What defines the boiling point of a substance?

The boiling point of a substance is defined as the temperature at which its vapor pressure equals the external pressure surrounding the liquid. At this point, the molecules within the liquid have enough energy to overcome the intermolecular forces holding them together, allowing them to enter the gas phase. This concept highlights the relationship between vapor pressure and temperature: as a liquid is heated, its vapor pressure increases; when it equals the external pressure, boiling occurs.

The other options describe different phenomena and properties of substances. The lowest temperature at which a liquid evaporates does not signify boiling, as evaporation can occur at various temperatures below boiling point. The melting point refers specifically to the transition from solid to liquid, which is distinct from boiling. Lastly, the temperature needed for chemical reactions to occur is not related to the boiling point, as reactions can happen at various temperatures and are facilitated by factors such as concentration and catalysts, rather than boiling.