Pure water’s pH is only 7.00 at a specific temperature of 25.0 °C. Students (and teachers too) hear that number so often that they forget where it comes from. And forgetting its origins makes one forget that if the temperature deviates significantly from 25.0 °C, you will get unfamiliar numbers for the pH of pure water.
At any temperature, pure water will always have the same concentration of ions resulting from a very slight splitting of the life-essential molecule into a positive hydrogen ion and a negatively charged hydroxide ion. The product of each ion’s concentration will equal its so-called Kw of 1.01 × 10-14 at 25.0 °C. To calculate the concentration of H+ ions, you first take the square root 1.01 × 10-14 and then take the negative logarithm of H+, the definition of pH. It yields 7.00.
But changing temperature usually affects any equilibrium constant, including Kw. In this case raising temperature helps water split up. You get more ions, thus a higher ion product. Kw becomes 5.48 × 10-14 at 50 ° C, Lowering the temperature has the opposite effect on equilibrium, and Kw becomes 0.29 × 10-14 at 10° C.
When you recalculate pH of pure water at 50 ° C and 10° C, we obtain pH’s of 6.63 and 7.27, respectively. The temperature does not make water either slightly acidic or alkaline. It’s just that the middle or neutrality point of the pH scale at those different temperatures changes. The 7.00 is not set in stone. The middle point of the pH scale is a setting derived from what the Kw happens to be at 25.0° C.
The pH is also 7.00 for aqueous solutions whose solutes at that same temperature do not affect the ions that water itself produces. When the temperature changes for those solutions, the pH will change accordingly. In our bodies, if temperature was the only factor, then out physiological pH would be below 7.00. But the presence of bicarbonate ions eats up some of the hydrogen ions, setting the physiological pH at about 7.4. The pH of the extracellular fluid of tumour cells, as determined by probing microelectrodes, is acidic. That truth has been known for at least 3 decades, and of course nothing one eats will have any impact on the pH of that fluid.
Often teachers have to reiterate to get ideas across. So indulge me. 🙂 Does temperature affect neutrality of pure water? No. The concentration of hydrogen ions will be equal to that of hydroxide ions as long as no solute interferes with one of them. Does raising temperature raise the concentration of OH– ? Yes. Of H+? Yes. Will that in turn affect pH? Of course, by definition.