The pH scale measures the acidity or basicity of a solution. It ranges from 0 to 14. At a pH of 7, a substance is neither acidic or basic and is considered neutral. Pure water is has a pH of 7.

Acidic and basic are two extremes that describe a chemical property of chemicals, especially in the case of solutions.

The term pH was first was introduced in 1909 by the Danish chemist Søren Sørensen (1868–1939) as a convenient way of expressing acidity of a solution. He derived a mathematical formula to do this.

pH = -log_{10}[H+]

Although the original symbol used by Sørensen was p_{H}^{+}. The mathematical formula makes the pH scale logarithmic. This means that a pH of 5 is ten times more acidic than pH 6 and 100 times more acidic than pH 7. On the basic side, (alkaline side) of the pH scale, it is a similar case. A pH of 10 is ten times more alkaline than a pH of 9.

The measurements of pH are important in fields beyond chemistry like biology, medicine, environmental science, engineering, nanotechnology and many more. Many processes only occur at a specific pH.

The “p” in pH does not mean “power” of hydrogen as many sources will state. It was one of two letters that Sørensen used to identify different variables in his work to find a simple way of characterising the acidity or alkalinity of solutions. The other letter he used was “q”.