Why does salt dissolve in water and not gasoline?
(Lansing State Journal, March 22, 1995)
Water and salt both consist of what we call polar molecules. This means electrons in the molecule are not equally shared between the atoms of the molecule, causing a partial charge on the molecule's atoms.
Water, for instance, contains two hydrogen atoms and one oxygen atom. The electrons in the water molecule spend more time around the oxygen atom than around the hydrogen atoms.
Since electrons are negatively charged, the oxygen part of the molecule is partially negatively charged and the hydrogen parts of the atom are partially positively charged.
In salt, which is made of sodium and chlorine, the shared electrons spend almost all of their time on the chlorine side, making the chlorine negatively charged, and very little time on the sodium side, making the sodium positively charged.
When salt is put into water, the positively charged side of water molecules surround the negatively charged chlorine, and the negatively charged side of the water molecules surrounds the positively charged sodium. This breaks the sodium chloride bond and the salt is dissolved.
Gasoline contains non-polar molecules, which means the atoms in those molecules share electrons equally.
This also means there is no partial charge on those molecules to be attracted to the charges on the sodium or chlorine molecules of salt. Therefore, salt doesn't dissolve in gasoline.