pH – IGCSE Chemistry Definition

pH appears on every IGCSE Chemistry paper. Paper 1 tests colour changes with indicators, Paper 2 asks for definitions, and Paper 4 connects pH to strong/weak acids and to neutralisation titration curves. Understanding the scale thoroughly is non-negotiable.

The 0620 definition

pH is a measure of the acidity or alkalinity of a solution, on a scale from 0 to 14.

• pH < 7: acidic (more H⁺ ions than OH⁻ ions)
• pH = 7: neutral (equal H⁺ and OH⁻ ions, e.g. pure water)
• pH > 7: alkaline (more OH⁻ ions than H⁺ ions)

Measuring pH

Method	What it tells you	Precision
Litmus paper	Acid or alkali only	Low — just acidic/alkaline
Universal indicator	Approximate pH number	Medium — colour matches to a chart
pH meter / probe	Exact pH value	High — to one or two decimal places

Universal indicator gives a continuous colour change: red (pH 1) → orange → yellow → green (pH 7) → blue → purple/violet (pH 14).

pH and H⁺ concentration (Supplement)

pH is related to H⁺ concentration. Each step down the pH scale represents a tenfold increase in H⁺ concentration. A solution at pH 1 has ten times more H⁺ ions than a solution at pH 2.

This is why strong acids and weak acids at the same concentration have different pH values. A 0.1 mol/dm³ strong acid (fully ionised) has a lower pH than a 0.1 mol/dm³ weak acid (partially ionised).

pH during neutralisation

When you add alkali to acid:

1. pH starts low (acidic)
2. pH rises as H⁺ ions are consumed
3. At the equivalence point, pH = 7 (for strong acid + strong alkali)
4. Excess alkali pushes pH above 7

This forms the basis of titration calculations.

Worked exam question

A student tests four solutions with universal indicator. Solution A is red, B is green, C is yellow, D is purple. (a) Arrange in order of increasing pH. [1] (b) Identify which solution is neutral. [1] (c) State which solution could be sodium hydroxide. [1]

Mark scheme

(a) A, C, B, D [1]

(b) B (green = pH 7) [1]

(c) D (purple = strongly alkaline) [1]

Common exam mistakes

• Saying “pH measures how strong an acid is” — pH measures acidity (H⁺ concentration), not strength. A dilute strong acid and a concentrated weak acid could have the same pH.
• Reversing the scale — lower pH means more acidic, not less.
• Forgetting that water has a pH of 7 because it contains equal concentrations of H⁺ and OH⁻, not because it contains none of them.