Acid reactions — three fixed patterns, each making a salt
Hydrochloric, sulfuric and nitric acids react in three patterns, each making a salt (the spec excludes nitric acid + metal): acid + metal gives salt + hydrogen; acid + base (a metal oxide or hydroxide) gives salt + water; acid + metal carbonate gives salt + water + carbon dioxide. The acid fixes the salt's second name — hydrochloric to chloride, sulfuric to sulfate, nitric to nitrate. Note which extra product pairs with each reactant type.
Proton transfer — acid donates, base accepts
The 4SD0 spec defines acids and bases by proton transfer, where a proton is a hydrogen ion, . An acid is a proton DONOR; a base is a proton ACCEPTOR — quote donor and acceptor exactly. In a neutralisation the base accepts the proton the acid donates: . The bases in 4SD0 are metal oxides, metal hydroxides and ammonia; an alkali is any base that also dissolves in water.
Solubility rules — learn the exceptions
Soluble: all sodium, potassium and ammonium salts, and all nitrates. Chlorides are soluble except silver and lead(II). Sulfates are soluble except barium, calcium and lead(II). Carbonates are insoluble except sodium, potassium and ammonium. Hydroxides are insoluble except sodium, potassium and calcium (calcium hydroxide slightly soluble). They also show which solids are insoluble — copper(II) oxide, magnesium carbonate, zinc — good starting solids for a preparation.
Drawn from real examiner reports.
The solid is in excess, not the acid
When making a soluble salt from an insoluble reactant, the insoluble solid (metal, base or carbonate) is added IN EXCESS so all the acid is used up; the leftover solid is then filtered off, leaving a pure salt solution. Candidates often say the acid is in excess — but excess acid would stay dissolved in the filtrate, contaminate the crystals and could not be filtered out.
A base is not always an alkali
A base is the general term — a metal oxide, metal hydroxide or ammonia that neutralises an acid. An alkali is narrower: a base that is SOLUBLE in water. Every alkali is a base, but not every base is an alkali. Copper(II) oxide is a base (it neutralises acid) but NOT an alkali, because it is insoluble in water. Only call a base an alkali once you know it dissolves.
Name the salt from the acid
The salt's second name comes from the acid: hydrochloric gives a chloride, sulfuric gives a sulfate, nitric gives a nitrate. Writing "magnesium hydrochloride" or "zinc sulfuric" scores nothing — the salts are magnesium chloride and zinc sulfate. Match the metal (or ammonium) to the acid's anion name, never to the acid's own name.
Anion formula and brackets
Get the anion right: the sulfate ion is (four oxygens, 2− charge), so copper(II) sulfate is , not or . And a number outside a bracket multiplies everything inside it — is one and two ions. Build every salt formula from balanced charges.
Digest: a number outside a bracket multiplies everything inside it; Pb(NO3)2 is one Pb2+ and two NO3- (Jun23 Q6aii-aiii).
Crystallise — never evaporate to dryness
Grow hydrated copper(II) sulfate crystals (, blue) by heating the filtered solution to the point of crystallisation, then cool slowly. Do NOT evaporate to dryness — that drives off the water of crystallisation and leaves white anhydrous powder, not blue crystals. "Evaporate the water" alone loses the mark.
Digest: candidates confuse making crystals with a drying step when preparing a solid salt (Jun23 Q9bi).
Base the calculation on the solid
In reacting-mass questions the acid is in excess, so the insoluble solid is the limiting reactant — base your moles on the solid. Use the Edexcel data-sheet masses (, never 64), keep full figures through the working and round only at the very end.
Write-and-balance scaffold for equations
For equation questions: (1) name the salt from the acid and write its formula from balanced charges; (2) add the second product by reaction type — hydrogen (metal), water (base), water + carbon dioxide (carbonate); (3) balance, as sulfuric acid is diprotic (two ).
Choose the preparation route from solubility
For a soluble salt from an insoluble reactant: add the insoluble solid in excess to warm dilute acid, stir until no more reacts, filter off the excess solid, then crystallise the filtrate. Use the solubility rules first to confirm the salt is soluble and the solid is insoluble.
The three reactions of acids (state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous):
| Reactant | Products | Example |
|---|---|---|
| Acid + metal | salt + hydrogen | |
| Acid + base (metal oxide/hydroxide) | salt + water | |
| Acid + metal carbonate | salt + water + carbon dioxide |
(The spec excludes nitric acid + metal, so do not write that reaction.)
Define an acid in terms of proton transfer.
Copper(II) oxide, , is a base used to prepare copper(II) sulfate.
Show that the relative formula mass () of copper(II) oxide is 79.5.
(: Cu = 63.5, O = 16.)
