The temperature at which the vapor pressure of a liquid equals the external pressure of 1 atm is called the boiling point of the liquid.

Here's why:

* Vapor pressure: The pressure exerted by the vapor of a liquid in equilibrium with its liquid phase.

* Boiling: The process where a liquid changes into a vapor.

* Equilibrium: When the rate of vaporization (liquid to gas) is equal to the rate of condensation (gas to liquid).

When the vapor pressure of a liquid reaches 1 atm (atmospheric pressure), the liquid starts to boil because the pressure of the vapor is sufficient to overcome the atmospheric pressure.