A strong acid is an acid that ionises completely in water to release a high concentration of hydrogen ions.
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A strong acid is an acid that ionises completely in water to release a high concentration of hydrogen ions, H⁺. The key word is completely: almost every acid molecule splits up and gives its hydrogen to the water, so a strong acid of a given molarity produces the maximum possible amount of H⁺. This is why strong acids react quickly, conduct electricity well and give the lowest pH readings.
A worked example fixes the idea. Hydrochloric acid ionises fully, HCl → H⁺ + Cl⁻, so a 1 mol dm⁻³ solution contains close to 1 mol dm⁻³ of hydrogen ions. Sulfuric acid and nitric acid behave the same way and are the three strong acids you meet most often in SPM. Because the ionisation is written as a single forward arrow, you can read straight off the equation that the acid has gone almost entirely to ions.
The confusion to avoid is mixing up strong with concentrated. Strength describes how completely an acid ionises; concentration describes how much acid is dissolved in a given volume. A strong acid can be dilute, and a weak acid can be concentrated. A dilute hydrochloric acid is still a strong acid because the little that is there is fully ionised, while a concentrated ethanoic acid is still a weak acid because only a small fraction of it ionises. Using “strong” when you mean “concentrated” is a classic marking-scheme trap.
Within this chapter the strong acid pairs with the weak acid, and the same completeness idea reappears for alkalis. Our teachers train you to separate strength from concentration on sight, so you never explain a low pH by the wrong reason.
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