What is the normal boiling point of a substance defined as?

The normal boiling point of a substance is defined as the temperature at which its vapor pressure equals atmospheric pressure. At this point, the liquid phase of the substance transitions into the gas phase. This definition is significant because it indicates the temperature at which a substance can change states under standard atmospheric conditions, thereby allowing for comparison between different substances. Understanding vapor pressure is crucial in this context; as a substance is heated, its vapor pressure increases, and when it matches the surrounding atmospheric pressure, boiling occurs.

In contrast, the other definitions provided refer to different physical processes. The point at which a substance solidifies is known as the freezing point, while melting describes the transition from solid to liquid. These phases do not relate to the boiling process, which involves a change from liquid to gas. Thus, recognizing the specific conditions surrounding boiling and its correlation with vapor pressure and atmospheric pressure is key to understanding the concept of normal boiling points in chemistry.