Eli5 How does water evaporate without ever reaching 100 degrees celsius?
In: Chemistry
If you mean transition to steam, boiling will occur at lower temp with altitude/elevation. At 5000 ft above sea level, water boils at 95 C and so on.
The body of water is made of molecules. Individual molecules on the surface can be given enough heat to reach evaporation before that heat transfers to the surrounding molecules.
If you’ve ever swam in a lake, you’ll notice that the surface feels nice and warm but a few feet down it can be noticeably colder. Water is not a perfect conductor of thermal energy.
Temperature is a measure of the *average* energy of the molecules in a substance. At 100c the energy of the average water molecule is high enough for them to overcome the bonds keeping them in liquid form. Some molecules won’t have quite enough, some will have more than enough, but *most* will evaporate, so the water boils.
Below 100c the *average* molecule can’t evaporate, but the occasional molecule on the surface of the water might happen to gain enough energy to evaporate on its own.
Heat in this context is the energy of the individual molecules. The higher the temperature the more energy, and the easier it is for them to overcome their connection to the other molecules. But this can also happen at lower temperatures.
Since this effect also removes energy from the system this effectively cools it. To see this in action wrap a bottle in a wet towel & leave it out in the sun for 15+ minutes.
Because the water molecules are moving about and some at the surface move fast enough to break away and float off. Heating the water increases the speed of the molecules and increases the vaporisation.
100 degrees C (212 degrees Fahrenheit) is the temperature at which the _vapor pressure_ of water equals one “standard atmosphere”.
Breaking that down a minute: liquids usually have a ‘vapor pressure’, meaning how much of that liquid compound in vapor form is in equilibrium with that liquid, so that as much is condensing into the liquid as is evaporating from it. As might be obvious, as the temperature rises, a liquid’s vapor pressure will also rise, and as temperature falls so will vapor pressure.
Different liquids have different vapor pressure graphs.
Now add in that the atmosphere is pressing DOWN on everything, with the weight of all the air above that stuff; on average, at sea level this is 14.7 pounds per square inch = one ‘atmosphere’.
(This sounds like a lot. And it IS. But we evolved down here and are used to it, and maintain the same pressure inside our bodies, so there’s no net ballooning up or crumpling in. Meanwhile, if you go underwater, every 33 feet down, approximately, is another atmosphere of pressure. Which is why scuba divers take _pressurized_ air or oxygen, in tanks, to breathe with.)
Given that, you can see that at sea level, water starts boiling when the internal vapor pressure equals (or exceeds) the outside atmospheric pressure. But even below boiling temperature, it still has a vapor pressure … and if the humidity ( = water vapor amount in the air) isn’t too high, it will still evaporate, though more slowly than by boiling, until there’s enough humidity in the air to be condensing back in just as fast.
–Dave, fun fact: a liquid’s vapor pressure at its freezing point is NOT necessarily zero
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