Final answer:
The boiling point is the temperature at which the liquid's vapor pressure equals the external pressure, with the normal boiling point being defined at a standard atmosphere pressure of 1 atm (101.3 kPa).
Step-by-step explanation:
The temperature at which the liquid vapor pressure is equal to the external pressure on the liquid is known as the boiling point. At the boiling point, a liquid turns into a vapor. The normal boiling point of a substance refers to the boiling temperature when the external pressure is one atmosphere (1 atm or 101.3 kPa), which is a standard pressure.
This point varies with the external pressure, such that at higher altitudes, where atmospheric pressure is lower, the boiling point decreases. The vapor pressure of a liquid increases with temperature, and when it becomes equal to the external pressure, the liquid will start to boil.