What Is Kw? The Ion Product of Water Explained
Pure water is supposed to be the definition of neutral. So why does a pH meter dipped into boiling distilled water read about 6.1 — and why is that water still neutral? The answer is one small constant that quietly underpins the entire pH scale. The short answer: Kw is the ion product of water , the equilibrium constant for water reacting with itself: Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C . Because that product is fixed at a given temperature, knowing either ion concentration always gives you the other. Water reacts with itself Water is amphiprotic — it can donate a proton and accept one. Given enough molecules, some of them do both to each other: 2 H₂O ⇌ H₃O⁺ + OH⁻ This is called autoionisation (or self-ionisation). It happens in every sample of water, and in every aqueous solution, all the time. It is just extremely rare: at 25 °C only about one water molecule in 555 million is ionised at any instant. The equilibrium constant for that reaction, with the solvent water fold...