<P> Pure water containing no exogenous ions is an excellent insulator, but not even "deionized" water is completely free of ions . Water undergoes auto - ionization in the liquid state, when two water molecules form one hydroxide anion (OH −) and one hydronium cation (H 3O +). </P> <P> Because water is such a good solvent, it almost always has some solute dissolved in it, often a salt . If water has even a tiny amount of such an impurity, then it can conduct electricity far more readily . </P> <P> It is known that the theoretical maximum electrical resistivity for water is approximately 18.2 MΩ cm (182 kΩ m) at 25 ° C . This figure agrees well with what is typically seen on reverse osmosis, ultra-filtered and deionized ultra-pure water systems used, for instance, in semiconductor manufacturing plants . A salt or acid contaminant level exceeding even 100 parts per trillion (ppt) in otherwise ultra-pure water begins to noticeably lower its resistivity by up to several kΩ m . </P> <P> In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.05501 ± 0.0001 μS / cm at 25.00 ° C. Water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process, as very little current is conducted . In ice, the primary charge carriers are protons (see proton conductor). Ice was previously thought to have a small but measurable conductivity of 1 × 10 S / cm, but this conductivity is now thought to be almost entirely from surface defects, and without those, ice is an insulator with an immeasurably small conductivity . </P>

Why is the boiling point of water a physical property