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Why Do Microwaves Only heat up Water?

  1. Dec 25, 2010 #1
    I understand that microwaves cause water in food to heat up, but I don't understand why it is only water. Why won't the say lipids or protein molecules in food heat up why is it only water. And on a related topic is there a way to use frequency to not heat up water but to lengthen the bonds between the hydrogen and oxygen in water?
  2. jcsd
  3. Dec 25, 2010 #2
    Who says lipids don't heat up in a microwave?
  4. Dec 26, 2010 #3
    pzona do they?
    Microwave is acting by switching the electric field, so that molecules that have dipole moment such as water start vibrating, moreover hydrogen bonds between water molecules break and when they reform heat evolving is supplied to surrounding molecules which causes the whole dish to heat up.
  5. Dec 26, 2010 #4
    The simplest explanation is that of resonances. Think of a wine glass. In order to shatter it you need exactly the right frequency to do so. The same is true for heating up things with radiation, the heating is caused by the radiation of a certain frequency resonating with the bonds within a molecule - and microwaves just happen to be perfect for the bonds found within water.
  6. Dec 26, 2010 #5
    Microwave ovens heat up a lot more than just water. They also heat sugars, fats, waxes, and can even efficiently heat some types of glass. These substances heat efficiently because they are very polar, electrically.

    This is a common misconception. Microwave ovens work on the principles of dielectric heating, not any form of resonance. The microwave radiation causes the molecules to rotate back and forth with the electric field to generate heat. It doesn't really have anything to do with the bonds.
  7. Dec 26, 2010 #6
    Anything with a dipole moment can be heated in a microwave. Microwave chemistry is a pretty neat area of research and has led to things such as microwave digestion for sample preparation.
  8. Dec 27, 2010 #7
    Water heats well in a microwave oven because it is a molecular dipole and has a significantly high dielectric loss factor. That makes it susceptible to being affected by the oscillating electrical field. Other strongly polar molecules such as nitrobenzene and chloroform also heat well, whereas symmetrical molecules such as benzene and carbon tetrachloride are microwave transparent and don't heat - they are used to make microwave thermometers. Lipids don't heat well because they have a very low dielectric loss factor and have a non-polar nature. However they do heat, especially in larger volumes. Solid materials such as sugar and salt don't heat well, but they will when they had dissolved in water, although, for sugar, the mechanism is primarily an effect upon the specific heat capacity.
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