What Is pH in Water? A Plain-Language Explanation

pH is a number that describes how acidic or basic (alkaline) a liquid is, on a scale that usually runs from 0 to 14. Pure water at 25 degrees Celsius sits at 7, which is neutral. Lower numbers mean more acidic, higher numbers more basic. Most tap water falls somewhere between about 6.5 and 8.5, and by itself the figure tells you very little about whether water is clean or safe.

How the pH scale works

The letters stand for “potential of hydrogen”. The number reflects the concentration of hydrogen ions in the liquid. The scale is logarithmic, so 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.

Some familiar reference points:

• Lemon juice and vinegar: strongly acidic, far below 4
• Natural rain: slightly acidic, typically around 5.6, because carbon dioxide from the air dissolves in it to form a weak acid
• Pure water: 7 at 25 degrees Celsius
• Seawater: mildly basic, usually a little above 8
• Household ammonia and bleach: strongly basic, well above 10

Temperature shifts the neutral point slightly, which is why the 25 degree condition is usually stated.

Why tap and bottled water vary

Water picks up minerals and gases along its route. Water that flows over limestone dissolves calcium carbonate, which pushes pH upward. Water from granite areas or peaty soils tends to be softer and a little more acidic. Dissolved carbon dioxide lowers pH; photosynthesis in lakes can raise it during the day.

Treatment plants also adjust pH on purpose. Slightly basic water is less likely to corrode metal pipes, so many utilities finish their treatment around the neutral-to-mild-basic range.

What counts as normal for drinking water

In the United States, the Environmental Protection Agency lists a pH range of 6.5 to 8.5 as a secondary standard. Secondary standards are non-enforceable guidelines about taste, colour, odour and effects on plumbing, not about health. The World Health Organization notes that it has no health-based guideline value for pH, but says the level matters for water treatment and corrosion control, and commonly cited operating ranges are similar.

In plain terms: pH outside that range may make water taste metallic or bitter, may stain fixtures, or may signal that pipes are being attacked. It does not by itself mean that the water contains a harmful contaminant, nor that a neutral reading means it is free of one.

How pH is measured

There are three common ways:

1. Test strips. Cheap, quick and rough. Good for a ballpark figure.
2. Liquid reagent kits. A colour change is compared against a chart. Slightly more precise.
3. Electronic meters. A probe gives a digital reading and needs regular calibration with buffer solutions to stay accurate.

A common mistake is testing water that has been sitting in an open glass. Dissolved carbon dioxide from the air can slowly lower the reading, so test soon after collecting a sample.

pH versus water quality

A single pH number cannot tell you about bacteria, lead, nitrate, pesticides or other contaminants. Two waters with the same pH can be very different in safety. Public water systems in the US publish an annual consumer confidence report that lists detected contaminants; a laboratory test is the way to check a private well.

pH and the human body

The body keeps blood pH within a very narrow range, roughly 7.35 to 7.45, using the lungs and kidneys. Stomach acid is strongly acidic by design and neutralises most of what is swallowed. For these reasons, mainstream health bodies do not treat the pH of ordinary drinking water as something that meaningfully changes the body’s acid-base balance. Claims that a particular water pH can treat, prevent or cure disease are not supported by established medical guidance.

Frequently asked questions

Is a pH of 7 the best for drinking water?

Not necessarily. Neutral water is simply neutral. Safe drinking water commonly ranges from about 6.5 to 8.5, and quality depends mostly on what else is dissolved in it.

Can pH affect the taste of water?

Yes. Lower pH can taste sour or metallic, while higher pH can taste soapy or bitter. Mineral content also strongly affects taste.

Does boiling change the pH?

It can shift slightly because dissolved gases leave the water, but the change is small and varies by source.

Further reading

The U.S. Geological Survey explains the topic in more depth on its pH and water page. On this site, the plain-language guide to pH levels and your water covers related background, and the drinking water news page collects reports on local water problems. The site home page lists other topics.

This article is general information. For questions about your own water supply or health, speak to your local water utility or a qualified health professional.