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Autoionization of Water and Kw

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Water reacts with itself, H2O + H2O ⇌ H3O+ + OH−, giving Kw = [H3O+][OH−]; at 25 °C this defining value is 1.0 × 10⁻¹⁴.

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In pure water at 25 °C, [H3O+] and [OH−] each equal 1.0 × 10⁻⁷ M, satisfying Kw.

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Kw holds in any aqueous solution at a given temperature, acidic or basic, not only pure water.