A sample of pure water is neutral because it contains

A sample of pure water is neutral because it contains the same number of H₃O⁺ ions as OH⁻ ions

Explanation:

Neutral pH is 7.

By definition pH = - log [H₃O⁺]

Water ionizes according to this equilibrium equation:

2 H₂O (l) ⇄ H₃O⁺ + OH⁺

So, the ionization (equlibrium) constant is:

Kw = [H₃O⁺ ] [ OH⁺]

At 25°C, Kw = 1 * 10⁻¹⁴

Hence: [H₃O⁺ ] [ OH⁺] = [H₃O⁺ ] ² = 1 * 10⁻¹⁴ ⇒ [H₃O⁺ ] = 10⁻⁷

⇒ pH = - log (10⁻⁷) = 7