|MadSci Network: Chemistry|
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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