Add acid and H⁺ soars while OH⁻ plummets—but their product stays locked at 1.0 × 10⁻¹⁴. Drag the slider to change pH and watch the ion concentrations dance around the invisible constant.
The ion product of water, Kw = [H⁺][OH⁻], always equals 1.0 × 10⁻¹⁴ at 25°C in any aqueous solution—pure water, acid, or base. When you add acid, the surge in H⁺ ions forces OH⁻ down to keep the product constant. Add base and OH⁻ rises while H⁺ drops, but Kw never changes. This inverse relationship is why knowing one ion concentration instantly tells you the other: [H⁺] = Kw / [OH⁻]. Water self-ionizes to produce equal tiny amounts of both ions, giving pH 7, but the product remains locked at 1.0 × 10⁻¹⁴ regardless of what you dissolve in it.
pH equals negative log of [H⁺] so pH 3 has 10× more H⁺ than pH 4, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C in all aqueous solutions, and pH + pOH always equals 14 at 25°C.