The pH Scale and Indicators

This lesson covers the pH scale, how it relates to acidity and alkalinity, the use of indicators, and the important distinction between strong and weak acids (Higher tier). This content is part of the Edexcel GCSE Chemistry specification (1CH0).

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The pH Scale

The pH scale is a numerical scale from 0 to 14 that measures how acidic or alkaline a solution is. The scale is based on the concentration of hydrogen ions (H⁺) in solution.

pH Range	Classification	Examples
0–2	Strong acid	Battery acid (pH 0), stomach acid (pH 1–2)
3–6	Weak acid	Vinegar (pH 3), orange juice (pH 4), black coffee (pH 5), milk (pH 6)
7	Neutral	Pure water, sodium chloride solution
8–11	Weak alkali	Baking soda solution (pH 8–9), milk of magnesia (pH 10), ammonia solution (pH 11)
12–14	Strong alkali	Limewater (pH 12), sodium hydroxide solution (pH 13–14)

Key Rules