pH as a chemical measure
The pH of an aqueous system describes the activity of hydronium ions. At school level it is usually written as pH = -log10[H3O+]. This concentration form is a useful approximation for sufficiently dilute solutions. Because the scale is logarithmic, a change of one pH unit corresponds to a tenfold change in hydronium-ion activity. At 25 °C, neutral pure water has pH close to 7 because [H3O+] = [OH−]. Neutrality can occur at a different numerical pH when temperature changes, so “pH 7” and “neutral” are not always identical.