Conductivity of Water

cepheid

Does "pure" i.e. distilled water conduct any electricity? I thought that the answer was no...that the conductivity of water was a result of impurities. But I became confused by post # 3 here:

http://www.physicsforums.com/showthread.php?t=64920

which explains that it has something to do with the polar nature of water molecules. Can someone clarify this?

Integral

Distilled water may have low conductivity, put to get the conductivity down to the megohm range it must be deionized. Very low conductivity water can only be maintained in specialized systems, plastic pipes and continuous circulation are key elements. It is rumored (but not supported by MSDS) that very low conductivity (>10megohm) DI water can be fatal to drink because it (supposedly) disrupts the ion balance in the digestive system.

cepheid

Thanks for the detailed response. Just to get back to basics...so conductivity in water is due to the presence of dissolved ions, right? Do water molecules themselves, despite their polarity, not constutute a conductive medium i.e. will they not move in the presence of an applied field? I'm thinking no...

Gokul43201

Distilled water is reasonably conductive (less than or of the order of a megaohm-cm). Completely deionized ("pure") water has an AC resistivity of about 18 megaohm-cm at 60 Hz. This residual conductivity comes from the natural ionization of water (it's pH is 7, so there are 10^{-7} moles of H+ ions in a liter of water). However, tap water is about 4 or 5 orders of magnitude more conductive than pure water. This conductivity comes primarily from dissolved ions.

So, it's perfectly acceptable to say that the conductivity of "water" comes essentially from dissolved impurities.

Integral

The water molecule is intrinsically neutral so by itself cannot be a current carrier. I do not think that the post you mention is on target. A molecule can be polar (magnetically) but not ionized. IIRC The polar nature of water is due to the asymmetric arrangement of the constituent atoms, this is different from Ionization. With an application of a magnetic field water molecules would tend to align themselves, but this is not a current. To the best of my knowledge currents in water are due to dissolved ionic material (even a tiny bit of salt makes water very conducive).