autoprotolysis of water
/ aw-toh-proh-TOL-ih-sis /
Even a glass of perfectly pure water is not as quiet as it looks. Every so often two water molecules bump together and one passes a hydrogen ion to the other, like neighbours quietly trading at the fence. Autoprotolysis of water is this gentle, ceaseless self-reaction by which water makes a trickle of ions all on its own.
Specifically, autoprotolysis is the reaction in which one water molecule donates a hydrogen ion to another, producing one hydronium ion and one hydroxide ion. At equilibrium the product of their concentrations is a fixed number called the ion product of water, Kw, about ten to the minus fourteen at room temperature. Because the two ions are made in equal numbers, pure water has equal amounts of each, giving the familiar neutral pH of 7.
This reaction matters because it is the silent backdrop to every aqueous acid-base calculation: it links pH and pOH, and it sets the floor that means no water solution is ever truly free of hydrogen or hydroxide ions. The honest caveat is that Kw rises sharply with temperature, so the neutral pH of pure water is exactly 7 only near 25 degrees Celsius — hotter water is neutral at a pH below 7, even though it is not acidic.
In pure water at room temperature, both the hydronium and hydroxide concentrations are about ten to the minus seven moles per litre — their product is the constant Kw of ten to the minus fourteen, and pH plus pOH adds up to 14.
Water quietly splits into equal hydronium and hydroxide, setting neutral pH 7.
Neutral means equal hydronium and hydroxide, not pH 7 specifically. Since Kw grows with temperature, the neutral point slides below pH 7 in warm water and above it in cold — neutrality tracks the ions, not the number.