pH is a way of describing how acidic or basic a water-based solution is.
On the familiar scale, a pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic, also called alkaline.
But the most important detail is easy to miss.
The pH scale is logarithmic.
What does pH actually measure?
In simplified terms, pH is related to the activity of hydrogen ions in a solution.
Acidic solutions have greater hydrogen-ion activity. Basic solutions have less.
The exact chemical definition gets more technical, but for everyday use, pH is a compact number that tells us where a solution falls on the acid-base spectrum.
Why is pH 7 called neutral?
Pure water at ordinary conditions is often described as having a pH near 7.
At that point, the balance between hydrogen ions and hydroxide ions is neutral.
Acids push the value lower. Bases push it higher.
Common examples help make the scale less abstract. Lemon juice is acidic. Baking-soda solutions are basic. Pure water sits near the middle.
The scale is not a simple ruler
A pH of 4 is not just “a little more acidic” than a pH of 5.
Each whole-number step represents about a tenfold change in hydrogen-ion activity.
So a solution at pH 4 is about ten times more acidic by that measure than a solution at pH 5, and about one hundred times more acidic than pH 6.
That logarithmic behavior lets a relatively short number scale describe an enormous range of chemical conditions.
Does pH always run from 0 to 14?
The familiar classroom scale runs from 0 to 14 because that covers many ordinary water-based solutions.
But pH can technically fall below 0 or above 14 in highly concentrated solutions.
For everyday explanations, 0 to 14 is useful.
It is not an absolute wall built into the universe.
Why does pH matter?
Chemical reactions can change dramatically with pH.
Living organisms also depend on particular pH ranges. Enzymes, cells, soil chemistry, lakes, swimming pools, food, medicine, and industrial processes can all be affected.
Aquarium owners monitor pH because fish and other aquatic organisms may tolerate only certain ranges.
Gardeners care because soil pH changes how available some nutrients are to plants.
Pool owners care because pH affects comfort, corrosion, and how well disinfectants work.
Your body regulates pH very carefully in places where large changes would be dangerous.
How is pH measured?
Litmus paper and universal indicator can provide color-based estimates.
Electronic pH meters use electrodes to produce more precise measurements.
Many classroom demonstrations use indicators because the color changes make an invisible chemical property visible.
Does acidic mean dangerous?
No.
Acidic and basic describe chemistry, not automatic danger.
Orange juice is acidic. Your stomach contains strong acid. Some household acids are mild.
Likewise, a base can be harmless or dangerously caustic depending on concentration and substance.
pH is one useful piece of information, not a universal safety rating.
So pH is not just a mysterious number printed on pool test strips.
It is a compact chemical language.
And because the scale is logarithmic, moving one little number can mean a much bigger change than it looks.
A related explanation is Why does salt melt ice?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why does salt melt ice? and Why does soap remove grease?