What is the pH scale?

The pH scale measures how acidic or alkaline a water-based solution is, running from 0 to 14. A value of 7 is neutral, like pure water; values below 7 are acidic, such as lemon juice or vinegar, and values above 7 are alkaline or basic, such as soap or bleach. The scale is based on the concentration of hydrogen ions and is logarithmic, meaning each whole step represents a tenfold change: a liquid at pH 4 is ten times more acidic than one at pH 5. Controlling pH is vital in biology, agriculture and industry, and the pH of human blood is kept within a narrow range around 7.4.