# Why does evaporation take place? [duplicate]

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I was looking at at puddle when I thought that this puddle will evaporate tomorrow but then it occurred to me that the boiling temperature of water (aka to turn into gas) is $100$ degrees under 1 atmospheric pressure of earth. Then I thought the temperature around me was 25 degrees which is way away from temperature of boiling therefore it should in theory or at least in my perspective never evaporate the water, yet nature disagrees and does this so.

Therefore I proposed an hypothesis (Still in Middle School so please excuse me) the water turns into gas as a result of the photons "hitting" the atoms and transferring its energy into the water atoms which in turn moves the atoms giving it $KE$ (Kinetic Energy) and this happens over a millions of times which results in water particles gaining this energy and therefore evaporating without substantial notice or feel of it bubbling or showing any sign of it as this happens very slowly.

However I as a middle school student am not too considerate or confident on my hypothesis so would you say my hypothesis is correct or wrong and can you explain why this happens?

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## marked as duplicate by BMS, Brandon Enright, DavePhD, Kyle Kanos, Emilio PisantyJun 24 '14 at 0:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Side note: 25 degrees is not 1/4 of 100 degrees unless you are using a temperature scale zeroed at absolute 0. You are never allowed to use multiplication or division with temperatures unless they are absolute. –  Chris White Jun 23 '14 at 19:10
...and absolute here refers to the kelvin scale. –  BMS Jun 23 '14 at 19:16
Possible duplicate of How does water evaporate if it doesn't boil? –  BMS Jun 23 '14 at 19:18
@BMS actually, other scales zeroed at absolute zero work as well, e.g. Rankine. –  Tim S. Jun 23 '14 at 20:46
Learned something! Thanks. –  BMS Jun 23 '14 at 23:08

## 2 Answers

Temperature is a measure of the average kinetic energy of particles, characterized by a Maxwell-Boltzmann distribution. Basically, that's a fancy way to say, if something is at 25°C, a large percentage of its particles have a temperature close to 25°C, but some have a temperature farther away. When some of those particles that happen to have a temperature of 100°C are at the surface of the water, they have enough energy to break away and evaporate. This cools the rest of the puddle down, but the puddle is constantly in interaction with other substances around it (ground, air) which put that heat back into the puddle. So, the puddle maintains a constant temperature (roughly) and shrinks smaller and smaller as particles near the surface get enough kinetic energy to evaporate.

Your photon hypothesis is not bad, but typically any photons that are absorbed are quickly re-emitted by the absorbing atom.

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The particles who break away and "evaporate" can also re-enter the liquid water, however some particles will diffuse away into the rest of the gas. However if the water en gas are a closed system, then the effective evaporation will stop if the relative humidity of the gas reaches 100%. –  fibonatic Jun 23 '14 at 20:26

I believe evaporation/condensation is modeled by the Claperyon equations, and as the previous answer stated it's really all about the flow of energy towards a minimum energy equilibrium. But if you run your evaporation experiment in the dark vs. light (but keeping surface temperature the same) you will likely refute your hypothesis about photons. The energy needed to evaporate is already in the molecules jiggling about. Actually there is condensation and evaporation happening all the time at the air-water interface, and for air that is not saturated with water, it's more likely that a molecule at the surface will fly out into the air than a water molecule from the air will condense into the water. But if the air is saturated with water the probabilities are about equal - the rate of condensation is about the same as the rate of evaporation. For different liquid-gas interfaces the rates are affected by certain quantum mechanical properties of the gas/liquid, the pressure and the temperature.

Hope that helps.

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