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
I think something like this even holds for plastic dishes, especially old ones. I have had empty Melamine dishes scorch, blister, and split, and give off a horrendously sickening stench, quite suddenly as the char phenomenon has positive feedback. I have even seen specs for rate of weight gain of submerged plexiglass. joerg>Even if the magnetron survives you might get
more than you ask for. For example, a tiny flame will ionize the air, rendering it conductive and absorbent to the microwaves. Spectacular blazing flames may well follow.
There are some web sites about these effects. The experiments are usually done outside the building, for very good reasons.
Finally, a use for used birthday candles. --rwg