Resistance of Water
Name: Camilla L.
If you use a dc source one can see the resistance between two metallic wires dipped into flowing water.
What is the phenomena behind?
Pure water has a very high resistivity, but it is finite. The value is
2.5x10^+5 ohm meters at 20C. and 1 atm pressure. In contrast, liquid mercury
has a resistivity of 9.58x10^-7, and at the other extreme glass has a
resistivity of ~ 10^-12 (depending upon the type of glass). The reason for
water's resistivity is that it is a self-ionizing liquid. The reaction is:
2H2O == H3O(+1) + OH(-1). These ions act as charge carriers for electric
current. Actually measuring this quantity is challenging because the water
must be EXTREMELY pure. Soluble impurities leached from the electrodes, the
walls of the container, or on dust particles can introduce small amounts of
various ions that will lower the resistivity significantly.
Ultra-pure (distilled) water is not a very good conductor of electricity. However, water
containing dissolved ionic compounds will conduct water because the cations (+ ions) and
anions (- ions) are freed from their attachment to each other be the dissolution process.
If your source of flowing water contains variable amounts of ions in solution, its
electrical conductivity will likewise vary because the aquated ions are the charge carriers
in solution. When the ion concentration is elevated, the water will show higher conductivity
than when the ion concentration is lower.
Click here to return to the Physics Archives
Update: June 2012