A logarithmic scale from 0 to 14 that measures how acidic or basic (alkaline) a solution is, based on the concentration of hydrogen ions.
- pH 7 is neutral (pure water); below 7 is acidic; above 7 is basic or alkaline.
- The scale is logarithmic: each unit change is a tenfold change in acidity, so pH 4 is ten times more acidic than pH 5.
- Acids release hydrogen ions (H+) and turn blue litmus red; bases release hydroxide ions (OH-) and turn red litmus blue.
- Everyday values: lemon juice and gastric acid are strongly acidic, blood is mildly basic (about 7.4), and soaps are basic.
- Applications: antacids (mild bases) neutralise excess stomach acid, soil pH affects crop growth, and acid rain (low pH) damages buildings and aquatic life.
The pH scale, the neutral value of 7, the acid-base litmus tests, and examples like antacids and acid rain are frequent everyday-chemistry questions.
A lower pH means a stronger acid; a higher pH means a stronger base. The scale is logarithmic, not linear, so a one-unit gap is a tenfold difference.
0 to 14 scale: below 7 acidic, 7 neutral, above 7 basic; each unit is a tenfold change.