2.31-2.32 - Ions in acids - alkalis and neutralisation
Acids and alkalis are not only identified by indicator colours or pH values. In water, their behaviour can be explained using ions: acids are sources of hydrogen ions, and alkalis are sources of hydroxide ions. Neutralisation happens because the ions from an alkali can remove the acidic effect of the ions from an acid.
Ions in water
An aqueous solution is a mixture where a substance is dissolved in water. This matters because the specification statements about acids and alkalis in this lesson are about aqueous solution, not dry substances.
Aqueous solution
An aqueous solution is a solution in which water is the solvent.
When an acid is dissolved in water, the solution is a source of hydrogen ions, written as H+. When an alkali is dissolved in water, the solution is a source of hydroxide ions, written as OH-.
[DIAGRAM: asset_name: Ions in acids, alkalis and neutralisation - diagram 01; asset_slug: c31_ions_in_acids_alkalis_and_neutralisation__diagram_01; recommended_method: image_gen; description: Monochrome two-beaker schematic comparing an aqueous acid as a source of H+ ions with an aqueous alkali as a source of OH- ions, with dissolved ions shown in water and no colour coding.]
The word source is useful. It means the solution provides those ions for reactions. It does not mean that the only particles present are H+ or OH-; other ions are usually present as well, but H+ and OH- are the ions that explain the acidic or alkaline behaviour in this lesson.
Recognising acid and alkali ions
You do not need a long list of acids and alkalis for this specification point, but you should be able to connect common examples to the ion model.
| Aqueous solution | Important ion for this lesson | Why it matters |
|---|---|---|
Hydrochloric acid, HCl(aq) | H+ | It is an acid because it is a source of hydrogen ions. |
Nitric acid, HNO3(aq) | H+ | It is an acid because it is a source of hydrogen ions. |
Sodium hydroxide, NaOH(aq) | OH- | It is an alkali because it is a source of hydroxide ions. |
Potassium hydroxide, KOH(aq) | OH- | It is an alkali because it is a source of hydroxide ions. |
In hydrochloric acid, chloride ions are also present. In sodium hydroxide solution, sodium ions are also present. These other ions balance charge, but they are not the reason the solution is acidic or alkaline. The acid idea is linked to H+; the alkali idea is linked to OH-.
Take care with the words hydrogen ion and hydrogen gas. A hydrogen ion is H+. Hydrogen gas is H2. They are not the same particle and they are not interchangeable in exam answers.
Continue this lesson.
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