pH Scale

Summary: A logarithmic scale (0–14) measuring the concentration of H⁺ ions. pH < 7 acidic, pH = 7 neutral, pH > 7 alkaline. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

The pH scale is a logarithmic scale, typically ranging from 0 to 14, that quantifies the acidity or alkalinity of an aqueous solution based on the concentration of hydrogen ions (H⁺): solutions with pH less than 7 are acidic (higher H⁺ concentration than OH⁻), pH exactly 7 is neutral (equal H⁺ and OH⁻ concentrations, as in pure water), and pH greater than 7 is alkaline (higher OH⁻ concentration than H⁺). Because the scale is logarithmic to base 10, each unit change in pH represents a tenfold change in hydrogen ion concentration — a solution of pH 3 has 10 times the H⁺ concentration of pH 4, 100 times that of pH 5, and 1000 times that of pH 6. The pH of a solution can be measured using several methods: universal indicator solution or paper changes colour across the full spectrum from red (strong acid) through orange, yellow, green (neutral), blue, to purple (strong alkali); a pH meter or probe gives a precise numerical reading; and specific indicators such as litmus, methyl orange, and phenolphthalein change colour at defined pH ranges and are used in titrations. The pH of common substances reflects their chemical composition — stomach acid has a pH around 1-2, lemon juice around 2-3, pure water is exactly 7, and household cleaning products like bleach and ammonia are around 11-12. In IGCSE exams, students must understand the logarithmic nature of the pH scale, be able to interpret indicator colour changes to estimate pH, and relate pH values to the relative concentrations of H⁺ and OH⁻ ions in solution.

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.