text: DNA is polar due to its highly charged phosphate backbone. Its polarity makes it water-soluble (water is polar) according to the principle "like dissolves like".
  Because of the high polarity of water, illustrated by its high dielectric constant of 80.1 (at 20 °C), electrostatic forces between charged particles are considerably lower in aqueous solution than they are in a vacuum or in air.
  This relation is reflected in Coulomb's law, which can be used to calculate the force acting on two charges  and  separated by a distance  by using the dielectric constant  (also called relative static permittivity) of the medium in the denominator of the equation ( is an electric constant):
  At an atomic level, the reduction in the force acting on a charge results from water molecules forming a hydration shell around it. This fact makes water a very good solvent for charged compounds like salts. Electric force which normally holds salt crystals together by way of ionic bonds is weakened in the presence of water allowing ions to separate from the crystal and spread through solution.
  The same mechanism operates in the case of negatively charged phosphate groups on a DNA backbone: even though positive ions are present in solution, the relatively weak net electrostatic force prevents them from forming stable ionic bonds with phosphates and precipitating out of solution.
  Ethanol is much less polar than water, with a dielectric constant of 24.3 (at 25 °C). This means that adding ethanol to solution disrupts the screening of charges by water. If enough ethanol is added, the electrical attraction between phosphate groups and any positive ions present in solution becomes strong enough to form stable ionic bonds and DNA precipitation. This usually happens when ethanol composes over 64% of the solution. As the mechanism suggests, the solution has to contain positive ions for precipitation to occur; usually Na, NH or Li plays this role