MadSci Network: Chemistry

Re: What is the definition of boiling water?

Date: Tue Jan 20 09:37:02 1998
Posted By: Jerry Franzen, Faculty Chemistry
Area of science: Chemistry
ID: 885162438.Ch

The actual definition of the boiling point of water is the temperature when 
the vapor pressure of the water equals the atmospheric pressure (the 
pressure of the air pushing down on the surface of the water.) Think of it 
this way: In the liquid, the water molecules are attracted to one another 
by their very nature and further held together by the pressure of the 
atmosphere pushing down on the surface of the liquid.  As they are heated 
they gain energy to begin to pull away from one another.  When they gain 
enough energy, they are able to separate and to overcome the force of the 
atmospheric pressure and form the gas (water vapor) that makes up the 
bubbles you see in the boiling process.  The standard temperature at which 
this occurs is 100 C or 212 F.

As for your question, there are several parts.  In the very early stages of 
heating, you may see some small bubbles even though the water is only warm. 
This is some air that is dissolved in the cold water and is being expelled 
by the heating.  This is not boiling.  As the water nears the boiling 
point, some areas of the container close to the heat source may be hot 
enough to cause some boiling in that area - produce some local bubbles. If 
you want to be sure that all of the water is "at the boiling point", you 
should wait until there is a large amont of bubbles rising to the surface. 
This is what people refer to as a "rolling boil." 

I hope that this has been helpful. If you have further questions, you can 
cantact me at:

Dr. Jerry Franzen
Chemistry Department
Thomas More College
Crestview Hills, KY 41017

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