Asked by: Adam King, Huddersfield
The pH scale typically stretches from zero to 14, passing through a neutral pH7 (freshly distilled water). Strong acids have a low pH, while alkaline chemicals, such as bleach and liquid drain cleaner, have a high pH.
The scale was invented in 1909 by a Danish biochemist called Søren Sørensen. It describes how many hydrogen ions (protons) are present in a solution: the higher the pH, the lower the hydrogen ion concentration, and vice versa. But the scale does not have fixed limits, so it is indeed possible to have a pH above 14 or below zero. For example, concentrated hydrochloric acid can have a pH of around -1, while sodium hydroxide solution can have a pH as high as 15.