How does the polar character of water molecules help to explain water's relatively large dielectric constant? (Figure 19.19)

The capacity of an insulating material to hold energy in the form of an electric field, produced due to the formation of dipoles when the material is placed between two metallic plates of the capacitor is given by the dielectric constant.

Because the hydrogen atoms are not aligned with the oxygen atom in the water molecule. This generates a charge separation in the molecule, with the hydrogen atoms having a net positive charge and the oxygen atom having a net negative charge, causing water molecules to behave like electric dipoles with a large dipole moment.

The average of all dipole moments determines a material's polarization, which is related to the dielectric constant, hence a big dipole moment of a molecule corresponds to a large dielectric constant.