2.344CH1 · Section 2: Inorganic chemistry

Acids, bases and salt preparations

Key learnings

•
Acids react with metals, bases, carbonates
Hydrochloric, sulfuric and nitric acids react in three patterns (nitric acid + metal excluded):
- acid + metal → salt + hydrogen ( $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$ )
- acid + base → salt + water ( $\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}$ )
- acid + carbonate → salt + water + $\text{CO}_2$ ( $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$ )
Acid = proton donor; base = proton acceptor
The spec defines acids and bases by proton transfer (a proton is a hydrogen ion, $\text{H}^+$ ). An acid is a proton (H⁺) donor; a base is a proton (H⁺) acceptor. Dissolving hydrogen chloride in water donates a proton to water: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$ In neutralisation the base accepts it: $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ . Quote donor / acceptor exactly (statements 2.35–2.36).
•
Solubility of the salt sets the prep route
Use the solubility rules to pick the route:
- Soluble salt from an insoluble base/metal/carbonate (2.39): add the insoluble solid in excess, filter off the excess, crystallise the filtrate.
- Soluble salt from acid + alkali (2.40C): both soluble, so titrate for the exact volumes, then repeat without indicator.
- Insoluble salt from two soluble solutions (2.41C): mix to precipitate, then filter, wash and dry the residue.

Common mistakes

Drawn from real examiner reports.

Evaporating to crystallise an insoluble salt

For an insoluble salt made by precipitation (e.g. lead(II) sulfate), the salt is the solid that forms — collect it by filtering and keep the residue. Candidates wrongly "heat the mixture to evaporate some liquid so crystals form on cooling", but evaporating the filtrate crystallises the soluble by-product, not the salt asked for — capping them at one mark.

June 2024 Paper 2C examiner report Q6(a) (pure dry lead(II) bromide): "candidates thought that they needed to heat the mixture to evaporate some of the liquid and on cooling crystals will form... This limited them to a maximum of one mark."

Not washing the residue, or only saying "dry"

After filtering off an insoluble salt, two points are routinely dropped. You must wash the residue with distilled water to remove soluble impurities, and you must name a drying method — "dry it" alone is not enough (e.g. a warm oven, or pressing between filter papers). The full route is filter → wash → dry (named method).

June 2024 Paper 2C examiner report Q6(a): marks were lost "for not washing the residue" and for writing only "dry" with no drying method named.

Saying the ACID is in excess

In the soluble-salt-from-an-insoluble-reactant route, the insoluble solid (metal, base or carbonate) is added in excess so that all the acid is used up; the excess solid is then filtered off. Candidates wrongly say the acid is in excess — but leftover acid would stay dissolved, contaminate the crystals, and could not be filtered out.

June 2024 Paper 1CR examiner report Q7(a)(ii): "Over half the candidates mentioned that zinc is in excess. A few lost the mark for stating that the acid was in excess." The solid reactant — not the acid — must be in excess.

Calling every base an alkali

An alkali is not just "a base" — it is a base that is soluble in water (giving $\text{OH}^-\text{(aq)}$ ). A base is any metal oxide, metal hydroxide or ammonia that neutralises an acid. Every alkali is a base, but not every base is an alkali — copper(II) oxide is a base but not an alkali because it is insoluble.

(common Edexcel chemistry error)

Naming the salt after the acid, not the anion

The salt name comes from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. Writing "magnesium hydrochloride" or "zinc sulfuric" scores nothing — the correct names are magnesium chloride and zinc sulfate. The metal (or ammonium) gives the first word; the acid gives the second.

(common Edexcel chemistry error)

Forgetting the solubility exceptions

Solubility predictions fail when the exceptions are forgotten. Silver chloride and lead(II) sulfate are insoluble (used as precipitates in 2.41C). Most carbonates are insoluble, but sodium, potassium and ammonium carbonates are soluble. Learn each rule with its exception: chlorides soluble except silver and lead(II); sulfates except barium, calcium and lead(II).

(common Edexcel chemistry error)

Exam tips

Choose the route from solubility first
Decide the route before writing anything. Is the target salt soluble or insoluble? Soluble + from an insoluble reactant → excess solid, filter, crystallise. Soluble + from acid + alkali → titration. Insoluble → precipitate, filter, wash, dry.
Name the salt, then balance by type
To write the equation: (1) name the salt (chloride/sulfate/nitrate + metal or base); (2) add the other product(s) — $\text{H}_2$ (metal), $\text{H}_2\text{O}$ (base), $\text{H}_2\text{O} + \text{CO}_2$ (carbonate); (3) balance — sulfuric acid is diprotic.
Spell out excess, drying and state symbols
In a preparation answer, spell out the examinable details: state which reactant is in excess and why, name a drying method rather than only "dry", and include state symbols when asked. On any mass calculation use the data-sheet $A_r(\text{Cu}) = 63.5$ , never 64.

Revision notes