This was asked by one of my students in my class. I know it is the pH of 7, but others say it can be 6.55 pH

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    $\begingroup$ And I say it's 6.2. What others say is not an argument. Atmospheric gases are dissolved in distilled water (mainly oxygen, nitrogen, argon, carbon dioxide). Due to dissolved carbon dioxide, distilled water has a weakly acid medium and its pH is typically between 6 and 7. It vastly depends on how it's stored and what the ambient conditions are. $\endgroup$ – andselisk Dec 6 '17 at 22:13
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    $\begingroup$ Does it depend on the temperature? $\endgroup$ – Lynn Dec 6 '17 at 22:16
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    $\begingroup$ Of course it does chemistry.stackexchange.com/questions/39608/… $\endgroup$ – Mithoron Dec 6 '17 at 22:18
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    $\begingroup$ ok. thanks for the info. i'll pass this on to my classmate. $\endgroup$ – Lynn Dec 6 '17 at 22:29
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    $\begingroup$ @Gert - At 0 °C pKw = 14.95, so pH = 7.48 $\endgroup$ – MaxW Dec 6 '17 at 23:46

keep in mind that water (distilled, deionized, or tap) is NOT "pure" (i.e., pH equal to 7). The moment it comes in contact with air, CO2 gas begins dissolving into it, forming carbonic acid. The actual pH, therefore, will often be slightly less than 7.

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  • $\begingroup$ The pH of water in equilibrium with aerial CO2 is reportedly 5.65.( or less due raising CO2 level ) $\endgroup$ – Poutnik May 23 '19 at 8:57

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