This has been bothering me for a while now

Any thermodynamics majors around?

Evaporation =\= phase change. The water does not turn into steam. In fact, no heat (or at least very little) is required at all. What haopens, plainly, is that some molecules may escape from the surface and into the air by sheer chance.

Evaporation is not a phase change. It's just vaporization occuring at the surface of a liquid.

>>8020813
Shouldn't there still be some kind of catalyst for that to happen?

Also, does that mean that the surface will cool down?

>>8020820
Ok.

Suppose, however, that the room temperature would suddenly rise to over 100C. If evaporated water isn't the same thing as steam, what fundamental change would there occur between the two states?

>>8020828
>the two states
The evaporated liquid and steam, that is.

>>8020761
Stat mech pls

>>8020813
This guy had it right.

Very little potential is required for the water to evict itself from the surface of the liquid. The potential is taken from the temp of the air, but the net change in temperature is negligible since required energy is so low

>>8020798
>Any thermodynamics majors around?
does that even exist?

>>8020822
>Shouldn't there still be some kind of catalyst for that to happen?

EE here btw, just so you can thank us later


The system is not in equilibrium when you start. The vapour pressure in the atmosphere is 0 at the start, so until equilibrium vapour pressure is reached, water in liquid form will keep evaporating. And don't forget that water will also condense elsewhere (but evaporate as well).


Ok so some molecules at the surface of liquid water start jumping in the atmosphere because they have higher than average energy.

Which means the average energy inside the liquid is now... lower. The water has just cooled a little from this process.

Another equilibrium is broken now: the water is colder than its container (or the table or whatever).
So heat is transfering from that container to the water, effectively cooling it.

water vapor =/= steam

Okay, thank you all. If you don't mind any extra questions, however:

Suppose you introduced water vapour to boiling temperature. How exactly would its form change? Is vapour just a tiny cluster of liquid water (or any substance for that matter), while steam/gas is essentially free molecules?

>>8020905
water vapour is water in gas phase
steam is water in gas phase
so nothing would change just the temperature

>>8020917
Then where does the phase change occur?

>>8020923
evaporation and steamification are both phase changes, not sure what >>8020813 is up to but it's bs

>>8020905
steam is just water vapor that's shoved out of hot water

enthalpy of vaporization describes the additional energy required to shove a particle into the gas phase at any temperature, not just as the boiling point. when water molecules are evaporating off the surface of cold water, they still need that extra vaporization energy to make it out, which is why it's a cooling process. it also happens much slower, because the probability that you have a water molecule near the surface with enough energy to make it out into the air is lower

the "boiling point" is really the point where it's so easy for evaporation to occur that you're balancing the incoming heat against the evaporative cooling effect. it's possible to heat the liquid above the boiling point if you can reduce the rate of evaporation somehow (hard to do unless you're in a super smooth container or you pressurize it)

>>8020761
The air is initially dry. That means, that partial pressure of water vapour is 0. So basically the water will ecaporate until phase equlibrium between water vapour and liquid water is established, The water will cool throughout the process.

>>8020761
might want to understand what the boltzmann distribution means. not all the molecules have the same thermal energies in equilibrium

But why does the laundry need heat from the sun in order for the clothes to become dry?

>>8021486
it doesn't need heat from the sun.
It just needs enough heat from the air around it so that it doesn't freeze.
it just helps because it gives extra kinetic energy.

Is it right to say that temperature represents the average kinetic energy of all the molecules in a certain area? And that those that evaporate below the boiling point are those with higher than average kinetic energy?

>>8020820
>evaporation is not phase change
why? gas and liquid are different states of matter, not in a solution and they're divided by a surface.

>>8020761
differences in concentration are a driving force on their own. heat transfer is actually titled "heat and mass transfer" for that reason.

our entire model for the thermal sciences is patchwork as fuck and mostly based of empiricism.

>>8021486
so that the evaporation process completes before mold starts growing on your clothes

>>8021576
Yes