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Wikipedia says that a substance that has a larger surface area will evaporate faster, as there are more surface molecules that are able to escape. I think the rate of evaporation should decrease as there are more surface molecules. I want to know that what is the surface area of a liquid? And also that why the rate of evaporation increases with increase in surface area of the liquid?

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Why would having more surface molecules make the evaporation rate decrease? –  Keith Thompson Feb 26 '13 at 19:29
    
I was not sure.. I just thought that when you've more of something, it will last longer.. –  Samama Fahim Feb 27 '13 at 0:07
    
If you have a tall glass of water with a given fixed surface area, it will lose a constant amount of water per unit of time (in an identical environment); the deeper the water, the longer it will last. But for a given volume, the large the surface area that's exposed to air, the more quickly it will evaporate. –  Keith Thompson Feb 27 '13 at 0:40

4 Answers 4

what is the surface area of a liquid?

The surface of a liquid is the interface between the liquid and (usually) the air surrounding it. The surface area is the amount of that liquid that is exposed to the air, or touching the air.

And also that why the rate of evaporation increases with increase in surface area of the liquid?

If more liquid is exposed to the air, then more of it can evaporate.

I think the rate of evaporation should decrease as there are more surface molecules.

To be particular, depending on how you define rate of evaporation, you could say that it stays constant not depending on the surface area. However, with more of the liquid exposed, there is more liquid evaporating at a time (at the given rate $\frac{V}{A\cdot t}$).

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Good clarification about the rate of evaporation -- not absolute amount. –  Asphir Dom Feb 26 '13 at 21:33
    
Thank you askewchan for the clarification. –  Samama Fahim Feb 27 '13 at 0:03

Because in a liquid, the molecules are all about the same distance apart (since most liquids are not very compressible).

So the top surface of liquid in your coffee cup has a certain area and a certain number of molecules in that area.

Two cups of coffee will have twice as much area, so twice as many molecules, so twice as much evaporation.

Now connect the two cups by a narrow tube below the surface. What you have then is a single quantity of liquid with twice as much surface area as one cup, and, of course, twice as much evaporation.

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Thanks Mike Dunlavey for such an example to clarify the concept! –  Samama Fahim Feb 27 '13 at 0:05

The surface area affects evaporation because if more area is exposed to air, allowing water molecules acquire more heat energy from the surroundings. Due to the increased heat energy (kinetic energy), there is more rapid movement of the water molecules which helps them to overcome the force of attraction and evaporate.

The progression is essentially more area -> more heat absorption -> increased kinetic energy -> increased movement -> faster evaporation.

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Akanksha - did you mean to comment on your own answer to say it was excellent? I am guessing you meant it to go on one of the other ones. .. –  Rory Alsop Apr 30 '13 at 18:43

A liquid is a material where the bonds between the molecules are too strong for most molecules to escape but weak enough for the molecules to move around. Evaporation occurs when molecules get enough energy from heat to escape the liquid. They get this heat from motion and the air. An increased surface area means that more liquid will be exposed to air at one time, and therefore, more water can evaporate in a given time period.

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