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In chemistry, pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution.


pH is defined as approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. The neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases.

Pure water is neutral, pH 7 at (25 °C), being neither an acid nor a base, although this value is difficult to achieve under atmospheric conditions, as pure water rapidly absorbs carbon dioxide from air, forming carbonic acid which lowers the pH, usually between 5.8-6. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively.

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