Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.
A strong acid completely ionises in water; a weak acid only partly ionises. Strong examples: hydrochloric, nitric and sulfuric acids. Weak examples: ethanoic, citric and carbonic acids. The diagram compares the same concentration of two simplified acids; HA means an un-ionised acid particle.
Concentrated or dilute describes the amount of acid per volume of solution. Strong or weak describes the proportion ionised. A strong acid can be dilute; a weak acid can be concentrated. Adding water dilutes an acid; it does not change its identity from strong to weak.
For acids of the same concentration, the stronger acid gives more H⁺ ions and a lower pH. Each fall of 1 pH unit means 10 times greater H⁺ concentration. pH 2 has 10 times more H⁺ per volume than pH 3, and 100 times more than pH 4. A rise of 1 pH unit means one tenth the H⁺ concentration.
Strong = fully ionised; weak = partly ionised. One pH unit is a factor of 10 in H⁺ concentration.
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