Microwaves can excite water molecules through a process called dielectric heating. This phenomenon occurs due to the interaction between the electric field of the microwave radiation and the dipolar nature of water molecules.
Water molecules have a positive charge at one end (hydrogen) and a negative charge at the other end (oxygen), creating a dipole. When microwaves pass through a substance like water, the alternating electric field of the microwaves causes the water molecules to rapidly rotate. This rotation attempts to align the dipole of the water molecule with the changing electric field of the microwaves.
The rotation of water molecules causes collisions and friction between neighboring molecules. This friction generates heat, leading to an increase in the internal energy of the water molecules, which manifests as an increase in temperature. The heating effect is more pronounced in liquid water than in ice or water vapor because the molecules in a liquid are closer together and have more opportunities for collisions.
The specific frequency of microwave radiation (around 2.45 GHz) used in microwave ovens is chosen to efficiently excite the rotational motion of water molecules. The microwave energy is absorbed by the water molecules, rapidly heating them and eventually heating the surrounding substances through conduction or convection.
It's important to note that while water is the primary target for microwave absorption, other polar molecules and substances with mobile charged particles can also be affected by microwave radiation and exhibit dielectric heating to some extent. However, the effect is most pronounced in water due to its high dipole moment and abundance in many food items.