From: Dan Asimov <dasimov@earthlink.net> To: math-fun <math-fun@mailman.xmission.com> Sent: Wed, November 11, 2009 3:48:44 PM Subject: [math-fun] Physics/chemistry question I believe I've seen paper catch fire in a microwave oven when it's been on too long and the water has evaporated away. But how is this possible, considering that water boils at well below the kindling point of paper? --Dan _____________________________________________________________________ I use paper in the microwave to sop up spills. I've never experienced the conflagration of originally dry paper, but I have seen paper that has been wet catch fire. Here is my guess for the mechanism. Water molecules are attracted to the cellulose molecules in the paper fibers. Because of this attraction, the effective boiling point for bound water exceeds that of bulk water, the boiling point being the temperature at which the vapor pressure of the water equals one atmosphere. The bound water is conductive and heats, leading to char. The carbon char is also somewhat conductive leading to further heating. If in addition, as in my case, there are several layers of paper, the heat is confined, and finally the paper bursts into flame. -- Gene