Water at $50~^\circ\mathrm{C}$ has a $\mathrm{pH}$ of $6.63$, while the pH of water at $0~^\circ\mathrm{C}$ is $7.47$.

The temperatures are comparable to a chilled, iced drink (at $0~^\circ\mathrm{C}$) and a hot drink (at $50~^\circ\mathrm{C}$). Would these differences in acidity be noticeable? Would they be noticeable if the drink was more acidic, such as orange juice?


First, the pH of water changes with rising temperature because the total number of $\ce{H+}$ ions (well, solvated hydronium ions, really) increases, not because it has become more alkaline. The water is still neutral, having an equal number of hydroxyl ions as hydronium ions. Heat simply increases the ionization.

Second, it depends on the individual taster, but a sample of water at $\mathbf{20~^\circ C}$ with a $\mathrm{pH}$ of $6.63$ (perhaps acidified with citric acid) would probably taste sour in comparison with a sample of water at $\mathbf{20~^\circ C}$ with a $\mathrm{pH}$ of $7.47$ (perhaps with a bit of baking soda, $\ce{NaHCO3}$).

See Temperature Dependent[sic] of the pH of pure Water for a more detailed explanation.

  • $\begingroup$ So the scale adjusts, but the acidity doesn't? Is that what you're saying? So even with a temperature change, though the pH value will change, all liquids will taste the same in terms of acidity (or sourness)? $\endgroup$ – StopReadingThisUsername Sep 1 '16 at 8:25
  • $\begingroup$ No. The scale doesn't adjust. pH is just a measure of the concentration of $\ce{H+}$ ions in solution. Pure water spontaneously dissociates a little bit into $\ce{H+}$ and $\ce{HO-}$, and does so more with higher temperature. So for any given temperature, there is an equal concentration of $\ce{H+}$ and $\ce{HO-}$ in pure water, and it is neutral. When the temperature rises, both concentrations rise and the water is still neutral. $\endgroup$ – SCH Sep 1 '16 at 12:34

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