Vapor pressure, boiling point, normal boiling point
 

Terms:
 

Liquid properties depend on intermolecular forces. The freezing point, boiling point, density, viscosity all are controlled by intermolecular attractions. The higher the attractions (forces) the higher the melting point, boiling point, density and viscosity.

Evaporation is a process where a liquid converts to the gas through the gradual escape of molecules from the liquid to the gas at temperatures below the boiling point. The number of escaping molecules exert a partial pressure that is less than the opposing atmospheric pressure. The molecules of the liquid will gradually escape into the atmosphere if the container is open.

  

Vapor pressure is the partial pressure exerted by gas phase molecules that are in equilibrium with a condensed state. This means liquids and gases have vapor pressure.

The boiling point is the temperature at which the vapor pressure equals the opposing atmospheric pressure. This means liquids can boil at almost any temperature if the opposing pressure is adjusted. Autoclaves and pressure cookers elevate the pressure and raise the boiling point for a liquid. The higher the pressure exerted on the liquid the higher the boiling point for the liquid.

The boiling point can be reduced by lowering the opposing pressure. This happens naturally at high elevations in mountain areas and cities like Denver and Mexico City.

The normal boiling point is the temperature at which the vapor pressure for a liquid equals 1 atmosphere.

Intermolecular forces and properties of liquids
Boiling point and liquid volatility are two physical properties that depend on intermolecular forces. The stronger the intermolecular forces the less volatile the substance. High intermolecular attractions produce a higher normal boiling point. The illustration above shows
 
 
 
 
diethyl ether CH3 CH2O CH2CH3 with a low boiling point of 36.5oC This is a nonpolar molecule with London forces between molecules.
 

ethanol CH3 CH2O H with a high boiling point of 78.3oC. This is a polar molecule with dipole-dipole forces and some hydrogen bonding between molecules.

water H2O with a high boiling point of 100oC. This is a polar molecule with dipole-dipole forces and a great deal of hydrogen bonding between molecules.

 

High boiling points and strong intermolecular forces go hand in hand.
 

WHAT IS MEANT BY NORMAL BOILING POINT? answer is here

Online Introductory Chemistry
Dr. Walt Volland revised March 31, 2005 all rights reserved 1998-2005

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