Final answer:
The boiling point is the temperature where the vapor pressure of a liquid equals the atmospheric pressure. The normal boiling point occurs at a temperature where the vapor pressure equals 1 atm (101.3 kPa), like water boiling at 100°C under standard conditions.
Step-by-step explanation:
The temperature at which the vapor pressure of a liquid is equal to atmospheric (or external) pressure is called the boiling point. At the boiling point, the liquid's equilibrium vapor pressure equals the pressure exerted by its gaseous surroundings. In the case of a liquid in an open container, this is typically the pressure due to Earth's atmosphere. The normal boiling point specifically refers to the temperature where a liquid's vapor pressure is equal to 1 atm (101.3 kPa), a standard pressure.
The normal boiling point is a constant value because it is defined relative to this standard atmospheric pressure of 760 mm Hg (or 1 atm or 101.3 kPa). However, because atmospheric pressure can change based on location, the boiling point of a liquid can also change with external pressure. For instance, water's normal boiling point is 100°C at 1 atm pressure, and this temperature will drop at higher altitudes where atmospheric pressure is lower.