Ph value:

A unit of measure of the acid of alkaline condition of a substance.

A neutral solution (such as pure water) has a ph of 7; add solutions are less than 7; basic, or alkaline, solutions are more than 7.

The ph scale is a logarithmic scale.

A substance with a ph of 4 is more than twice as add as a substance with a ph of 5.

Similarly, a substance with a ph of 9 is more than twice as alkaline as a substance with a ph of 8.