pH is a measure of the acidity or alkalinity of a solution. Pure water is neutral because the concentration of hydrogen ions (H⁺) equals the concentration of hydroxide ions (OH⁻) produced by the auto- ionization of water (H₂O ⇌ H⁺ + OH⁻). At standard temperature (25°C), in pure water, [H⁺] = 10⁻⁷ mol/ L. The pH is defined as -log₁₀[H⁺]. Thus, the pH of pure water is -log₁₀(10⁻⁷) = 7. Multiple-distilled water is highly purified water with most impurities removed. While it's difficult to achieve a perfect pH of 7 in practice (e.g., due to dissolved CO₂ from the air forming carbonic acid, making it slightly acidic, or dissolved substances from the container), the theoretical and practical pH of pure, neutral water is very close to 7.