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The ratio of hydrogen to hydroxide is higher than it normally is. Water naturally dissociates into hydrogen and hydroxide ever so slightly in equal amounts unde

• logfromblammo 11y ago
  The auto-dissociation constant Kw is 10^-14 mol^2 L^-2. That means that concentration of hydronium [H3O+] ion multiplied by the concentration of hydroxide [OH-] ion is equal to the constant. In pure water, there are no other ions to change the balance, so you get 2 H2O <-> H3O+ + OH-, and each concentration is the square root of the dissociation constant. So 10^-7 mol/L H3O+ and 10^-7 mol/L OH-. The abbreviation "pH" stands for "power hydrogen", and is the negative log10 of the H3O+ concentration. 10^-7 mol/L H3O+ is pH 7, which is neutral. There is also a lesser-known term pOH, which is pretty much pOH + pH = 14 under normal conditions. But that water doesn't stay perfectly neutral for long. CO2 from the atmosphere dissolves into it, using CO2(g) <-> CO2(aq) K_CO2 = 2.0e-3, P_CO2 = 3.6e-4 CO2(aq) + H2O(l) <-> H2CO3(aq) K = 1.3e-3 H2CO3(aq) + H2O(l) <-> H3O+(aq) + HCO3-(aq) Ka1 = 2.0e-4 HCO3-(aq) + H2O(l) <-> H3O+(aq) + CO3--(aq) Ka2 = 4.7e-11 Solving the equilibrium gives you a pH of 5.65 (10^-5.65 mol/L H3O+). Some natural groundwater contains other dissolved minerals, including carbonates (limestone is CaCO3), which pushes the CO2 equilibrium in the other direction, making water more basic (higher pH).