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A microwave oven heats food primarily by:

1. giving rotational energy to water molecules
2. transferring electrons from lower to higher energy levels in water molecules
3. giving translational energy to water molecules
4. giving vibrational energy to water molecules

Correct answer: giving rotational energy to water molecules

Solution

Microwaves carry too little energy to excite electronic transitions. Water molecules are polar, so the rapidly oscillating microwave electric field exerts a torque on their dipole moments, driving rotational motion. The resulting molecular collisions convert this rotational energy into heat, warming the food. Thus microwaves act by giving rotational energy to water molecules.

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