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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, 2017 at 22:13
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    $\begingroup$ Does it depend on the temperature? $\endgroup$
    – Lynn
    Dec 6, 2017 at 22:16
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    $\begingroup$ Of course it does chemistry.stackexchange.com/questions/39608/… $\endgroup$
    – Mithoron
    Dec 6, 2017 at 22:18
  • $\begingroup$ Yep, it does, and quite significantly. The solubility of $\ce{CO2}$ decreases with the increase of the water temperature (A page ripped off from CRC Handbook which you can use to support your explanation to the students). $\endgroup$
    – andselisk
    Dec 6, 2017 at 22:20
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    $\begingroup$ So for pure distilled water at $25\ \mathrm{^\circ C}$ the pH would be 7.00. But the $K_\mathrm w$ is a function of temperature and hence the pH of pure water will vary with temperature too. The problem is that ordinary distilled water will contain some dissolved $\ce{CO2}$ which will lower the pH. You have to take extra precautions not to get $\ce{CO2}$ dissolved in the distilled water. $\endgroup$
    – MaxW
    Dec 6, 2017 at 23:12

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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, 2019 at 8:57

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