When i was reading solublity of ionic compounds,they mentioned that polar compunds are fairly soluble in polar solvents and water is better solvent due its high dielectric constant.Can anybody explain what is it and how does it affect solublity.

  • 2
    $\begingroup$ There is expected some a priori effort, like Wikipedia and/or Google search. $\endgroup$
    – Poutnik
    Jun 12 '19 at 3:43
  • $\begingroup$ 2 comments about this question: (1) Dielectric constant is an average value but solvents can have different values along their structure; (2) water is a fantastic solvent for polar compounds but maybe water is not such a wonderful solvent and life (as we know it) evolved around water and created mostly water-soluble compounds. $\endgroup$
    – SteffX
    Jun 12 '19 at 14:40
  • $\begingroup$ That statement is not exactly wrong, but it also does not explain a lot. I recommend reading about the chemical properties of water and what a dielectric constant ist, separately, the connection will then become clear. $\endgroup$
    – Karl
    Jun 12 '19 at 22:50

The dielectric constant $\epsilon$ (known also as permittivity) is a measure of the extent to which a substance is polarized under an applied (external) electric field. Polarization amounts to net separation of charge across the substance. A "dielectric" is the common name used to refer to a substance placed between the plates of a capacitor and used to store electrical energy in the form of polarization, that is, as charges separated across the material. A capacitor is the non-chemical analogue of a voltaic cell, that is, a form of storing electrical energy.

A substance with a high dielectric constant is easily polarized. Polarization allows countercharges to be placed around an ion, resulting in Coulombic interactions between solvent and ion, promoting solubilization of the ion by competing with interionic interactions. Similarly, a polar solvent - one with a high dielectric constant (relative permittivity $\epsilon_r>\approx15$) - will solubilize polar molecules by forming stabilizing interactions with the solute that compete with solute-solute interactions.

Water is a good solvent for polar molecules for the above-mentioned reasons. Water is sometimes referred to as the "universal" solvent, but it is generally a good solvent for polar solutes, not apolar ones.


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