The pH of pure water is 7 because of the equilibrium between the hydrogen ions (H+) and hydroxide ions (OH-) that are naturally present in water.

Here's a breakdown:

* Water is amphoteric: This means it can act as both an acid and a base.

* Self-ionization: Water molecules can donate and accept protons (H+), leading to a small but significant amount of H+ and OH- ions forming in pure water:

* H2O + H2O ⇌ H3O+ + OH-

* Equilibrium: This reaction reaches an equilibrium, meaning the rate of forward and reverse reactions are equal.

* Equal concentrations: At 25°C, the concentration of H+ and OH- ions in pure water is equal at 10^-7 moles per liter.

* pH scale: The pH scale is a logarithmic scale that measures the concentration of H+ ions. A pH of 7 indicates a neutral solution, where the concentration of H+ and OH- ions is equal.

Therefore, pure water has a pH of 7 because the concentration of H+ and OH- ions is equal at equilibrium.