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Water can conduct electricity, and some solids can conduct. Why can't ice? Are ice molecules too packed together to let valence shell electrons bounce across each other to create electrical charge? Does ice stop conducting completely at absolute zero?

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    $\begingroup$ Ice can conduct electricity, but not very well. Electricity is conducted by free ions in water. The ions come from dissolved salt and from electrons released by disintegrated water molecules. Unless the valence electrons are freed from the water molecules, they don't play a part. In ice, the ions don't travel easily, as they are locked into crystals. Here is a succinct explanation: van.physics.illinois.edu/qa/listing.php?id=16435 $\endgroup$ – Ernie May 10 '15 at 21:14
  • $\begingroup$ Welcome to Physics.SE! Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. Thanks! $\endgroup$ – Gonenc May 10 '15 at 21:16
  • $\begingroup$ Valence electrons (as usually defined) are not the carrier of electric current (as valence bounds are paired electrons and thus correspond to filled bands, which do not conduct). $\endgroup$ – Sebastian Riese May 10 '15 at 22:30
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    $\begingroup$ @ernie: that comment probably could be turned into a good answer ;) $\endgroup$ – Kyle Kanos May 10 '15 at 23:58
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Conduction in water is mostly ionic - for pure water you have a very small fraction of ionized molecules (about 2 parts in 10$^{-7}$), so conductivity for pure water is poor. Add a little electrolyte (for example NaCl) and conduction improves. But in an ice crystal, the molecules / ions cannot move, so the main conduction mechanism is disabled. In that case you rely on occasional conduction band electrons - but there aren't many of those around. The band gap is about 7.8 eV source which means that the number of electrons excited into the conduction band at 0C will be extremely low - the fraction given by the Boltzmann factor $e^{-E/kT}=e^{-322}$

Contamination and impurities can bring the band gap down significantly - but pure ice is a good insulator.

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    $\begingroup$ One other comment, @Floris. If you slowly freeze salt water, the ice is composed of fresh water. This means that even in the presence of electrolytes, the ice's electrolyte concentration will be somewhat lower (sometimes MUCH lower) than the solution that it came from. $\endgroup$ – David White Jun 18 '16 at 1:01
  • $\begingroup$ @DavidWhite good point $\endgroup$ – Floris Jun 18 '16 at 1:10

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