What is the pH of pure water?
Pure water has a pH of 7 at 25 degrees Celsius, which is neutral. Values below 7 are acidic and values above 7 are basic, or alkaline.
The pH scale measures how acidic or basic a solution is, usually from 0 to 14. It is based on the concentration of hydrogen ions. In pure water, a few water molecules split into hydrogen ions and hydroxide ions in equal amounts, which balances out to a neutral pH of 7.
The scale is logarithmic, so each step represents a tenfold change. Something with a pH of 5 is ten times more acidic than something with a pH of 6 and a hundred times more acidic than pure water. Lemon juice has a pH of about 2, while household bleach is around 12 or 13.
Real-world water
Water is rarely perfectly pure. Rainwater is usually slightly acidic, around pH 5.6, because it absorbs carbon dioxide from the air. Tap water typically ranges from about 6.5 to 8.5. The pH of pure water also changes slightly with temperature, dropping a little as water gets hotter.
Did You Know?
Human blood is kept within a narrow range, around pH 7.35 to 7.45. Even small changes outside this range can make a person seriously ill.