Boiling point
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals
the external environmental pressure surrounding the liquid.
The boiling point of a liquid may also be defined as the temperature at which it
changes state from a liquid to agas throughout the bulk of the liquid. At that point, bubbles of
vapor which form below the surface of the liquid contain vapor at a pressure which matches the
external pressure. Thus, they are not crushed by the surrounding liquid and their buoyancy
causes them to rise through to the surface of the liquid and give the familiar appearance of a
boiling liquid as seen in the adjacent photograph of boiling water.[1][2]
A liquid in a vacuum environment has a lower boiling point than when the liquid is at
atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than
when the liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies
with and depends upon the surrounding environmental pressure.
The boiling point increases with increased pressure up to the critical point, where the gas and
liquid properties become identical. The boiling point cannot be increased beyond the critical
point. Likewise, the boiling point decreases with decreasing pressure until the triple point is
reached. The boiling point cannot be reduced below the triple point.
, Liquids may change to a vapor at temperatures below their boiling points through the process of
evaporation. Evaporation is a surface phenomenon in which molecules located near the vapor-
liquid surface escape into the vapor phase. On the other hand, boiling is a process in which
molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the
liquid. Liquids are converted into their vapor phase very much more rapidly by boiling than by
evaporation.
The normal boiling point
The normal boiling point (also called the atmospheric boiling point) of a liquid is the special
case in which the vapor pressure of the liquid equals 101.325 kilopascals (kPa).[3][4]
Usually, boiling points are published with respect to the atmospheric pressure (101.325 kPa or 1
atm) at sea level, which is also known as the "standard atmosphere".[5] In other words, published
boiling points of liquids are usually the normal boiling points of the liquids.
Estimation of the normal boiling point
The heat of vaporization is the amount of heat required to convert or vaporize a saturated liquid
(i.e., a liquid at its boiling point) into a vapor.
If the heat of vaporization and the vapor pressure of a liquid at a certain temperature are known,
the normal boiling point can be estimated using this rearranged form of the Clausius-Clapeyron
equation:[6][7]
Saturation temperature and pressure
A saturated liquid contains as much thermal energy (i.e., heat) as it can without boiling into a
vapor and, conversely, a saturated vapor contains as little thermal energy as it can without
condensing into a liquid.
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals
the external environmental pressure surrounding the liquid.
The boiling point of a liquid may also be defined as the temperature at which it
changes state from a liquid to agas throughout the bulk of the liquid. At that point, bubbles of
vapor which form below the surface of the liquid contain vapor at a pressure which matches the
external pressure. Thus, they are not crushed by the surrounding liquid and their buoyancy
causes them to rise through to the surface of the liquid and give the familiar appearance of a
boiling liquid as seen in the adjacent photograph of boiling water.[1][2]
A liquid in a vacuum environment has a lower boiling point than when the liquid is at
atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than
when the liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies
with and depends upon the surrounding environmental pressure.
The boiling point increases with increased pressure up to the critical point, where the gas and
liquid properties become identical. The boiling point cannot be increased beyond the critical
point. Likewise, the boiling point decreases with decreasing pressure until the triple point is
reached. The boiling point cannot be reduced below the triple point.
, Liquids may change to a vapor at temperatures below their boiling points through the process of
evaporation. Evaporation is a surface phenomenon in which molecules located near the vapor-
liquid surface escape into the vapor phase. On the other hand, boiling is a process in which
molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the
liquid. Liquids are converted into their vapor phase very much more rapidly by boiling than by
evaporation.
The normal boiling point
The normal boiling point (also called the atmospheric boiling point) of a liquid is the special
case in which the vapor pressure of the liquid equals 101.325 kilopascals (kPa).[3][4]
Usually, boiling points are published with respect to the atmospheric pressure (101.325 kPa or 1
atm) at sea level, which is also known as the "standard atmosphere".[5] In other words, published
boiling points of liquids are usually the normal boiling points of the liquids.
Estimation of the normal boiling point
The heat of vaporization is the amount of heat required to convert or vaporize a saturated liquid
(i.e., a liquid at its boiling point) into a vapor.
If the heat of vaporization and the vapor pressure of a liquid at a certain temperature are known,
the normal boiling point can be estimated using this rearranged form of the Clausius-Clapeyron
equation:[6][7]
Saturation temperature and pressure
A saturated liquid contains as much thermal energy (i.e., heat) as it can without boiling into a
vapor and, conversely, a saturated vapor contains as little thermal energy as it can without
condensing into a liquid.
