Why does pure water have pH value?

Answer 1

The water molecule is made up of hydrogen and oxygen, H2O. One of the hydrogen atoms is able to leave the water molecule, leaving behind OH- and H+. This happens, to a certain degree, in pure water such that in every liter of water, 10-7 moles of water will dissociate into OH- and H+. This is described by the equilibrium constant of water deprotonation.

pH is calculated as -log10[H+]. Because [H+] = 10-7, pH = 7. We consider this "neutral" because it is the pH that our bodies, and most of the rest of the biological organisms found in nature, use.

Answer 2

Pure water undergoes a self-ionization process where water molecules can donate and accept protons, creating equal amounts of hydronium ions (H3O+) and hydroxide ions (OH-) in solution. This equilibrium leads to a concentration of 1 x 10^-7 mol/L for both ions, resulting in a pH of 7, which is considered neutral.