What defines the boiling point of a liquid?

The boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid equals the environmental or atmospheric pressure surrounding the liquid. At this temperature, the molecules of the liquid have enough energy to overcome the intermolecular forces holding them together, allowing them to transition into the gas phase. This is a fundamental principle in physical chemistry and thermodynamics, as it relates to the behavior of liquids while considering changes in pressure and temperature.

In practical terms, if the atmospheric pressure is lowered (such as at higher altitudes), the boiling point of water decreases. Conversely, increasing the pressure raises the boiling point. This concept is essential in various applications, including cooking, chemical processes, and understanding various natural phenomena.

The other options describe different physical properties or phenomena that do not pertain to the definition of boiling point. For example, the first option refers to melting, which is the transition from solid to liquid, unrelated to the boiling point. The third option pertains to flammability and does not address the thermodynamic conditions of phase changes. Lastly, the fourth option describes a critical point, which is a specific thermodynamic state where a substance cannot exist as a liquid or vapor phase under higher temperatures and pressures, rather than defining the boiling point