Boiling Point Definition in Chemistry

What Boiling Point Is and What Affects It

This is boiling water. The temperature of the water may be at its boiling point or above it.
This is boiling water. The temperature of the water may be at its boiling point or above it. David Murray and Jules Selmes / Getty Images

Boiling Point Definition

The boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. Therefore, the boiling point of a liquid depends on atmospheric pressure. The boiling point becomes lower as the external pressure is reduced. As an example, at sea level the boiling point of water is 100 °C (212 °F), but at 2000 meters (6600 feet) altitude the boiling point is  93.4 °C (200.1 °F).

Boiling differs from evaporation. Evaporation is a surface phenomenon that occurs at any temperature in which molecules at the liquid edge escape as vapor because there is not enough liquid pressure on all sides to hold them. In contrast, boiling affects all molecules in the liquid, not just ones on the surface. Because molecules within the liquid change to vapor, bubbles form.

Types of Boiling Points

​Boiling point is also known as saturation temperature. Sometimes boiling point is defined by the pressure at which the measurement was taken. In 1982, the IUPAC defined the standard boiling point as the temperature of boiling under 1 bar of pressure. The normal boiling point or atmospheric boiling point is the temperature at which the vapor pressure of the liquid equals the pressure at sea level (1 atmosphere).