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Resistivity of Water

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Light, Truman S., Stuart Licht, Anthony C. Bevilacqua, and Kenneth R. Morashc. The Fundamental Conductivity and Resistivity of Water [pdf]. Electrochemical and Solid-State Letters. Vol. 8, No. 1 (2005): E16-E19. "At 25°C the accepted values with their uncertainties for conductivity, κ and its reciprocal, resistivity, ρ, are:
κ = 0.05501 ± 0.0001 μS/cm at 25.00 °C
ρ = 18.18 ± 0.03 MΩ·cm at 25.00 °C"
18.18 MΩ·cm
Light, Truman S. and Stuart L. Licht. Conductivity and Resistivity of Water from the Melting to Critical Point [subscription required]. Analytical Chemistry. Vol. 59, No. 9 (1987): 2327-2330. "The temperature at which the water may be used and measured varies considerably, but is not usually at 25 "C. Therefore the resistivity, which theoretically is 18.2 mΩ cm [sic] only at 25 "C, may be automatically compensated to the standard temperature of 25 "C to permit ready judgment of the ionic impurity level." 18.2 MΩ·cm
Millipore. Measuring the Conductivity of Ultrapure Water. Water Line. No. 1 (June 1998): 3. "The curve below indicates how the resistivity of ultrapure water varies as a function of temperature. For instance, at 25°C, the resistivity of ultrapure water is equal to 18.2 MΩ.cm whereas at 10°C, the resistivity will be greater than 40 MΩ.cm." 18.2–40 MΩ·cm
Calder, Vince. Resistance of Water. Argonne National Laboratory. 22 April 2004. "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 [sic] (depending upon the type of glass)." 25 MΩ·cm

Every child has had the experience of someone telling him/her not to get near a body of water during a thunderstorm, whether it be from their parents, a teacher, or a life guard. When the child asks their elder for explanation they would simply claim "If you do you'll get hit by a bolt of lightning because water conducts electricity!" Exactly how true is this? Could it be that convention is wrong?

We can further delve into this question by finding the resistivity of water. Resistivity is an intrinsic property of a material that is measured as its resistance to current per unit length for a uniform cross section." Many people have conducted tests for this value for one reason or another (mostly for filtration systems) and have found the resistivity of water to be approximately 182,000 Ωm at 25 °C. This makes pure water a horrible conductor. It would take a huge potential difference to just run a 1 A current through a body of water. But does this mean that you're safe in a body of water during a thunderstorm? Yes, if you are certain that it is distilled water. But since your skin is full of salts and other impurities, any body of water you may be standing near is almost surely an electrolytic solution (good conductor).

Sam Tetruashvili -- 2006