Question

Why does the microwave oven heat up a food item containing water molecules most efficiently?

Answer 1

Microwaves are non-ionizing electromagnetic radiations with frequencies ranging from $0.3\,GHz$ to $300\,GHz$. A household microwave oven is a device used for cooking foods which operates at a frequency $2.45\,GHz$. Microwave heating is most effective on polar molecules. Microwaves cause polar molecules to rotate and produce thermal energy.

Complete answer:
A microwave oven is an electric oven that heats and cooks food by exposing it to the microwaves in a process called dielectric heating. The materials containing polar molecules have an electrical dipole moment. When these materials (polar molecules) are placed in an oscillating field such as a high-frequency electromagnetic field (microwave in our case), the dipole moments (and hence the molecules) attempt to align with the field. Since the field is oscillating, the polar molecules rotate continuously by aligning with the field.

This causes frictional, vibrational, and collision interaction with other molecules. By this process, the energy transfer from the field to the material increases the kinetic energy or heat energy. This entire process of heating is called dielectric heating. Water molecules, fats, and sugar are examples of polar molecules but microwave heating is more effective on water because fats and sugar are far less polar than water molecules. Since the water molecules are polar molecules, microwaves raise the temperature of water most efficiently.

Note: Microwave heating is more effective on liquid water than on ice. Because In ice, movement of molecules is restricted. Do not try any experiment on the microwave oven at home. Please follow the safety guidelines to avoid any physical damages.