An expression representing the negative logarithm of the effective hydrogen-[[ion]] concentration or hydrogen-ion [[activity]] (in gram equivalents per liter). The pH value is a unit of measure of the [[acid]] or alkaline condition of a substance. A neutral solution (as pure water) has a pH of 7; acid solutions are less than 7; basic, or alkaline solutions are above 7. The pH scale is a logarithmic scale; a substance with a pH of 4 is more than twice as acid as a substance with a pH of 5. Similarly, a substance with a pH of 9 is much more than twice as alkaline as a substance with a pH of 8.