In simple terms
pH tells you how acidic or basic a water based liquid is. Lemon juice is acidic, a baking soda solution is basic, and pure water sits between them.
The scale runs from 0 to 14. Seven is neutral, below 7 is acidic, and above 7 is basic, also called alkaline.
How it works
The US Environmental Protection Agency defines pH as the negative base 10 logarithm of the hydrogen ion concentration in moles per liter.
Because it is a logarithm, each whole step is a tenfold change. Water at pH 5 is ten times more acidic than water at pH 6, and a hundred times more acidic than water at pH 7.
Useful ranges are set by purpose, not by nature. US water quality criteria for fresh water suggest 6.5 to 9, and most aquatic organisms do best between 6.5 and 8. Normal rainfall is already slightly acidic, around pH 5.6, because carbon dioxide from the air dissolves into it.
How readily a water body shifts depends on its buffering capacity, meaning its ability to soak up added hydrogen ions without much change in pH.
Why it matters
pH decides what dissolves. The EPA notes that it alters the chemistry and availability of pollutants, and can make metals and nutrients more toxic to living things.
It also governs ordinary processes: soil fertility, pool treatment, brewing, food preservation and drinking water safety.
Where you’ll see it
- Water quality reports from utilities and regulators
- Soil test results for farms and gardens
- Pool and aquarium test strips
- Labels on shampoos and cleaning products
Example
A river station records pH 6.2 after heavy rain. That sits below the 6.5 floor in US freshwater criteria, so staff check for acid drainage upstream.
Often confused with
pH is not buffering capacity. pH is the acidity right now. Buffering capacity is how much acid the water can absorb before that number moves.
Key facts
- The EPA defines pH as the negative base 10 logarithm of hydrogen ion concentration in moles per liter.2
- The scale runs from 0 to 14, with 7 neutral, below 7 acidic and above 7 basic.1
- Each whole pH unit is a tenfold change, so water at pH 5 is ten times more acidic than water at pH 6.1
- Normal rainfall has a pH of about 5.6, slightly acidic because of carbon dioxide from the atmosphere.1
- US water quality criteria for fresh water suggest a pH range of 6.5 to 9, and most aquatic organisms do best between 6.5 and 8.2
Related concepts
Quick checkHow much more acidic is water at pH 4 than water at pH 6?Show answer
One hundred times, because each whole unit is a tenfold change.
Sources
- U.S. Geological Survey, Water Science School. pH and Water. 24 August 2026 (accessed 16 September 2026)
- U.S. Environmental Protection Agency, CADDIS. pH. 22 January 2026 (accessed 16 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue