Re: Why does water remain a gas below 100 degrees C°

Water remains a gas below 100 deg C due to saturation pressure and  temperature.  “The term saturation temperature designates the temperature at which vaporization takes place at a given pressure, and this pressure is called saturation pressure for the given temperature" - Van Wylen, Gordon J. & Sonntag, Richard E., Fundamentals of Classical Thermodynamics (3^rd ed.) p. 36.  Temperature is a measure of the mean molecular velocity for a given distribution.  The higher the mean velocity, the higher the temperature.  Since not all the molecules in a body of water are moving at the same velocity, the faster ones my be able to escape; especially from the surface of the water.  A good example of the relationship between pressure and vaporization temperature, is altitude.  Notice that at high altitudes, water boils at a lower temperature.  The lower pressure over the surface of the body of water permits the molecules to escape easier.  Temperature, pressure, and volume of a fluid are closely related to the state of the fluid.  A fluid kept at constant pressure will expand in volume as it is heated.  If that volume were to then be decreased by compressing the fluid - no longer at a constant pressure - the gas would once again condense.  All the way back to pure liquid if enough pressure is added. This is the method used to create liquid hydrogen and liquid oxygen for rocket fuel.  Each gas is cooled and compressed until it is a liquid.  

Water may actually exist in all three states at once.  This is known as the triple point of water.  The triple point of water occurs when the temperature is 0 deg C, and the pressure is .00603 atmospheres.  

Now you may understand that defining H20 as liquid in the range 0 < Temp < 100 degrees  is quite an over simplification.  It is a good rule of thumb; however, the mean temperature does not reflect the individual molecular velocities.  It is just an average over the distribution.

Note: Heat is the transfer of temperature; therefore, heat is not a property.  It only occurs at the interface between two bodies of different temperatures.  Heat transferred to a system is considered to be positive.  

-Dennis K. Van Gemert

[note added by MadSci Admin: There is a VERY GOOD previous answer posted on the MadSci network here. Go to the excellent Web page at the University of Florida referenced in that answer for some GREAT phase diagrams. - - admin]