Question

Water has a large dielectric constant, but it is rarely used in capacitors. Explain why.

Short answer

The reason is, that while being a weak conductor, pure water is a superb ionic solvent. The conductivity of the water is increased by the dissolved ions, causing the charge to flow between the capacitor plates until it discharges.

Step-by-step solution

Defining Dielectric constant  

The dielectric constant is an index that tells us how easily a dielectric will get polarised when placed in an external electric field. The more easily a material gets polarized, the higher will be the value of the dielectric constant for it.

Cause of despite its huge dielectric constant, water is rarely utilised as a dielectric in capacitors

Water molecules are polar and have a high relative permittivity value. If water is utilised as a dielectric between the capacitor plates, the water molecules will have a large value of conduction at a given voltage due to their polar characteristics. This high degree of conduction transforms a water molecule's dielectric characteristic into a conductor. As a result, water cannot be employed as a dielectric between the capacitor plates.