Ice in a Microwave

The microwave oven emits microwaves that are absorbed by water molecules in food. Microwaves make the polar water molecules rotate or oscillate, and their “friction” within the material converts some of this kinetic energy into thermal energy to raise the temperature of the food.

Suppose you made an ice block that had liquid water trapped in a large cavity inside and then you placed the block into a microwave oven. Could the trapped water be brought to a boil while the ice remained ice?

Answer

Yes! The water molecules in the liquid state rotate a bit in the microwaves and transfer energy to the surrounding molecules to make them jiggle randomly. The water molecules in ice are locked into crystals and are unable to rotate. (Note: The actual details of molecular bonding in the ice are more complicated and show that a minuscule amount of rotation is possible, but an insignificant amount to change the ice to water.) Using microwaves, therefore, one can boil water inside an ice block!

Boiling the water inside the ice block is an example of selective energy absorption. Numerous examples of selective absorption occur in the natural world. For example, the green leaves of plants have chlorophyll A and B molecules that selectively absorb bluish and greenish light for photosynthesis. At an even smaller scale, nuclei are very selective in absorbing gamma rays of specific energies. At the macro scale of meters, we know that rooms can absorb and amplify sound energy at selected resonance frequencies. Some materials are even useful for just the opposite behavior, such as window glass, which has no selective absorption in the visible part of the electromagnetic spectrum. You can take your pick, but the game is played by the rules of nature.

The selective absorption by water molecules (and some other molecules) in a microwave environment is a little different from the other examples given above. At the water molecule’s resonant frequencies in the microwave region of the electromagnetic spectrum, the applied field changes so rapidly that very little energy is transferred to the nearby molecules. Microwave ovens actually operate at a frequency that is lower than the frequency at which the absorption is greatest. The food needs to be heated throughout, and by lowering the applied frequency a bit, more microwaves penetrate farther inside, past the outer layer.