Relative permittivity of water

Water's static dielectric constant of about 80 is enormous for a small molecule. It comes from the permanent dipole of the bent H–O–H geometry combined with hydrogen bonding, which lets whole clusters reorient together in an applied field. The practical consequence is Coulomb's law divided by 80: the electrostatic attraction holding Na⁺ to Cl⁻ drops eightyfold when the pair is surrounded by water, which is why table salt dissolves and why water is the universal solvent of biology.

The value is strongly temperature-dependent — about 87.9 at 0 °C, 80.1 at 20 °C, 78.4 at 25 °C and 55.6 at 100 °C — because heat disrupts the hydrogen-bond network that makes cooperative alignment possible. It is also frequency-dependent: above roughly 20 GHz the molecules can no longer follow the field, and the resulting dielectric loss is precisely what heats food in a microwave oven. Treat any single figure here as a typical value tied to its temperature.