From the Greek meaning 'heavy with wine'
A blog devoted to science and reason
Written after a glass or two of Pinot Noir.

Sunday, November 27, 2011

Don't Ask Marilyn a Physics Question

Marilyn needs a physics class. A chemistry class would help, too.  In this morning's Parade magazine*, Marilyn vow Savant answered the question "Is it true that if water is 100 percent pure, it will not conduct electricity?" with "Yes."

Uh, no.

Electricity is the motion of electrically charged particles.  Most commonly, it is the motion of electrons in metal wires that most people think of as electricity.  However, in batteries and other electrochemical applications, both positive and negative ions are moving.  How well a substance conducts electricity depends on a number of factors such as the number of charged particles, the distance the charges have to travel, the area through with they move, the temperature of the material, complex interactions with the atoms and molecules, and the applied voltage.

Tap water is a poor conductor; copper is about 10 billion times better**.  Adding ions makes it somewhat better.   Copper is only 10 million times better than sea water. In other words, sea water is around 1000 times better at conducting electricity than drinking water.

So it makes some sense to think that if one were to remove all impurities from water than it wouldn't conduct at all.  Except…

Water autodissociates.  Even in pure water, there are hydronium ions (H30+) and hydroxide ions (OH-).
Chemists have a way to express how much hydronium ions are in solution; it's called pH, the logarithm (base 10) of the H3O+ concentration.  Pure water at room temperature has a pH 0f 7.  

Therefore, pure water will conduct**.

There are other substances that one might think don't conduct electricity - air and glass, for example.  Under normal conditions they don't, but if I were to apply enough of a voltage, even these materials will conduct.  Remember this the next time you watch a lightning storm.

*As of noon on Nov. 27, neither Parade.com nor www.marilynvossavant.com have a link to this column.  When one appears, I will update the blog.  UPDATE: The link to the column is http://www.parade.com/askmarilyn/2011/11/Sundays-Column-11-27-11.html.

** Pure water has a conductivity of 5.5 X 10^-6 S/m.  For comparison, copper has a conductivity of 5.96 X 10^7 S/m.  

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