Definition · Plain-language
The pH scale
The pH scale measures how acidic or alkaline a solution is, running from 0 to 14 based on the concentration of hydrogen ions.
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What pH measures
pH is a measure of the concentration of hydrogen ions (H⁺) in a solution, which determines how acidic or alkaline it is. The more hydrogen ions present, the more acidic the solution and the lower the pH. The "p" stands for a mathematical operation (the negative logarithm) and "H" for hydrogen ion concentration. Most common solutions fall on a scale from 0 to 14, though values slightly outside this range are possible for very strong, concentrated acids and bases.
Acidic, neutral and alkaline
The scale has three regions. A pH below 7 is acidic, with more hydrogen ions than hydroxide ions; lemon juice and stomach acid sit here. A pH of exactly 7 is neutral, where hydrogen and hydroxide ions balance — pure water at room temperature is the standard example. A pH above 7 is alkaline, or basic, with more hydroxide ions; soap and oven cleaner lie at this end. Indicators such as litmus or universal indicator give a quick colour reading, while a pH meter measures the value precisely.
Why the scale is logarithmic
The pH scale is logarithmic, not linear, which has an important consequence: each whole-number change in pH represents a tenfold change in hydrogen ion concentration. A solution of pH 4 is ten times more acidic than one of pH 5, and a hundred times more acidic than one of pH 6. This is why small-looking pH differences can matter so much in biology and the environment, and why the scale can express an enormous range of concentrations in convenient single-digit numbers.
Key facts
At a glance
- Definition: a measure of acidity or alkalinity from hydrogen ion concentration
- Range: usually 0 to 14
- Acidic: below 7 (more H⁺ ions)
- Neutral: exactly 7 (pure water at 25 °C)
- Alkaline: above 7 (more OH⁻ ions)
- Logarithmic: each unit is a tenfold change in H⁺ concentration
Common misconceptions
What people often get wrong
Often heard: The pH scale is linear, so pH 4 is only a little more acidic than pH 6.
Actually: The scale is logarithmic. Each whole unit is a tenfold change, so pH 4 is a hundred times more acidic than pH 6, not slightly more.
Often heard: pH can only ever be a value between 0 and 14.
Actually: 0 to 14 covers most everyday solutions, but very strong, concentrated acids or bases can give pH values slightly below 0 or above 14.
Often heard: A pH of 7 is always neutral, at any temperature.
Actually: Neutral pH equals 7 specifically at 25 °C. Because water’s ionisation changes with temperature, the exact neutral point shifts slightly at other temperatures.
Going deeper
