Re: Would a substance with no water molecules ever get hot in a microwave?

Your question:

If microwaves work by changing the polarity of water molecules 
and creating friction...heat, then, it would seem that a 
substance, free of water would never get hot...or is it the 
hydrogen atoms.  Finally, would it only be theoretically possible 
to test that? If not, how would you do it?

Dear WONDERING,

This is a very good question because I have met many people with advanced 
degrees in science who have asked this very question.  I think I have a 
brief answer that you will understand and may challenge you to study 
further.  I am assuming that you are 13 or 14 years of age.

It is true that microwaves interact with water due to the polarity of the 
water molecule.  Interaction of energy with a molecule's natural polarity 
is called a dipole interaction.  The reason why microwaves have a dipole 
interaction is they don't have enough quantum energy to interact with the 
other parts of molecules (inner atomic electrons, outer atomic electrons 
and bonding electrons) as do higher or more energetic forms of 
electromagnetic energy.  Gamma rays, x-rays, ultraviolet light and visible 
light are all more energetic than visible light.  Microwaves can't change 
the electronic structure around atoms or between atoms, but they can 
interact with electronic differences between atoms.

Why do they only interact with water?  Simply, they do not only interact 
with water.  They act on the polarity differences between many materials!  
An important point to make here is, if the molecular dipoles are 
restricted because they are frozen into place, the microwave dipole 
interaction is much smaller than would be predicted.

What are some other polar molecules that interact with microwave energy 
and will get hot in a microwave?  You have many around the house.  Be sure 
to get your parents approval before experimenting with any of these 
materials.  A relatively safe one is...butter!

Butter is made of fat.  Fat is an organic molecule that has dipoles 
between carbon atoms and oxygen-hydrogen groups known as hydroxyls and 
also between carbon and oxygen atoms that are doubly bonded, known as 
carbonyls. These dipoles interact strongly with microwave energy. The 
interaction is easy to see.  A few pads of cold butter placed on a saucer 
and exposed to microwave energy in a microwave overn for several seconds 
will melt.  Melted butter will continue getting hotter.  Cooking oils can 
be heated to very high temperatures in a microwave oven, so if you try one 
of these, watch the temperature carefully.  NEVER DO THESE EXPERIMENTS 
WITH PLASTIC CONTAINERS.  THE OIL OR BUTTER CAN EASILY GET HOT ENOUGH TO 
MELT THE CONTAINER. USE PYREX BOWLS OR MEASURING CUPS.

Other substances that microwaves heat include: dried meat and fruits, 
wood, most decorative stones, tire rubber.  BE CAREFUL, THESE SUBSTANCES 
CAN GET SO HOT THEY WILL BURN IN JUST SECONDS!  SOMETIMES THE SMELL CAN BE 
TERRIBLE! Do not try substances such as shellac, laquer paints or anything 
else that can easily cause a fire.  If anything else starts to burn or 
smoke, immediately turn the microwave oven off and keep the microwave door 
closed.

Microwaves also interact with poorly bound or nearly free electrons (as I 
call them) in semiconductors such as graphite.  Pencil lead is made from 
graphite.  If you remove the wood from a pencil lead ( or just get a few 
mechanical pencil leads) and put it (them) in a pyrex measuring cup and 
turn the microwave oven on for just a few seconds, the leads will get very 
hot.  BE CAREFUL WITH PENCIL LEADS!  THEY CAN REACH TEMPERATURES IN EXCESS 
OF 500 CENTIGRADE IN LESS THAN A MINUTE!

If you carefully try these experiments with your parent's approval and 
supervision, or if you can do these at school with teacher supervison, you 
will see that materials that don't contain water can be heated with 
microwave energy.

However, materials that are not polar and not semiconductors that do not 
heat include: polyethylene (empty milk container), pure candle wax, quartz 
(very clean, dry sand), polystyrene (foam cup), liquid nitrogen, and dry 
ice (solid carbon dioxide).

Just for fun, try the following experiment. What do you think will happen?

The next time a regular incandesent light bulb burns out around the house, 
don't just throw it away.  Get a clear glass that is big enough so that 
the light bulb (at least a 60 watt bulb) will fit inside the glass 
completely with the metal stem pointing toward the closed bottom of the 
glass.  Now invert the glass with the light bulb inside it (so that the 
bottom of the glass and the light bulb stem point up) and put this in a 
microwave oven on top of a ceramic or glass microwave safe surface.  
Darken the room. Set the oven timer for 4 to 6 seconds.  NO MORE!  The 
bulb will get hot and will not burst (generally) if you follow the 6 
second maximum rule.  What else do you observe?  Remember to watch very 
closely!

REMEMBER, ALWAYS GET PERMISSION FROM AN ADULT TO DO THESE EXPERIMENTS.  
ALL OF THESE EXPERIMENTS CAN LEAD TO VERY HOT TEMPERATURES, FIRE, NOXIOUS 
FUMES AND SMOKE. ALWAYS HAVE A FIRE EXTINGUISHER HANDY.  IF A FIRE STARTS, 
KEEP THE MICROWAVE OVEN DOOR CLOSED.  IF POSSIBLE, DO THESE EXPERIMENTS IN 
A SCIENCE LAB IN A PROPERLY VENTILATED HOOD WITH PROPER SUPERVISION FOR 
MAXIMUM SAFETY.

I hope I have answered your question and that you get to try a few of 
these experiments.  Some of this could be science fair material.

Dr. Peterson
xpetersonx@aol.com