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Science and Society

Microwave Ovens and Molecular Bonds
microwave oven rapidly cooks foods BOX TABLE 9.1 Positive
Athat contain water, but paper, glass, Negative hydrogen
and plastic products remain cool in the Approximate ranges of visible light, oxygen end
oven. If they are warmed at all, it is from the infrared radiation, and microwave end
heat conducted from the food. The explana- radiation A
tion of how the microwave oven heats water
but not most other substances begins with Radiation Frequency Range (Hz)
the nature of the chemical bond. Visible 4 × 10 to 8 × 10 14
14
A chemical bond acts much as a stiff light Maximum
spring, resisting both compression and Infrared 3 × 10 to 4 × 10 14 torque
11
stretching as it maintains an equilibrium radiation
distance between the atoms. As a result, a Microwave 1 × 10 to 3 × 10 11
9
molecule tends to vibrate when energized radiation
or buffeted by other molecules. The rate of B
vibration depends on the “stiffness” of the
spring, which is determined by the bond water molecule, the type of covalent bond
strength and the mass of the atoms mak- holding the molecule together.
ing up the molecule. Each kind of molecule The difference in electronegativity be-
therefore has its own set of characteristic tween a hydrogen and oxygen atom is 1.4,
vibrations, a characteristic natural fre- meaning the water molecule is held to gether Moved Aligned
quency. by a polar covalent bond. The electrons are by with
torque
field
Disturbances with a wide range of strongly shifted toward the oxygen end of
frequencies can impact a vibrating sys- the molecule, creating a negative pole at the
tem. When the frequency of a disturbance oxygen end and a positive pole at the
matches the natural frequency, energy is hydrogen end. The water molecule is thus a C
transferred very efficiently, and the system dipole, as shown in Box Figure 9.1A. BOX FIGURE 9.1 (A) A water molecule
undergoes a large increase in amplitude. The dipole of water molecules has two is polar, with a negative pole on the oxygen
Such a frequency match is called resonance. effects: (1) the molecule can be rotated by end and positive poles on the hydrogen
When the disturbance is visible light or the electric field of a microwave (see Box end. (B) An electric field aligns the water
some other form of radiant energy, a res- Figure 9.1B), and (2) groups of individual mol- dipoles, applying a maximum torque at right
onant match results in absorption of the ecules are held together by an electrostatic angles to the dipole vector. (C) Electrostatic
radiant energy and an increase in the mo- attraction between the positive hydrogen ends attraction between the dipoles holds groups
lecular kinetic energy of vibration. Thus, of a water molecule and the negative oxygen of water molecules together.
a resonant match results in a temperature end of another molecule (see Box Figure 9.1C).
increase. One model to explain how microwaves temperature over the boiling point of wa-
The natural frequency of a water heat water involves a particular group of ter, so a microwave oven does not brown
molecule matches the frequency of infra- three molecules, arranged so that the end food (a conventional oven may reach tem-
red radiation, so resonant heating occurs molecules of the group are aligned with the peratures almost twice as high). Large food
when infrared radiation strikes water mol- microwave electric field, with the center mol- items continue to cook for a period of time
ecules. It is the water molecules in your ecule not aligned. The microwave torques the after being in a microwave oven as the en-
skin that absorb infrared radiation from center molecule, breaking its hydrogen bond. ergy is conducted from the water molecules
the sun, a fire, or some hot object, result- The energy of the microwave goes into doing to the food. Most recipes allow for this con-
ing in the warmth that you feel. Because of the work of breaking the hydrogen bond, tinued cooking by specifying a waiting pe-
this match between the frequency of infra- and the molecule now has increased poten- riod after removing the food from the oven.
red radiation and the natural frequency of tial energy as a consequence. The detached Microwave ovens are able to defrost
a water molecule, infrared is often called water molecule reestablishes its hydrogen frozen foods because ice always has a thin
heat radiation. bond, giving up its potential energy, which layer of liquid water (which is what makes
The frequency ranges of visible light, goes into the vibration of the group of mol- it slippery). To avoid “spot cooking” of small
infrared radiation, and microwave radiation ecules. Thus, the energy of the microwaves pockets of liquid water, many microwave
are given in Box Table 9.1. Most microwave is converted into a temperature increase of ovens cycle on and off in the defrost cycle.
ovens operate at the lower end of the micro- the water. The temperature increase is high The electrons in metals, like the dipole water
wave frequency range, at 2.45 gigahertz. enough to heat and cook most foods. molecules, are affected by the electric field of
This frequency is too low for a resonant Microwave cooking is different from a microwave. A piece of metal near the wall
match with water molecules, so something conventional cooking because the heating of a microwave oven can result in sparking,
else must transfer energy from the micro- results from energy transfer in polar water which can ignite paper. Metals also reflect
waves to heat the water. This something else molecules, not conduction and convection. microwaves, which can damage the radio
is a result of another characteristic of the The surface of the food never reaches a tube that produces the microwaves.

9-15 CHAPTER 9 Chemical Bonds 243