Pure water has 10^-7 moles of hydrogen ions per liter; this is represented as 7 on a negative logarithmic pH scale.

A substance that increases concentrations of hydrogen ions has a pH lower than 7 and is called an acid.

A substance that lowers concentrations of hydrogen ions has a pH greater than 7 and is called a base.

Thus a solution with a high concentration of hydrogen ions has a low pH and is called an acid.

And a solution with a low concentration of hydrogen ions has a high pH and is called a base.

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