Question

Liquid dielectrics that have polar molecules (such as water) always have dielectric constants that decrease with increasing temperature. Why?

Short answer

The restored potential energy will decrease so the dielectric constant will decrease.

Step-by-step solution

About polar molecules and liquid dielectric 

A polar molecule is usually formed when the one end of the molecule is said to possess more number of positive charges and whereas the opposite end of the molecule has negative charges, creating an electrical pole

A liquid dielectric is a dielectric material in liquid state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric liquids are used as electrical insulators in high voltage applications.

Determine why dielectric constants decrease with increasing temperature

As we know by increasing the temperature the kinetic energy that the electron in the molecules of the dielectric liquid have been increased so the electron, moves faster and the polarity if the molecules will increase so the strong induced electric fileld will create which resist the external electric filed then the restored potential energy will decrease so the dielectric constant will decreases.

Therefore the restored potential energy will decrease so the dielectric constant will decrease.