In chemistry, pH is a numeric scale used to specify the acidity or alkalinity of an aqueous solution. It is roughly the negative of the logarithmto base 10 of the concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion. Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are alkaline or basic. Pure water is neutral, being neither an acid nor a base.