What is defined as the boiling point?

The boiling point of a substance is defined as the temperature at which its vapor pressure equals the atmospheric pressure surrounding it. At this point, the energy provided to the liquid causes molecules to escape into the gas phase, resulting in boiling. This concept is crucial in understanding phase changes, as the boiling point can vary depending on the external pressure. For instance, at higher altitudes where atmospheric pressure is lower, water will boil at a temperature below 100 degrees Celsius. This definition encompasses not just a specific temperature for all liquids but is contingent on the pressure applied, distinguishing it from the other choices provided. Thus, identifying the boiling point as the temperature where vapor pressure equals atmospheric pressure accurately reflects the physical properties of liquids under various conditions.