Water is the substance that has an equal number of hydrogen ions (H⁺) and hydroxide ions (OH⁻). This balance is what makes water neutral on the pH scale, with a pH of 7.
In pure water, a small fraction of water molecules naturally dissociate into hydrogen ions and hydroxide ions. This process is known as self-ionization of water. The chemical equation for this process is:
H₂O ⇌ H⁺ + OH⁻
At 25°C (77°F), the concentration of hydrogen ions and hydroxide ions in pure water is equal, each at a concentration of 1 × 10⁻⁷ moles per liter. This equilibrium ensures that water remains neutral, neither acidic nor basic.
Understanding this balance is crucial in chemistry, as it forms the basis for the pH scale and helps in predicting the behavior of substances in aqueous solutions.