[Physics] What happens at the molecular level during evaporation

Question

During boiling the liquid gets its energy from the heat source so it can break the force between molecules and turn into gas while evaporation happens at the top of the liquid at any temperature. Where does it get it's energy, I've read that when molecules bump they exhange kinetic energy and if one gets enough energy they turn into the gas state if it is correct why does it only occur how is it affected by humidity and wouldn't the air molecule bump into the liquid molecules so they go downwards not upwards. Also how would humidity affect evaporation if they have enough energy to escape why would saturated conditions prevent them from escaping. I've read a lot of answers but some of them deny other answers. It is been frustrating me that I can't even under stand simple concepts.,

Answer 1

Where does an evaporating molecule get enough energy to escape?

In any system of a large number of molecules, the molecules are going to have a variety of kinetic energies (even if they all start out at the same kinetic energy, random collisions transfer energy and ensure that a few molecules "get lucky" by being boosted in the same direction multiple times). Based purely on the statistics of such systems, we have a relatively good idea of the distribution of kinetic energy among the particles of a fluid. This distribution is called the Maxwell-Boltzmann distribution*. Two examples of this are shown below (the vertical axis can be thought of as the relative number of particles having a particular speed/kinetic energy):

As you can see, there is a "tail" of this distribution that indicates that a few particles have very high kinetic energy, and these are the particles that escape the liquid. You can also see that at higher temperatures, the fraction of particles with very high kinetic energies substantially increases, which is why evaporation happens more quickly at higher temperatures.

*The Maxwell-Boltzmann distribution is technically only valid for an ideal gas; liquids have additional interactions (like van der Waals forces and, in the case of water, hydrogen bonding) that add correction terms to the Maxwell-Boltzmann distribution. In water, for example, substantial hydrogen bonding reduces the extent of the high-energy tail and causes water to evaporate slower than expected from this picture alone. Still, though, qualitatively the Maxwell-Boltzmann distribution provides the correct intuition.

How is this process affected by humidity?

In a container containing a liquid, there are actually two opposing processes at work: high-energy molecules of the liquid escape the surface and evaporate, and low-energy particles of vapor condense into the liquid. Humidity is the density of vapor, so evaporation works to increase the humidity, and condensation works to decrease it. There is always an equilibrium level of humidity where evaporation and condensation occur at the same rate, and the amount of liquid and vapor do not change with time. If the humidity is lower than this level, then evaporation will occur faster than condensation, and the net effect will be that the liquid evaporates; if the humidity is higher than the equilibrium level, then evaporation will occur slower than condensation, and the net effect will be that some of the vapor condenses. The equilibrium level of humidity is affected by temperature, because it depends on the ratio of high-energy molecules in the liquid to low-energy molecules in the vapor. High humidity doesn't prevent high-energy molecules in the liquid from escaping; it just ensures that more molecules are entering the liquid than are escaping it.

Wouldn't the air molecules bump into the water molecules and cause them to go downward, not upward?

Sometimes they do. But the density of the vapor above the liquid is always lower than the density of the liquid itself, so it's not particularly likely that a vapor molecule is in the right position to do this, and high-energy molecules leaving the liquid usually end up entering the gas.