What is the normal boiling point?

The normal boiling point is defined as the temperature at which a liquid boils under an external pressure of 1 atmosphere (atm). This condition allows for consistent and standardized measurement of boiling points across various substances and conditions. When a liquid reaches its normal boiling point, the vapor pressure of the liquid equals the atmospheric pressure, thus facilitating the transition from the liquid phase to the gas phase.

Understanding the context of other options helps clarify why the correct answer stands out. The second option refers to the condensation point, which is the temperature at which a gas transitions back to a liquid, but it does not define boiling. The third option relates to the melting point, which is the temperature at which solids turn into liquids, not boiling. The fourth option describes freezing, the temperature at which a liquid becomes a solid. While each of these terms is significant in thermodynamics, they do not provide the definition for the normal boiling point, which specifically involves the boiling of a liquid at a defined atmospheric pressure.