The dielectric constant of water and ice is high

Scientific Explanation

The static dielectric constant (relative permittivity) of water at 25 degrees Celsius is approximately 80 — an exceptionally high value among liquids. For comparison, methanol reaches 33, ethanol 25, acetone 21, and nonpolar liquids like hexane only about 2. Ice also has a remarkably high dielectric constant of roughly 92.

The reason lies in the strong permanent dipole moment of the water molecule (1.85 Debye) combined with its ability to form a cooperative network through hydrogen bonds. In an electric field, the dipoles partially align. Crucially, the hydrogen bonds between molecules promote correlated orientation: when one molecule rotates, the hydrogen bonds pull neighboring molecules partially along. This cooperative orientation amplifies the dielectric response far beyond what the individual molecular dipole moment alone would predict.

Mathematically, this cooperative effect is captured by the Kirkwood g-factor, which for water is approximately 2.7 — meaning the effective polarization is nearly three times that of an isolated dipole.

Everyday Relevance

The high dielectric constant is the primary reason water is such an excellent solvent for salts and polar substances. It weakens the electrostatic attraction between oppositely charged ions by a factor of 80, allowing ions to be easily extracted from crystal lattices. Table salt dissolves in water but not in oil — this difference is essentially due to this property. In biochemistry, the high dielectric constant is also decisive: it enables enzyme function, protein folding, and the transport of ions through cell membranes.