I am confused by the thermodynamic definition of boiling. It is stated that boiling occurs when the vapor pressure of a liquid is the same as the ambient atmosphere. Now suppose we deal with an open reaction tube filled with water. It is an open system, connected to the outside air pressure of 1 atm (101325 Pa).
Now we heat the water to 373.15 K. The water will begin to boil at this temperature. There isn't any water vapor in the air yet though before boiling (at least not the equilibrium vapor pressure).
So basically, does that mean that a liquid boils when it's value for the vapor pressure function at the specific temperature reaches ambient pressure, and doesn't use equilibrium vapor pressure for its definition?
To clarify, let's look at another example. Water in an open system at 90 °C is exposed to 1 atm of dry air and, let's say, 0.5 atm of water vapor.
To boil, does the water need to overcome the 1 atm of air and the 0.5 atm of water vapor together, or only the 1 atm of air, because the 0.5 atm water vapor is the same species as the water?