December 13, 2003
A: FROM MENTOR MOLLY WILLIAMS IN MI
There's a critical difference between ionic compounds and dipolar
molecules. In solution, an ionic compound separates into two
charged particles that can move independently in an applied electric
field, so the positive ion can move toward the negative electrode,
and the negative ion can move toward the positive electrode. An
electric current is the motion of charged particles, so in a
solution that contains ions, the charge carriers are the ions. A
dipolar molecule has a positive end and negative end, but the two
ends remain attached to each other, so the molecule is neutral
overall. In an electric field, a dipolar molecule will orient so
that its charged ends are pointed toward the oppositely charged
electrode, but there's no net force, so the whole molecule isn't
pulled toward either electrode. Furthermore, because there are no
free charged particles able to move toward the electrodes, there's
no electric current, and no electrolysis can occur. Water, for
example, is a dipolar molecule; the oxygen atom is negative, and the
hydrogen atoms are positive. Very pure water is not a good
electrical conductor. Chemists check the purity of their distilled
water by measuring its electrical conductivity.
However, there's an important role that dipolar solutions fill;
they're able to dissolve ionic compounds. If you put a little salt
(sodium chloride) in water, the positive sodium ions become
surrounded by water molecules with their negative (oxygen) ends
nearest the sodium ion; and the negative chloride ions are
surrounded by water molecules with their positive (hydrogen) ends
nearest the chloride ion. This makes it much easier to dissolve the
salt in water. When salt is dissolved in water, the solution has
free charged carriers (the ions, surrounded by water molecules), so
it can conduct electricity and be used for electrolysis.