquids.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
There is a similar thing roofers learn about when it comes to tending a kettle heating the asphalt.Flash point is arount 525°F or so. A kettle tender learns to manage the temperature by adding cold chunks to melt and adjusting the flame in the bottom and by openning or closing the lid.But every now and then a new guy gets the idea that he can quickly cool that hot stuff when it gets too close to overheat, by pouring in a cup of water.What happens is htat the water runs straight to the bottom, when it almost instantly turns to steam with many times the volume of the water.A steam explosion from the bottom of a roofing kettle is not a pretty thing. It is one of the first things I was taught to avoid and that I taught new guiys to avoid.
But some of them still try to be geniuses and invent their own solutions...
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