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What does pH actually measure?

The GCSE picture: the pH scale, hydrogen ions, and why each step is ten times.

  1. A glass of lemon juice10 cm
  2. A drop4 mm
  3. A droplet1 µm
  4. The ions1 nm
pH = −log10[H+]
What the simulation shows, in words

A square of 10,000 tiny blue dots stands for 10,000 water molecules. Crimson dots mark the hydrogen ions that go with them at the chosen pH. At pH 0 there are about 180, at pH 1 about 18, and at pH 2 about 2. From pH 3 upwards there is less than one in the square, and the readout says how many squares you would need to find one. A scale from 0 to 14 marks the pH, with everyday examples: lemon juice about 2, black coffee about 5, pure water 7, blood 7.35 to 7.45, sea water about 8.1 and bleach about 12.5 to 13.

What makes an acid an acid

When an acid dissolves in water, it releases hydrogen ions, H⁺. Hydrochloric acid does it completely:

HCl(aq) → H+(aq) + Cl−(aq)

Alkalis release hydroxide ions, OH⁻. When an acid and an alkali meet, the two ions join to make water. That is neutralisation:

H+(aq) + OH−(aq) → H2O(l)

The pH scale measures how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral and above 7 is alkaline. You measure it with universal indicator, which changes colour, or with a pH probe, which gives a number.

Each step is ten times

pH is not an ordinary scale. Every step down is ten times more hydrogen ions. A solution at pH 3 has ten times the H⁺ concentration of one at pH 4, and a hundred times that of one at pH 5.

The simulation shows this as a count. Imagine looking at 10,000 water molecules. At pH 1 about 18 of them have a hydrogen ion beside them. At pH 2 it is about 2. By pH 7, pure water, you would need more than 50,000 squares like that to find a single one. Move the slider and watch how fast the crimson dots disappear.

Strong is not the same as concentrated

  • A strong acid, such as hydrochloric, nitric or sulfuric acid, is completely ionised in water: every molecule releases its H⁺.
  • A weak acid, such as ethanoic acid (in vinegar), citric acid or carbonic acid, is only partly ionised. Most of its molecules keep their hydrogen.
  • Concentrated and dilute say how much acid there is in each dm³ of solution, which is a separate question.

So a dilute strong acid and a concentrated weak acid can have the same pH. For acids of the same concentration, the strong one has the lower pH.

Safety

Never taste or touch chemicals to test them, and never mix household cleaning products: some, such as bleach and acidic cleaners, release toxic gas together. pH testing with real acids and alkalis belongs in a school lab, with eye protection and a teacher.