The pH scale

A scale ranging from 0 to 14

pH (hydrogen ion concentration) is a dimensionless number used to classify an aqueous solution on a scale that generally ranges from 0 to 14.

• pH < 7: the solution is acidic
• pH = 7: the solution is neutral (this is the case for pure water at 25°C)
• pH > 7: the solution is basic

Everyday examples

Solution	Approximate pH
Lemon juice	2
Vinegar	3
Pure water	7
Seawater	8
Soap	10
Drain unblocker (soda)	13–14

The further away from 7, the more pronounced the effect

A pH of 2 corresponds to a solution that is significantly more acidic than one with a pH of 5. Similarly, a pH of 12 is significantly more basic than a pH of 8. We’ll see in the next lesson that this scale isn’t ‘linear’ in the intuitive sense: moving from one pH to another corresponds to multiplying or dividing a concentration by 10.

Measuring pH

pH is measured using pH paper (a colour indicator) for a quick estimate, or with a pH meter for a precise value to one or two decimal places.

Common misconception

Many pupils think that a pH cannot be negative or greater than 14. In reality, the scale from 0 to 14 is a practical convention for common aqueous solutions: solutions highly concentrated in strong acids can have a negative pH, and solutions highly concentrated in strong bases can exceed 14. What matters is the definition of pH (next lesson), not an artificial limit of the scale.