Acids, Bases, Salts & Titration

An acid is a source of hydrogen ions (H⁺) in water; a base neutralises it, and a soluble base is called an alkali (a source of OH⁻). When acid meets base the H⁺ and OH⁻ join to make water — that is neutralisation — and the rest of the acid and base form a salt.

Strong vs weak acids

A strong acid (hydrochloric, sulfuric, nitric) is fully ionised in water — every molecule splits to give H⁺. A weak acid (ethanoic, citric, carbonic) is only partially ionised, so at the same concentration it has a higher pH and reacts more slowly. Note: strong is about ionisation, concentrated is about how much is dissolved — they are not the same thing.

The reactions of acids

Making salts

• Soluble salt (acid + insoluble base/metal/carbonate): add the solid in excess so all the acid reacts, filter off the leftover solid, then crystallise the filtrate by evaporating some water and leaving it to cool.
• Soluble salt (acid + alkali): both are solutions, so use titration to find the exact volume that neutralises — then repeat without indicator and crystallise.
• Insoluble salt (precipitation): mix two solutions that contain the right ions; the insoluble salt forms as a precipitate which you filter, wash and dry. Example: barium chloride + sodium sulfate → barium sulfate (insoluble) + sodium chloride.

Titration

Titration finds an unknown concentration accurately. Use a pipette to measure a fixed volume of one solution into a flask, add an indicator, then run the other solution in from a burette until the end point (colour change). Record the volume added.

Exam technique: for a titration calculation always write the balanced equation, then moles of the known → mole ratio → moles of the unknown → concentration. Memory hook for salt names: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.