The pH of pure water

PH is a measuring scale in respect of acids and alkali. It is negative logarithm of hydrogen ion concentration. Why does pure water have a pH of 7? That question is actually misleading! In fact, pure water only has a pH of 7 at a particular temperature - the temperature at which the Kw value is 1.00 x 10-14 mol2 dm-6. This is how it comes about: To find the pH you need first to find the hydrogen ion concentration (or hydroxonium ion concentration - it's the same thing). Then you convert it to pH. In pure water at room temperature the Kw value tells you that: [H+] [OH-] = 1.00 x 10-14 But in pure water, the hydrogen ion (hydroxonium ion) concentration must be equal to the hydroxide ion concentration. For every hydrogen ion formed, there is a hydroxide ion formed as well. That means that you can replace the [OH-] term in the Kw expression by another [H+]. [H+]2 = 1.00 x 10-14 Taking the square root of each side gives: [H+] = 1.00 x 10-7 mol dm-3 Converting that into pH: pH = - log10 [H+] pH = 7 That's where the familiar value of 7 comes from. The variation of the pH of pure water with temperature The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. Using the simpler version of the equilibrium: The forward reaction absorbs heat. According to Le Chatelier's Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made.
	Note:  If you don't understand Le Ch atelier's Principle, you should follow this link before you go on. Make sure that you understand the effect of temperature on position of equilibrium. Use the BACK button on your browser when you are ready to return to this page.
According to Le Ch-atelier, if we increase the temperature of the water, the equilibrium will move to lower the temperature again. It will do that by absorbing the extra heat. That means that the forward reaction will be favoured, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of Kw as temperature increases. The table below shows the effect of temperature on Kw. For each value of Kw, a new pH has been calculated using the same method as above. It might be useful if you were to check these pH values yourself. T (°C) Kw (mol2 dm-6) pH 0 0.114 x 10-14 7.47 10 0.293 x 10-14 7.27 20 0.681 x 10-14 7.08 25 1.008 x 10-14 7.00 30 1.471 x 10-14 6.92 40 2.916 x 10-14 6.77 50 5.476 x 10-14 6.63 100 51.3 x 10-14 6.14 You can see that the pH of pure water falls as the temperature increases. A word of warning! Although the pH of pure water changes with temperature, it is important to realise that it is still neutral. In the case of pure water, there are always going to be the same number of hydrogen ions and hydroxide ions present. That means that the pure water remains neutral - even if its pH changes. The problem is that we are all so familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that you calculate the neutral value of pH from Kw. If that changes, then the neutral value for pH changes as well. At 100°C, the pH of pure water is 6.14. That is the neutral point on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14. Similarly, you can argue that a solution with a pH of 7 at 0°C is slightly acidic, because its pH is a bit lower than the neutral value of 7.47 at this temperature.