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Can someone please explain to me what does "pure" water mean? The definition vary so much everywhere I look. But I just want to see it from a chemistry/scientific perspective.

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    $\begingroup$ Interestingly, there is a classification system of what constitutes various degrees of "pure" water. $\endgroup$ – Tyberius Apr 15 '17 at 20:25
  • $\begingroup$ chemistry.stackexchange.com/questions/2870/… $\endgroup$ – Mithoron Apr 15 '17 at 20:33
  • $\begingroup$ chemistry.stackexchange.com/questions/575/… $\endgroup$ – Mithoron Apr 15 '17 at 20:36
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    $\begingroup$ Hello and welcome to Chemistry.SE. If you have any questions about how this site works, you can take the short tour or visit the help center. Regarding your question, I think you'll find that it's all a matter of degrees. There is no such thing as perfectly pure water (or most any other bulk substance for that matter) if you look closely enough. If you had the ability to see each individual molecule in a container of water, you would find billions of billions of non-water molecules even in the highest purity water. Hopefully someone will give you a good answer, though this is pretty broad. $\endgroup$ – airhuff Apr 15 '17 at 20:37
  • $\begingroup$ @Tyberius Your comment (+1) should be the answer, as it highlights there is not "pure water" per se, rather than different thresholds and criteria of purity, suitable (or not) for certain applications in mind. As in the comment by airhuff, too. $\endgroup$ – Buttonwood Apr 15 '17 at 21:02
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As suggested in the comments, there isn't, strictly speaking such a thing as pure water. Pure water (pure anything) would require that there isn't a single molecule of anything else, but water in the sample. As Buttonwood mentions above, if we even could separate water pure water somehow, water will form $\ce{H+}\text{ and } \ce{OH-}$ in attempt to reestablish equilibrium.

If we are a little less stringent about what "pure" means, we can define purity via various classifications that rely on our ability to experimentally determine certain properties of water. One factor that is commonly used to judge the purity of water is its resistivity. While we often think of water as being a good conductor, this is actually due to the prevalence of charged impurities dissolved in the water. The American Society for Testing and Materials International (ATSM) includes a resistivity of $\pu{18.2 M\Omega\cdot cm}$ ($\mathrm{M\Omega}$ means Mega-Ohms) as part of the definition for water of the highest purity standard (Type I).

But as mentioned in the link, even being in the highest category might not make the water pure enough for certain applications, such as trace metal analysis. So, even still, purity really depends on the context in which it is being used and the precision of experimental methods which are a proxy for purity.

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