Inspired by this column by Cecil, if the boiling point for a liquid can be lowered, then what exactly is the boiling point? To put it another way, what’s going on in the water at microscopic level 1 degree below the boiling temp and 1 degree above it?
Is “boiling” a set speed of the movement of molecules within a liquid? Like when water, orange juice, urine, or soda molecules move up to a certain mph, we call that boiling?
Boiling is when a substance is hot enough for the molecules it’s made of to overcome the forces attracting them to each other and fly away as a gas. Which isn’t an absolute because what we speak of as temperature is the average thermal energy, from which there will be substantial random variations. So what you actually get at a given temperature is what’s called the vapor pressure: how strongly the liquid is evaporating away at any moment. If you had the liquid in a closed container, then at a certain temperature you would have an equilibrium, a balance between how much of the liquid is evaporating into gas and how much gas is being recondensed into liquid. What we for convenience call the “boiling point” is usually the temperature at which the vapor pressure is equal to atmospheric pressure, or a standardized pressure close to the average atmospheric pressure.
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.
If you add energy to a liquid well below the boiling point, the liquid will get hotter. If you add energy to a liquid at the boiling point, more of the liquid will evaporate, while the temperature remains constant (in an open pot)
At about 8640 ft altitude where I am, the boiling point of water is: 195.357 degrees.
It’s when it get all bubbly from not watching it. 
Boiling is when the pressure in steam bubbles causes them to expand. Although we are told this happens at 100C, this is not precise. Whether a bubble with expand (or expand then collapse) is determined by the bubble’s size. If you find a container of water that only has miniscule bubbles, you will have to heat it to significantly above 100C for it to start boiling. Yet, a large steam bubble can travel through a region of cooler water without collapsing. Water at the bottom of a pot may be superheated, but water closer to the surface may be relatively cool.
The reason why pressure in a steam bubble varies with temperature, the reason why vapor pressure exists, is interesting. It lies in the fact that liquid water does as much to suck up water molecules from the air (like a dessicant) as it does to release them. If liquid water didn’t pull molecules out of the air, the pressure would rise indefinately and every bottle of water would explode. At any temperature, in a closed container, you get an equilibrium between these two processes. Yet the higher the temperature, the more evaporation overtakes condensation, and the higher the pressure that builds up.
At 211.9 F, you’ve got hot liquid water. It may be simmering a bit (correct me if I’m wrong) as small localized regions reach 212 F, become a gas, and escape from the pot.
At 212.1 F, you’ve got water vapor. Some individual molecles will cool to 212 F, become a liquid, and condense on the ceiling.
It takes little energy to increase from 210 F to 211.9 F, and little energy to increase from 212.1 F to 214 F. But it takes much more energy (and thus more time on the stove) to increase from 211.9 F (liquid) to 212.1 F. Just watch a thermometer climb and then hold steady in a pot of boiling water.
The actual boiling point will vary due to atmospheric pressure (and ambient humidity?). But there is very much a difference between just-below and just-above that point.
Ambient humidity directly above the pot is exactly 100% relative humidity. When it’s boiling, that equals 100% absolute humidity (the pressure of water vapor directly above the pot is equal to the pressure of the air a yard away).
Differences in humidity might affect the dynamics of the evaporation and boil, the speed at which the vapor moves away from and back over the pot, but they don’t affect the boiling point.
To raise one pound of water 1 degree F at 60 degrees takes one BTu. To convert one pound of water at 212 degrees F to one pound of steam at 212 degrees takes 970 BTUs.