MadSci Network: Chemistry

Re: Why bubbles do not form when heated and result in Superheating?

Date: Sun Jul 30 16:17:13 2000
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Chemistry
ID: 963899350.Ch

This is due to a phenomenon called nucleation.  Nucleation simply means an 
initial step that starts off any process.  Processes like bubble formation and 
crystallization sometimes require this nucleation step.  This step can be 
provided by the presence of impurities such as dust or irregularities in the 
walls of the containers i.e. the presence of any surface which can provide 
surface energy for the process to occur. When there are no impurities and the 
walls of the container is extremely smooth, no surface is available for the 
bubbles to nucleate. Also, since the bubbles are nothing but air pockets, some 
heat is transferred out of the liquid through these bubbles.  So in the absence 
of bubbles, very little heat actually is transferred away from the interior of 
the liquid and the liquid-vapor transition is increased. So the liquid is 
superheated and boils at a much higher temperature.  It is important that you 
realize that this phenomenon is not the same as Raoults law wherein the presence 
of a solute (something that dissolves in water, say) increases the boiling 
point of water.  

In crystallization, sometimes small crystals of the compound that is 
crystallizing can itself as as a nucleating agent. Impurities also act as 
nucleating agents. This can be illustrated easily by taking a saturated solution 
of salt in water (large volume). Attach a small salt crystal (pick one out of 
your salt container) to the tip of a fiber (say a hair from your head!) by means 
of gum and suspend it in the solution. You will see that after a couple of days 
the crystal attached to the tip of the fiber would have grown in size!! 
(provided you have reasonably strong hair and you use good glue!!)  This is 
because of the nucleating effect of the salt crystal in crystallizing salt. 
Both these phenomena and several others are governed by the same principle - 
nucleation and growth.

Hope this helps.

You can read a book called "Metastable Liquids" by P.DeBenedetti of Princeton 
University in which he points out the maximum boiling point of pure water 
(without impurities and in a container with extremely smooth surfaces) as much 
higher than 100C!! I dont remember the exact number.  Don't worry about the 
details in the book since it is written for postgraduate students I think. 

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