| MadSci Network: Chemistry |
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 606-344-3377 franzenj@thomasmore.edu
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